ContentsDesktop Power Supply from a PC................................................................................................ 2

Remote Sensing ............................................................................................................................10

Mouse over the plugs for pinouts and voltages .........................................................................11

The Fix ..........................................................................................................................................13

Power_OK or Power_Good ........................................................................................................14

Desktop Power Supply from a PC

Updated March 13, 2009

(See the narrative and disclaimer at the bottom of the page)

Do you have an interest in converting one of these:

into one of these:

A completed 145 watt ATX power supply with switch, binding posts, labels and feet. Noticethe zip ties in the ventilation slots

that hold the load resistor.

If you find building your own desktoppowersupply from a recycled PSU and afew parts from the local electronics store

appealing, then grab some tools, pouryourself a cup of coffee (or personal

preference) and let's get started. The LED(light emitting diode) was also salvaged

from an old PC. If you want to add a poweron indicator, LED's add a nice touch andcan easily be wired into the +5v rail. I do

strongly encourge you to read the contentsof this site and associated links before

beginning your conversion -- there are a number of hints included in the associated pages.

This ATX PS board has leads for +5 (RED), -5 (WHITE), +12 (YELLOW), -12 (BLUE)volts, Ground (BLACK) and switch (GREEN). Be warned that some DELL power suppliesmanufactured between 1996 and 2000 do not follow the industry standard pinout and colorcodes. The fan has also been unplugged for better viewing. Since this PS was converted foruse in the logic and robotics labs, the selected voltages were tapped. Other users may wantcombinations of +3.3 V (ORANGE), +5 V and/or +12 V if they are converting one of thenewer supplies. For R/C applications, the 5 volt output can also serve as a desktop source todrive receivers and servos. If used as a power source for the micro and sub-micro servos, youmust be careful not to drive the servo to either endpoint to avoid stripping the smaller gears inthese units. Most standard servos have sufficiently robust gear trains and will simply stall ifpushed to the mechanical stops..

Measured voltages on this particular PS (1996 P5-100 MHz Gateway) were about 5.15 and11.75 volts. The remaining leads have been clipped off at the circuit board.

View of the case top with fan, binding posts and switch. The switch (SPST) and binding postsare available at Radio Shack or other electronics suppliers.

Power supplies in today's computers are known as SWITCHMODE or Switching Modepower supplies and require a load to continue to operate after being switched on (the termswitching mode actually applies to the technique of A/C to D/C conversion and not to thepower up action). This load is provided by a 10 watt, 10 ohm wire wound load resistor(sandbar - about $0.80 at Radio Shack) across the +5 volt supply. While many of the newerpower supplies will Latch_On without a preload, you will find that adding the resistor will (1)increase the measured voltage on the 12 volt rail slightly and (2) help stabilize the voltagelevel in this rail by minimizing voltage drop when the powersupply is loaded with a charger.Some inexpensive power supplies may fail if forced on without a load although the DesignGuide states that the supplies should not be damaged if run without a sufficient load. Thesandbar resistor has been zip tied to the case with a small amount of heat sink compoundapplied to the flattest side of the resistor . I will also take a file and remove any stamping flashthat may remain around the ventilation slots. Without cooling, the resistor will get very hot

and may fail prematurely; with this arrangement, the resistor will remain barely warm to thetouch.

Be warned that many of the heat sink greases can be quite toxic and any excess should becleaned up and disposed of properly. Also be sure to thoroughly clean your hands and toolsafter use. While most heatsink compounds are rated to 160 to 170 C, some may dry out overtime and their effectiveness will diminish -- a periodic check for good contact between thecase and resistor is a recommended practice.

Additional comments

Disclaimer: The information presented should not be considered a "HOWTO" article,but merely a documentation of my conversion process. Modern PC Power Supplies can

produce high output current levels that may cause internal overheating in the PS ordamage to devices connected to them. Any individual attempting their own conversion is

cautioned to carefully research their PS specifications and to be mindful of theassociated voltages and power. DO NOT work on your opened power supply with it

plugged in!!!!

The PS in the picture is a 145 watt ATX salvaged from a 1996 P5-100 MHz Gateway -- Isalvage all usable parts from the older PC's before dumping them. This one is set up for alogic lab, hence the +5, -5, +12, -12 volt taps. We also use the +5 to drive servos in therobotics lab. This supply does not have a 3.3 V source, but the newer supplies do. INTEL hascontinued to modify the ATX specifications to include additional power connectors to supportthe increased power requirements of the newer motherboards. Before any modification isattempted, you should be sure of the type of power supply you are working with and theoutput currents being produced at each voltage level. Higher wattage supplies can generatefairly hefty levels of current and may overheat or damage devices attached to them. See theTable of Representative Current Levels for other power supplies.

Wiring coming off an industry standard circuit board will be:

ORANGE +3.3 VYELLOW +12 VBLUE -12 VRED +5 V

WHITE -5 V (May not be present on recently manufacturedsupplies)

BLACK GND

GREEN POWER-ON (Active high -- must be shorted toground to force power up)

GRAY POWER-OK What is this??PURPLE +5 V STANDBYBROWN +3.3 V REMOTE SENSING Design Guide Update

*** Note that the 1996-2000 Dell's did not completely follow this color coding -- check yourvoltage levels with a meter before wiring ***

The yellow, red and black wires will likely be grouped together with a clip. Some of the PS'swill have a detachable plug for the fan and some will have the fan permanently attached to thecircuit board. If the fan is attached, I usually clip the wires then re-solder and cover withheatshrink tubing -- this gives more working room while modifying the PS and allows me tolube the fan.

If you are going to use only the +12v and +5v, you may clip the other wires at the circuitboard level or leave the unused wires about an inch long, gather common colors together, slipa piece of heatshrink tubing over the bundle and shrink -- it is an easy way to corral andinsulate loose ends.

For the +5 / +12 volt PS, you will need the following combinations:

GREEN / BLACK Power on Switch (Use a SPST switch; a momentaryswitch will not work)

RED / BLACK Pre-Load Resistor (See text for recommended valuesand a possible substitution)

YELLOW / BLACK +12 volt sourceRED / BLACK +5 volt sourceORANGE / BROWN See the Design Guide Update

I use a single common post (GND -- black) for all voltage sources. Our loads are light and wedon't require separate grounds for each.

Leave 3 black wires -- switch, load resistor and common (GND) binding post

Leave 2 red wires -- 5 volt binding post and load resistor

Leave 1 yellow wire -- 12 volt binding post

Leave the green wire -- power on switch

If sense wires are present, refer to the Design Guide Update

If you expect to place high current demands on your powersupply, it may be prudent to runtwo wires to each binding post -- while it is very unlikely that the 18 AWG wire will overheat,there have been some instances of melted wires and connectors occurring on high demandmotherboards.

Cut everything else off even with the board or bundle together as noted above. I usually cutthe power harnesses so I can keep as much together as possible. The wires remaining in thepower supply should be left long and cut to length as needed. If you leave them too long, they

will get in the way when boxing it up, especially if the fan is internal rather than external. Besure that they stay out of the way of the fan blades.

Wire in your power switch between the green (PS_ON) rail and any DC ground (black). Theswitch (single pole, single throw) and binding posts can be found at local electronics supplyhouses or online. If your powersupply has a master switch, usually located near the AC plug,you may simply solder the green PS_ON directly to DC ground and use the master switch topower up. This works just as well and will save you the expense of a switch and time neededto install it.

Install the 10 ohm 10 watt pre-load resistor between DC ground and the +5V rail (red). Don'tforget to heatsink this resistor.

Attach your other rails, DC ground, +12v, and +5v if used, to the appropriate binding posts.These posts must not be grounded to the powersupply case, so be sure to check for anycontinuity between the case and post before trying to powerup the supply.

If you want to add a power on indicator light, now's the time to do it. LEDs are quiteinexpensive, have incredibly long lifetimes if run at 20 ma or less, produce essentially no heatand can be wired to the +5v rail. However, LEDs are current driven devices and will require adropping resistor to ensure that it does not burn out immediately. A 1/4 watt carbon filmresistor rated at 180 to 220 ohms wired between either of the leads and the PSU will worknicely. LEDs, being a diode, are also polarized and must be wired with the positive lead(anode) attached to +5v rail and the negative lead (cathode) attached to DC ground. LEDshave a flat molded into one side of the base --- this flat will be on the same side as thecathode. If your LED is new and has not had the leads shortened, the longest leg will be thepositive lead or anode, but locating the flat is the safest means of determining polarity.Although commercial mounting clips are available, a 3/16" ID rubber grommet works out justas well. Drill your case to accept the grommet, pop it into place and push the LED in until thebase bottoms out against the grommet. It will protrude about 1/8" for good visibility. I preferdiffuse lens to clear since they show up better when viewed from the side, but either lens stylewill add a little DIY pizzazz.

When reassembling the case, be sure to reattach the fan -- some supplies will not functionwithout the fan attached - in any event, you need the cooling. This PS in the pictures has thefan mounted on rubber shock mounts and is extremely quiet. I will also disassemble the fanand lube the bearings while I have the PS open. Since these are salvaged, the fans have beenin use for some time and normally the bearings are dry -- I use a high grade sewing machineoil from SINGER. Any light oil will work, just don't use WD40 --

As an aside, you can get 7 volts from the +5 V and +12 V outputs -- the +5 V is consideredthe negative (GND) and +12 the positive -- some geeks will use this combination to run theirfans at a lower speed to reduce noise.

I've followed all the instructions, but the output voltage on the +12 V side is still low -- whatcan I do?? Many of the R/C folks are converting power supplies for the purpose of drivingfield chargers and are finding that voltage levels below 12 volts are sometimes insufficient to

power their chargers. Read these TIPS for some options that may help increase this voltagelevel, provide a little theory, identify the connector pinouts found on most PC supplies andgive a few troubleshooting hints.

Is there any way I can get more amperage out of my converted PSU?

Updated: March 13, 2009

Improvements in battery technology, brushless motors and more robust speed controllers haveallowed "electrics" to expand into model sizes that were once the province of nitro and gasengines only. Obviously, as the motors became more powerful, the batteries required to drivethese motors also increased in capacity, measured by the amperage they are able to supply tothe flight system. To realize a reasonable charge time, modern battery chargers must be ableto deliver more current to these batteries than ever before. In the electronics environment, asin all other closed systems, there ain't no free lunch. Consequently, the chargers also need ahigher amperage power source than previously required. Converted PC power supplies maybe stretched to the limit by these demands for more current. Is there anything that can be doneto squeeze more amps from one of these PSU's?

There may be a possible fix to this problem, but your PSU must be one of the newer ATX12Vmodels for you to be able to apply the modification. Visit this page to see if a solution isavailable for your conversion.

Resistor Substitute

A viable alternative to using a power resistor is to substitute an 1157 automotive signal lampin its place. This is a dual filament lamp and its load, with both filaments powered, is usuallysufficient to maintain Latch_On and to raise the voltage on the 12v rail to an appropriate levelfor most needs. Your options are to solder a 5v line (red) to both positive pins on the lamp andground the base to DC ground or to pick up a twist-lock socket when you buy the lamp. Theadvantage of using a socket is the ease of replacement should the lamp fail. If you don't feelcomfortable with your soldering skills, it is also a little easier to work with the wiring on thesocket rather than the pins on the lamp. Just remember that the socket housing is the groundand the two wires in the base are to be attached to the 5v rail. More importantly, you must bevery careful that neither the bulb base nor socket housing touch any of the internalcomponents in the power supply. These lamps may be purchased at any automotive supplystore and most Walmarts.

I prefer the use of resistors since the final converted product is wholly self-contained and Ihave more control over the applied load, but the use of a lamp does simplify finding andinstalling components. It also makes a very obvious Power_On indicator!

I usually deal with on-line suppliers such as Jameco, Digikey, Mouser, etc. because we arebuying in larger quantities and Radio Shack is too expensive for large numbers of items.However, you should be able to convert your PC supply for $5.00 or $6.00 dollars -- less ifyou have a junk box of parts. I suppose you could add an LED indicator with a 220 ohmdropping resistor to the 5v rail to show the PS is turned on, but the fan is a pretty good hint.We have had supplies running 24/7 for months without problems -- just electricityconsumption.

The PS has some fairly hefty electrolytic capacitors and can still give a bit of a shockimmediately after being unplugged -- let it sit a couple of minutes before poking aroundinside. Obviously, you can get whacked if you are inside the case with it still plugged in --probably won't kill you, but you WILL turn it loose (never mind how I discovered this bit ofinformation).

If you have any questions, comments or corrections, feel free to mail me.

Remote Sensing

ATX Design Changes

As one might expect with an ever changing technology, component specifications andmodifications represent an ongoing development process. The operation of the microcomputerpower supply, while not dictated by INTEL, is nonetheless heavily influenced by the largestplayer in the motherboard chipset market. INTEL publishes a design guide specification forpower supplies to ensure they will be compatible with the most recent chipset offerings andregularly revises this guide as PC power needs change. Revisions may occur as frequently astwice a year and power supply manufacturers must likewise modify their designs to remaincurrent.

While manufacturers will advertise their power supplies as meeting ATX12V 2.2 compliancestandards -- this may or may not always be the case as some suppliers may be in partial, butnot full compliance. INTEL does state that not all ATX12V supplies must conform exactly totheir specification.

The major changes that have been recently implemented are summarized below:

Remove guidance and reference to the -5 VDC rail Increase the +12 VDC output capability Increase minimum measured efficiency Replace the 2x10 main power connector with a 2x12 connector to support the 75 watt

PCI Express requirements Assign a separate current limit for the 12V2 rail and specify a 2x2 connector Add remote sensing to the +3.3 VDC output to compensate for excessive cable drops

The industry standard color coding remains the same with two additions:

If your supply is equipped with the higher current rated 12V2 rail, the color coding for thiswill be yellow with a black stripe and will terminate in a 2x2 keyed MOLEX connector.

The second addition is of more importance to individuals converting their powersupplies. The +3.3 VDC remote sensing wire (brown or orange) is connected directly to theorange +3.3 VDC at pin 11 of the power connector (pin 13 if a 2x12). The sense wire willusually be of a lighter gauge (22 AWG) than the power wires (16 or 18 AWG) and its purposeis to monitor the voltage at the connector in order to provide feedback for voltagecompensation by the supply.

If your PS has two wires attached to the same pin on the motherboard power connector,orange +3.3 VDC and a brown sense wire on Pin 11 for example, then these two wires should

be joined when you make your conversion. Some power supplies may also have sense wiresrunning to the +5 VDC and +12 VDC connector pins. If there are multiple sense wires, thenthey will usually be of the same color as the primary supply wire, but of a lighter wire gauge.These sense wires will also terminate in a different location on the PS printed circuit boardthan the heavier supply lines. As with the +3.3 sense wire, these additional wires should alsobe connected to the corresponding supply lines. Failure to complete these connections mayresult in your power supply not latching into a Power_On mode when switched on.

BACK

Mouse over the plugs for pinouts and voltages

(Allow time for image to load on slow links)

Keep in mind that power is measured in WATTS and that an increase in power consumptionimplies there has been an increase in WATTAGE. If a resistor is used to add a load to acircuit, the power consumed will result in heat in the resistor and heat dissipation can becomea problem. If the power level becomes too high (resistors are manufactured with a statedresistance (OHMs) and WATTAGE capacity), it may result in an alteration of the internalresistance or, in the worst case, the resistor burning out.

Power (watts) is calculated as P = V2 / R. Thus, increases in the power level may beaccomplished by increasing the voltage or by reducing the resistance. Higher voltage levelscertainly would seem intuitively reasonable, but REDUCING THE RESISTANCE to raisepower comsumption??!!

AAAARRRRRRRGGGGGHHHHH -- This is too much like school, just tell me what to do!!!

If I wanted to increase the load in a circuit, wouldn't it seem more feasible to increase theresistance? This seeming paradox may be better explained by looking at the extremes. Bydefinition, a resistor restricts the electron flow through the circuit. If we can reduce theelectron flow, we can also reduce the amount of work being performed over a period of time.A high level of resistance allows only a small number of electrons to migrate -- materials withextremely high resistance levels are known simply as insulators -- glass, air, rubber, plastic,etc. If I inserted a piece of glass into my circuit and attached the battery leads to each end,nothing would happen -- high resistance, no electron movement, no load and no heat. On theother hand, materials with very low resistance are known as conductors; the lower theresistance, the better the conductor -- silver, copper, gold, aluminum. Placing a thin silver

wire across my battery leads might result in the wire beginning to glow or burning completelyin two, depending of course on the battery capacity. Hence, low resistance leads to highelectron flow, high load and possibly high heat generation. The physics of heat generation isenmeshed in free electron flow, valence shell electrons and other factors that will remainoutside this discussion.

It is suffice to say that using a resistor with a lower resistance rating will add a greater load tothe circuit and produce increasingly higher levels of heat, either within the resistor itself or inother parts of the circuit.

Many within the R/C community have been converting power supplies to drive their fieldchargers, but have not been satisfied with the measured voltage on the +12 V output -- inseveral cases, the voltage levels have been lower than expected leading to long charge times,power supply shutdowns or charger malfunctions. Relying on a little help from Mr. Ohm,Watt and Kirchhoff, we may be able to increase the amount of work expected from our PCpower supply, and in turn, get it to step up to the plate with an increased voltage level. TheATX switching mode power supplies require a static load to function and for many, a 10 ohm10 watt resistor on the +5 V output is sufficient, but voltage levels on the +12 V line may fallin the range of 11.5 to 11.75 volts, adequate for logic labs, but below desired levels forchargers. Five volt output generally holds around 5.09 to 5.15 volts.

Returning to our power calculation, a 10 ohm load across 5.15 volts consumes about 2.65watts of energy. P = V2 / R = 5.152 / 10 ~ 2.65 watts. By virtue of being a regulated powersupply, we cannot easily change the voltage levels -- the ATX power supply design guidespecifies a +/- 5% variation for the 3.3, 5 and 12 volt outputs and the internal circuitry isdesigned to maintain output within those specifications. Consequently, to increase theperceived load, the most easily controlled variable is resistance. If we replace the 10 ohmresistor with a 2 ohm resistor, what change can we expect in the load? As before, P = V2 / R =5.152 / 2 ~ 13.26 watts. This substitution has certainly increased the load, but it has alsointroduced another potentially destructive situation. The load resistor must now dissipate thisincreased energy and it does so in the form of heat. We can deal with heat to some extent, butwhen the load exceeds the rating of the resistor, our best efforts may not save a 10 wattresistor carrying a continuous 13 watt load. My options are to find a whopper of a resistorwith a high load rating or come up with a workable solution using easily obtainablecomponents.

Four laws will help us ramp up the perceived load and use off-the-shelf parts to successfullyimplement them.

1. The total resistance in a SERIES circuit equals the sum of the individual resistances: Rtotal =R1 + R2 + ... + Rn

2. The sum of voltage drops in a series circuit will equal the voltage source: Vs = Vd1 + Vd2 +... + Vdn

3. The voltage drop across a resistor in a series circuit is directly proportional to the size(Ohm rating) of the resistor: Vd1 = Vs x R1 / Rtotal

4. The total power consumed in a series circuit is equal to the sum of the individual powersused by each circuit component: Ptotal = P1 + P2 + ... + Pn

Using (1) above, I can connect two 1 Ohm resistors in a series to get a total resistance of 2Ohms. Using (2) and (3), I know the total voltage drop across both resistors will equal 5.15volts and since each resistor comprises half the total resistance, each will drop 2.575 volts.Using (4) and applying my power formula, each resistor will dissipate 6.63 watts of power,i.e. P1 = V2 / R1 = 2.5752 / 1 ~ 6.63 watts. Total power = 6.63 + 6.63 = 13.26 watts.

The Fix

By using two easily obtained 1 Ohm 10 Watt resistors (Radio Shack), we can wire them inseries across the +5 volt (red/black) output and increase the load on our power supply with anattendant increase in output voltage on the 12 volt line. Both resistors will be running at about65% of their rated wattage and will not be damaged by overload. However, they will get veryhot -- the single 10 Ohm resistor was dumping about 2.65 watts while each of the 1 Ohmresistors will generate nearly 2.5 times that. To keep them cooled down, it is stronglysuggested that both be attached to the PS case with heatsink compound to help reduce heatbuildup.

On the power supplies I have tested, all produced higher voltage levels, with increases of .15to .2 volts and total output of 11.85 to 12.06 volts.

If you are converting your PC power supply with the intention of driving a field charger, youmight consider the substitution indicated above to get a little more voltage. Be mindful thatthe conversion comes with a tradeoff in the form of more heat. However, a little care inheatsinking your resistors should provide a reliable and long lasting source for regulated DCpower.

Unexpected Shutdown -- I've made the recommended changes, but when I connect mycharger, the PS still shuts down. Now what?? The usage of a PC powersupply as a substitutefor a field charger power source falls far outside the intent of the original designspecifications. Once the PS is running and stable, the overload circuitry is tuned to detect highcurrent sinks and shut the PS down -- under normal usage, these sinks would be indicative ofan internal short in the PC. When a microcomputer is running, powersupply demands change,but these are minimal and are usually associated with optical, hard or floppy drive usage orUSB devices being attached. Some field chargers produce a high current sink when firstattached and generate a latch into the overload state -- as designed. The specifications statethat the PS will remain latched until the load is removed from the rail -- the PS will eitherautomatically reset or may require at least one PS_ON cycle. Even though your PS may havesufficient wattage to effectively drive the charger, the load change is the problem. Onemethod that has been effective is to attach the charger before powering up -- to the PS, thecharger now appears as a high draw motherboard and not as a potential short occurring afterthe PS is stable.

Doesn't work!!! -- When I tested my PS before making the conversion, it worked fine, butnow it won't power up at all. What should I try?? If the power supply were functioning beforebeing modified, there are two prime suspects to check out. A failure to correctly reconnect theremote sense wires can keep some PS from latching on. See the Design Guide Update forremote sensing information. The other likely culprit is a short causing the overload circuitry tocapture. Occasionally, this will cause a fan bump -- the fan will attempt to spin up, then stopand there will be no voltage on the output rails. Cycling the ON/OFF switch produces the

same result: a bump, then Latch_Off. Check your internal wiring to ensure none of the clippedends are shorted to the case or another rail. A second potential source for shorting is bindingposts. The majority of binding posts have either a plastic bushing that passes thru the case or ashoulder on the post and retaining washer that keeps the post centered. Occasionally, thesecond style will not be correctly centered and the bare threaded post will come into contactwith the PS case. With the PS unplugged and using your VOM or DMM, test each post in turnfor a short to the case -- none of the DC rails, including GROUND, should have case contact.The case is grounded on the AC side; the DC side is isolated. If one appears grounded to thecase, realign the post and test again.

Should none of modifications prove workable, your only remaining solution may be topurchase a standard benchtop powersupply from a retail vendor or one of the on-line auctionsites -- perhaps not as satisfying as a DIY project, but it will get you back into the air.

BACK

Updated: March 13, 2009

Power_OK or Power_Good

Myth or Fact

The primary purpose of the PC power supply is to supply stable current and voltage ofdiffering levels to the motherboard and various peripherals attached to our PC's. The ATXdesign guides are very specific with respect to voltage levels, acceptable variation, overloadcircuitry, plug design and even screw-hole placement on the PS cases themselves. As many ofyou are aware, the personal computers today do not function well with fluctuating voltagelevels and may be damaged if operated in a less than ideal electrical environment.

One of the design specifications ensures that the system will not function if voltages are notsufficient to operate the system properly. In short, the PS is designed to complete a series ofPower On Self Tests (POST) before the motherboard starts to powerup. These tests determineif all the voltages are up to design specification and stable before sending a signal to themotherboard. If the POST completes successfully, a POWER_GOOD or POWER_OK signalis sent to the processor over the POWER_GOOD line. This signal must be continuouslypresent for the system to operate and, if withdrawn (due to a brownout, for instance), willgenerate a system RESET. The system will remain in a continuous RESET mode until thesignal is restored. Since the processor initializes in the RESET mode, the system will not startuntil sensing the PWR_OK signal.

This signal is a +5V active (nominal) high, usually present within 100ms to 500ms afterapplying A/C to the powersupply. Active high means that as long as the PS is functioningproperly (active), the signal can be measured. There may be some variation in the voltagelevel on this line, but ranges of +2.4V to +6.0V are generally considered to be sufficient toforce the processor out of RESET.

Since the signal is generated by the PS for use by the MPU, the PWR_OK (gray) wire shouldnot be grounded, attached to any of the other output lines or tied to a resistor. It is not required

for the PS to function -- its sole purpose is to allow the motherboard to initiate the bootprocess and to continue to function in the absence of unstable or improper power levels.

Could this signal be of any use when converting a PC powersupply to desktop usage?Realistically, the answer is YES. A voltage on the PWR_OK line indicates that the PS hascompleted a successful POST and that the output voltages are stable and within designspecification. If you wanted to use an LED (light emitting diode) as an indicator that the PS ison, rather than tie it to one of the +5V or +12V lines, attach the PWR_OK line to the anode(+) side of the LED and place a 220 ohm resistor on the cathode (-) leg before grounding it.The cathode leg is normally shorter on new LEDs -- if the legs have been clipped, the cathodewill be on the same side as a flat spot on the LED base. The LED should be bright for normaloperation -- it could possibly glow faintly if the PS has withdrawn the signal simply due tobleedover.

After having read several news groups and had some inquiries relative to this function, Ithought I would add this short section to clarify the purpose and operation of the PWR_OKsignal. I hope it has been of use.

BACK

Gray = PG o da = “Power Good” demekmiş. Ana karta bilgi veriyor olabilir mi?

24-pin ATX12V 2.x power supply connector(20-pin omits the last 4: 11, 12, 23 and 24)

Color Signal Pin Pin Signal Color

Orange +3.3 V 1 13

+3.3V Orange

+3.3 V sense Brown

Orange +3.3 V 2 14 −12 V Blue

Black Ground 3 15 Ground Black

Red +5 V 4 16 Power on Green

Black Ground 5 17 Ground Black

Red +5 V 6 18 Ground Black

Black Ground 7 19 Ground Black

Grey Power good 8 20 −5 V (optional) White

Purple +5 V standby 9 21 +5 V Red

Yellow +12 V 10 22 +5 V Red

Yellow +12 V 11 23 +5 V Red

Orange +3.3 V 12 24 Ground Black

Setting Up a PC's Power supply to use with your Dc/DC battery charger

By Pat Harvey

Most of us have several battery chargers for our NiCad packs. Most of these chargers only operate on 12 volt DCinputs (your car battery). This is fine as long as you are at the flying field. This isn't so fine when you want tocycle a pack on the workbench or peak up a pack before you leave for the field (and the wife has the car). Wehave all had the need for a good source of 12 volt DC power, good clean power. I had a couple of simple powersupplies but one of my chargers would not run on either of them.

The power supplies from old PCs will provide good clean power that will run either (as well as bothconcurrently) of my chargers. One charger is an Astro 110D and the other is Dymond Super Smart Charger. Ifyou have the need - Let's get started. The basic tools you will need are a VOM, screwdrivers, soldering iron, wirecutter, pliers, drill and bits.

The first thing is to get an old PC power supply. If you have an old machine setting gathering dust that youshould have thrown out, you are in luck. If you don't have one, then stop by the local PC fix-it shop. Often theywill have machines that are destined for the dumpster that have perfectly good power supplies. Just offer to haulthe whole thing off for them and you may have a real bargain. If you can't get one for nothing, then many shopssell used power supplies at a fair price. I've had very good luck in getting free ones.

Open the PC case and take a look at the top of the power supply box. It will tell you how many amps at +12 voltsare available from this power supply. It should be at least 4 amps to be of much value. A 7 or 8 amp output on+12 volts is very common and if this box is too low power you may wish to look for a different power supply. Inorder to take the power supply out of the case do not cut any wires, simply unplug everything. Once you have thepower supply out of the PC case, you have to decide which of three basic types of power supplies you have. Ifthe switch for the power supply is a paddle switch on the side of the power supply itself you have an older ATstyle power supply. If it is a push button type switch, either on the side of the power supply or on an umbilicalcord them you have a newer AT style power supply. If it only has a "rocker" type switch (or no switch) it isprobably an ATX style power supply. The label may in fact have "ATX" on it. An ATX style will have a plugthat goes to the motherboard which has a double row of connections with 10 connections on each side. Onlybother reading the section that follows that applies to your style of power supply.

Older AT Style Power Supply (the ones with the paddle switch on the side)

You are in luck. This old style power supply (PS) is much simpler to make work and generally the case is largerso you have more room to work. Plug in the power supply and turn it on. The fan should be running. Use yourVOM and identify the correct color pair of wires for +12 volts. This is fairly easy. Pick a set of wires that ends ina plug with only 4 wires. This probably went to a disk drive (either hard disk or floppy). There will be 2 centerwires the same color (probably black) and the outside wires will be different colors (perhaps yellow and red).Use the VOM with one probe in a center wire and one probe in an outside wire. What you will find is that thecenter wires equate to a negative post on a battery and the outside wires are the positive posts. With enough trialand error you can identify most of the colors. Ones that I have seen are:

Yellow +12 volts

Black Common

Red +5 volts

Orange -5 volts

Blue -12 volts

White Power good .

There will be a lot of +12 volt wires, a lot of +5 volt wires, an awful lot of "Common" wires and only one or two-12 volt or -5 volt wires. Normally there is only one "Power Good" wire.

Now that you know which color is +12 volts and which is "Common" all you need to do is "design" your box.Since we plan to use this power supply as a substitute for a car battery I envision it with "Positive" and"Negative" posts, just like a battery. Pick two locations on the PS case that will allow your charger to be clippedonto without shorting out, and that you can run several wires to the inside of the PS case to those locations. Go tothe local hardware store and get:

2 rubber grommets (1/4 inch center holes is fine)

2 #10 machine bolts 1 1/2 inches long (these should go through the grommets without problem)

4 nuts for the bolts

4 flat washers for the bolts

4 large (probably 1/4 inch by 2 inch diameter) nylon (or other insulating) washers with small (1/4 inch) holes incenter

Now back at the shop. If you happen to have some Red Zagi tape and some Black Zagi tape then cover one sideof a Nylon washer with Red and one side of another Nylon washer with Black. Trim the tape from the uncoveredside with a sharp knife. Drill a 5/16 hole at each of your chosen locations. Put a rubber grommet in each hole.Next cut 3 or 4 of the +12 volt wires to length to reach the first hole. Solder these wires to the bolt (near thehead). "Ring terminals" are an excellent option rather than soldering directly to the bolt but either route willwork. Put a nut on the bolt and tighten it against the soldered wires. Put a metal flat washer on the bolt. Next putone of the nylon washers on the bolt. Shove the bolt through the grommet. If necessary you can trim the nylonwasher a bit if it conflicts with something but leave enough of the nylon washer to be sure the wires do notcontact the PS case. Put another nylon washer (the Red one if you covered one with Zagi tape) on the bolt. Putanother metal flat washer on the bolt. Put another nut on the bolt and tighten it up. You now should have"Positive" battery post that is fully insulated from the PS case.

Next cut 3 or 4 of the "Common" wires to length to reach the second hole. Repeat the same process you did withthe plus 12 volt wires this time using "Common" wires. Use the Black Nylon washer on this one if you coveredone with Zagi tape. You now have the "Negative" post for your new PS. Now all that is left is to cut off theexcess wires such that they will not short out. Put the cover back on the power supply and mark the posts as"Positive" and "Negative". You are done.

Newer AT Style Power Supply (the ones with the push button switch on the side or on a cord)

This power supply (PS) is a bit more complicated than the older ones and requires a bit more work. Not only thatthey tend to be smaller and there is less room to work inside the PS box.

Plug in the power supply and turn it on. The fan may be running or it may just start and then stop. Generally thefollowing colors will identify specific functions - generally:

Yellow +12 volts

Black Common

Red +5 volts

Orange Power good

Blue -12 volts

White -5 volts

Green or Grey Power Supply – On (PS-on)

Note: "PS-on" may not exist. If it exists it will be part of the double rowed plug that went to the motherboard ofthe PC.

If the fan is not running consistently turn the power off and temporarily connect "Power Good" to a +5 volt line.This should cause the fan to run consistently when the PS is turned on. If the fan is still not running you shouldlook for the "PS-on" line and connect it to a "Common" line. The PS-on line is in fact a switch to turn on (or off)the PS. Use your VOM and identify the correct color pair of wires for +12 volts. This is fairly easy. Pick a set ofwires that ends in a plug with only 4 wires. This probably went to a disk drive (either hard disk or floppy). Therewill be 2 center wires the same color (probably black) and the outside wires will be different colors (perhapsyellow and red). Use the VOM with one probe in a center wire and one probe in an outside wire. What you willfind is that the center wires equate to a negative post on a battery and the outside wires are the positive posts.With enough trial and error you can identify most of the colors. There will be a lot of +12 volt wires, a lot of +5volt wires, an awful lot of "Common" wires and only one or two -12 volt or -5 volt wires. Normally there is onlyone "Power Good" and one PS-on wire.

Make the connection from "Power Good" to +5 volts a permanent connection (solder it with a bit of heat shrink).

Now that you know which color is +12 volts and which is "Common" , next you need to "design" your box.Since we plan to use this power supply as a substitute for a car battery I envision it with "Positive" and"Negative" posts, just like a battery. Pick two locations on the PS case that will allow your charger to be clippedonto without shorting out, and that you can run several wires to the inside of the PS case to those locations.

You may wish to move the PS power switch into the case if it is an "umbilical" cord type switch. I normallychoose to move it into the hole that the "umbilical" cord comes out of the PS case through. This process is just amatter of unsoldering the wires, shortening them and re-soldering them. Be sure to solder the right same coloredwires back onto the same lugs on the switch. You will probably need to drill a couple of mounting holes in thePS case to hold the switch and mount the switch using these holes and a screw through each.

Go to the local hardware store and get:

2 rubber grommets (1/4 inch center holes is fine)

2 #10 machine bolts 1 1/2 inches long (these should go through the grommets without problem)

4 nuts for the bolts

4 flat washers for the bolts

4 large (probably 1/4 inch by 2 inch diameter) nylon (or other insulating) washers with small (1/4 inch) holes incenter

A 12 volt automotive light with a socket and wires (I use a small clearance type light with amber lens). A #1154or #1156 bulb also works well.

Now back at the shop. If you happen to have some Red Zagi tape and some Black Zagi tape then cover one sideof a Nylon washer with Red and one side of another Nylon washer with Black. Trim the tape from the uncoveredside with a sharp knife. Drill a 5/16 hole at each of your chosen locations. Put a rubber grommet in each hole.

Next cut 3 or 4 of the +12 volt wires to length to reach the first hole. Solder these wires to the bolt (near thehead). "Ring terminals" are an excellent option rather than soldering directly to the bolt but either route willwork. Put a nut on the bolt and tighten it against the soldered wires. Put a metal flat washer on the bolt. Next putone of the nylon washers on the bolt. Shove the bolt through the grommet. . If necessary you can trim the nylonwasher a bit if it conflicts with something inside the PS case but leave enough of the nylon washer to be sure thewires do not contact the PS case. Put another nylon washer on the bolt (use the Red Nylon washer if you coveredone with Zagi tape). Put another metal flat washer on the bolt. Put another nut on the bolt and tighten it up. Younow should have "Positive" battery post that is fully insulated from the PS case.

NNext cut 3 or 4 of the "Common" wires to length to reach the second hole. Repeat the same process you didwith the plus 12 volt wires this time using "Common" wires. Use the Black Nylon washer on this one if youcovered one with Zagi tape. You now have the "Negative" post for your new PS.

Next choose a location on the PS case to mount the automotive light socket. Mount the light and connect it to a"Common" wire and to a +5 volt (yes plus five volts) wire. You need a small "load" on the +5 volt side of the PSin order for it to put out it's maximum voltage on the +12 volt lines. Some folks use a 1 ohm 25 watt resistor. E-zone is full of opinions on this but I find a small load works fine. If you are not getting slightly over 12 volts ofoutput on the "posts" you can add a second light by connecting it to a "common" and a +5 volt wire.

Now all that is left is to cut off the excess wires such that they will not short out. Put the cover back on the powersupply and mark the posts as "Positive" and "Negative". You are done. The automotive light will serve as anindicator light that the PS is turned on.

ATX Style Power Supply (the ones with no switch or maybe a "rocker switch" on the side of the box)

The ATX power supply will have a rather long 20 pin plug that went to the PC motherboard that has thefollowing pin-outs:

Note that PW-OK (or PWR_OK) is the "Power Good" signal.

Conversion of an ATX style power supply proceeds just like the conversion of the newer AT style units exceptthat you can ignore the discussion about moving the switch. Please use those guidelines (above) with thisexception.