Auto Lead Acid Battery Capacity Logger. (Feb’15 Update)

Part NO: 2209668

By Ancel Bhagwandeen, mosaicmerc@yahoo.com

Difficulty: Intermediate

Time: 1 to 2 hours.

Required tools and parts

Soldering iron (40W). Solder wire (rosin core, about 0.8 to 1.2mm dia)

Hookup wire (#20 to #26awg) for jumpers etc. Heavier #16 AWG for battery clip connection and Lamp jumpers.

Wire flush cutter and stripper, hobby knife

Bird beak pliers.

An Antistatic mat OR aluminum foil sheet that is grounded for the MOSFET & PIC chip handling.

220 to 320 grit sandpaper to clean connectors & heavy wire for soldering.

Multimeter for debugging if required PIC Kit 2/3 programmer/Compatible programmer for PIC programming

Lead acid batteries are all around us. Cars, trucks, motorcycles, boats, jet skis, military & UPS systems,

medical backups, golf carts, solar storage and even airplane avionics all use them. Accurate measurement of

their ampere hour (Ah) capacity is still most accurately done by a discharge into a load.

This kit is flexible enough to test small batteries from 3.75Ah up to larger 99Ah - 125Ah units.

There is a lot of utility for the average user. Any 12V lead acid battery is a candidate from small

gel cells found in PC UPS ‘bricks’ to high end AGM/calcium/silver sealed batteries found in

luxury vehicles. A firmware upgrade to permit up to 250AH capacity testing is in the works.

Automatic shut off (and an optional relay trigger) prevents over discharge (and optional auto recharge with an

appropriate charger connected). Upload the full voltage discharge log into a spreadsheet via text import for

detailed analysis or for battery maintenance logging.

Building the KIT!

The printed circuit board is single sided, standard sized (4” x 6”), and well-spaced for easy assembly. Begin by

installing all resistors, diodes and the wire jumper/bridges excepting the two parallel, longest bridges

connecting the spade terminals. Parts placement is noted in the image in figure 1. Match with the PCB and

install & solder. Note that electrolytic capacitors (1uF) are polarized as are the diodes and LEDs. The +ve

polarity is marked in copper near the component pin, or can be followed in figure 1. The flat side on the LEDs

is the negative. The grey or dark stripe on the diodes & capacitors is the negative.

You can use the cuttings from the component leads as bare jumpers on the top side of the board. Note the

two thick, insulated, ‘power’ bridges in the pictured PCB bottom, figure 2. These are installed after all the

capacitors, diodes & resistors and must be #14, #16 or #18AWG insulated wire. Tin the thick copper traces

with solder as shown. Figure 3 shows the board with the I.C. Sockets and LEDs installed as well. Add these

now.

The Version 1.1 PCB has improved spacing for a MOSFET HEATSINK and soda can aluminum heat shields. Note

PICS at the end of this document. This is useful when running near max. loads to deflect lamp radiant heat

from the semiconductor parts. Also the IRFZ MOSFET is now an IRF3205 and the 4.7 ohm becomes a 22 ohm.

Figure 1 –TOP - parts placement version 1.0, note V1-1 pics for the newest PCB layout at end of PDF.

Figure 2 – Bottom wire bridges & tinned hi current traces - Version1.0

Now install & solder the 4 power resistors, the 2 pushbuttons and the 7 segment display (note

polarity marked with copper on PCB, the display decimal points are located near the board

edge). Install and solder the 4, ¼ inch, spade terminals for the lamps, the 7805 V. regulator

(facing the nearby 0.1uF capacitor) and the MOSFET transistor (handle this carefully as it is

static sensitive). This MOSFET is mounted facing the nearby 1uf Electrolytic capacitor, see

figure 4 assembly. Install the 1K Cermet trimmer potentiometer. This is used for voltage

calibration.

Install the two 3 pin terminal blocks; the 10A fuse holder, the 6 pin ICSP header, and the power

relay (optional) located near the power resistors. The center pin of the battery terminal block

near the Zener diodes is not used. The battery terminal block pin nearest the relay takes a #16

AWG wire with attached red alligator clip (used for the +ve battery connection). The opposite

pin on this 3 pin block goes via another 16 awg wire and black alligator clip to the –ve post of

the battery. Only connect these clips to the battery when ready to begin testing the unit. The

relay’s normally open, common (center) and normally closed pins are marked in copper under

the relay terminal block. They are rated for 15A D.C. or 15A A.C. For such high currents (>10A)

you must add 3 parallel #18Awg wires to the PCB copper traces connecting to the 3 pin

terminal block from the relay.

Figu re 3 - Some parts installe d version 1.0

Install the socketed PIC 16F690, 20 pin IC and the TC4420, 8 pin IC aligning the #1 pin to the

pad marked ‘1’ in the PCB copper. Lastly, install the 10A fuse into the fuse holder and mount

the 6-32 ‘feet’ in the perimeter holes from the PCB bottom. Solder or crimp two (#16AWG)

wires (each around 18” long) to the alligator battery clips and secure the floating ends to the

battery input terminal block observing the polarity! Note the completed assembly in figure 4

below. Note the Version 1_1 assembly with heat shields in place later on.

.

Mounting the Lamps

If you have obtained a pair of dichroic 12V lamps (standard track light, down lighters, GU5.3

base), you should crimp (use the birdbeak pliers) a female spade receptacle’s rear crimping

section to each of the base pins on each lamp. Four crimps in all. Remove any plastic insulation

first, as it can burn during operation! Twist the female spade receptacles to align with the male

spades on the PCB and press on. If the crimp doesn’t feel tight, you can pull the lamps pins

from the crimps, pinch (using birdbeak pliers) the exposed crimps a little and press the lamps

pins back into the tightened crimps. Note Figure 5.

Figure 4 Completed Assy – Version 1.0

50W, 12V lamps are suggested, a single 36W lamp can be used for loads as low as 3/8 Amp.

Wattage divided by voltage gives the peak current possible. The unit permits 7, 1/8 current

load steps; starting from a minimum 12.5% upward to 100% of the lamps’ rating.

WARNING: During normal operation these lamps will get extremely hot, they WILL burn skin

instantly. DO NOT touch them until they have been off for 10 minutes or more. Do not cover

these lamps or place them in close proximity to anything flammable or that might melt (such

as the battery case). Three inches of lamp clearance is reasonable. NOTE version 1-1 heat

shielding assembly as of Feb 2015 at end of this doc..

Figure 5 -12V Lamp with Crimp on .25” Terminals, mates with male .25” on PCB.

Online Lamp Source: http://www.goldmine-elec-products.com/products.asp?dept=1352

$5.00 for 10 pcs!

Use and Operation.

Program the 16F690, 20 pin microcontroller, with a Pickit2 or compatible programmer - (download and install

the code yourself). Code updates can be done at any time via the 6 pin ICSP header on the PCB.

Essential First Step- Calibration. The unit must be calibrated to closely match a reliable digital multimeter’s readout of the 12V battery’s

voltage to two decimal places.

Unloaded Calibration – compensation for resistor, voltage divider, tolerances.

1) Remove all lamps from the PCB for this initial, no load calibration.

2) Connect the alligator clips to a charged 12V (actually 12.5V or better) battery, else ‘Lo’ is displayed.

3) The digital display should show alternating (every 4 seconds) number pairs after about a second. The

green LED will light indicating normal operation. Reseat the battery clips if this doesn’t happen. Ensure

the battery posts are clean and not corroded.

4) The LED display’s number pair with the decimal lit is the Integer voltage (e.g. 12.)

5) The number pair with no decimal represents the first two decimal places of the voltage. E.g. 74; thus

the actual sampled voltage would be 12.74V in this example.

6) Using a small screwdriver, slowly adjust the trimmer potentiometer to bring the displayed voltage as

close as possible to the multimeter measured voltage of the battery.

Loaded Calibration – compensation for cabling and connection impedance losses.

1) Disconnect the positive alligator clip from the battery.

2) Install the lamps onto the spade terminals on the PCB.

3) Reconnect the positive alligator clip to the charged battery’s positive terminal.

4) The lamps will light dimly. Click and release button #1 (furthest from the display) several times until the lamps

are at their brightest. If you long click (1/2 sec.) the button it will reduce the lamps brightness and hence reduce

the load on the battery.

5) The lamps will get hot, avoid touching them.

6) The display will flash and display the actual current up to 9.9A drawn. The red LED will light indicating that the

current drain in Amperes is now being displayed on the blinking display.

7) After 8 seconds the blinking LED display reverts to the alternating voltage display and you must compare this to

the multimeter voltage readout (from the battery’s terminals). The green LED is lit once more. Red LED is off.

8) To ‘adjust’ the alternating voltage pairs to match the multimeter’s value, clicking Button 0 (nearest the LED

display) incrementally increases the displayed voltage. Holding down (1/2 second long clicks) this button causes

a decrease in the displayed voltage.

9) Keep monitoring the multimeter readout as the battery voltage may be slowly dropping due to the applied load.

Once the Kit displayed voltage closely matches the multimeter voltage the calibration is complete.

10) Disconnect the alligator clips. The calibrated kit is now ready to use.

Regular Operation

Once you have calibrated the unit as per the previous section you can put the system to work.

Here’s how:

1) Connect the clips to the charged 12V battery to begin. A ‘Lo’ display that does not

become a voltage display implies the battery‘s rest voltage is low and the battery needs

to be charged. Reverse connection does no damage.

2) The minimum (1/8th) load is applied once the battery appears charged and the

alternating voltage digit pairs are displayed. The green LED is lit.

3) Using button #1 (furthest from the LED display), click once quickly to increase the

current which will be displayed as a flashing, single decimal place, number. Continue

clicking until the desired load current is shown. A long (1/2 sec) click reduces the load

current. If the maximum current is not enough you will need to use higher wattage

lamps up to a maximum of 120Watts load or 10Amps current.

Figure 6 - OPERATIONAL!

4) With the desired current load selected (preferably: [battery expected Ah rating /10])

you can leave the kit to do its job. Ensure that nothing can come into contact with the

hot lamps. After the first 3.75 minutes the cumulative Ah replaces the voltage on the

display and the green LED begins blinking, indicating the calibration period is passed.

5) When the discharge cycle is complete the lamps are switched off and the rated Ah value

on the display flashes. The Red & Green LEDs blink alternately. Also, the relay (if

installed) will close its normally open contacts. This is useful to switch a connected

charger to cause an automatic battery recharge.

6) At this time using the Pickit2 UART feature (if you have a Pickit2) or a PL2303 or FTDI

RS232/USB cable, you can upload the full data log of the discharge cycle to a PC text file

for importation into a spreadsheet for graphing or documentation. See the next section

for details.

7) Note that the 2 digit display shows ‘Hi’ after 99 Ah is exceeded although the stored log

will continue to log up to 127 Ah.

RS232/USB Connection for data log uplink.

No battery or other power must be connected at this time!

Connect the USB/RS232 adapter input to the selected ICSP header pins as follows:

ICSP Pin2 to 5V supply (RS232/USB)

ICSP Pin 3 to ground (RS232/USB)

ICSP Pin 4 to the RS232/USB cable receive/ RX, line.

If you will be using a Pickit2: just plug in the PK2 onboard mating 6 pin female to

the 6 pin ICSP header.

Once 5V power is enabled the LED display shows ‘UP’, meaning it is ready to upload

the dataset.

Setup your PC terminal software, or Pickit 2 UART tool (5Vdd active), to receive at

2400,n,8,1. Then press Button #0 (the volt calibration button) to upload the data.

Check the data to ensure no corruption (especially at the beginning). Your PC

should have received a series of 3 digit numbers, generally descending voltage

values (note format in sample Excel file). You may repeat the upload as required.

Note the sample EXCEL data file for how these are used. A comma delimited,

columnar text import is used, no options.

Search ‘FTDI’ on www.JAMECO.com for appropriate USB/RS232/TTL

adapters/cables. Also PL2303 from EBAY etc. is an option.

Protective Features

1) Reverse protection rectifier protects circuitry from incorrect battery

polarity.

2) A Zener, sacrificial crowbar circuit blows the 10A fuse in the case of over

voltage (>16V). The zener diodes MUST be replaced after such an event or

the fuse will immediately blow again when replaced.

3) An under volt battery is detected. ‘Lo’ is displayed. Recharge before

testing.

DIY without the kit - Component Substitution

Should you choose to build this from parts on hand, I suggest 105C rated capacitors

be used. Do not substitute the 1uF Poly cap unless you have a ceramic or film

version; no electrolytic or tantalum here. Higher precision power resistors can

improve accuracy a little. Use a N-FET with a greater than 50A continuous rating to

negate needing a heat-sink. MOSFET 20Vds on upward is ok. Keep all other parts

rated at 16 VDC or better.

Automobile ‘fog’ lamps can be used as loads but you must use heavy gauge wiring

(>#18Awg) when connecting the lamps’ terminals. Use incandescent lamps.

The laser toner transfer method for the PCB artwork works fine. The included PCB

files can be used with the free EAGLECAD application download.

http://www.cadsoftusa.com/

If a perf-board assembly is attempted, ensure that the 0.1uf capacitors near the

IC’s are kept very close to their power pins. Use 18 AWG or thicker wiring for the

high current (thick trace) connections. Regular #24 to #28Awg hookup wire

elsewhere.

The dual zener crowbar, fuse, and the Relay with its associated 10k base resistor,

1uf Cap, 2n3904 NPN, 0.1uf and 1n4001 diode are all optional. The 4.7 ohm gate

resistor can be up to 22 ohm in value. A 15 to 30Watt precision 0.1 ohm resistor

can replace the 4 x block power resistors.

THEORY of OPERATION and limitations.

Depending on your knowledge of lead acid batteries, this link can get you up to speed as to the

different types and ratings such as CCA, Ah and RC which appear on the batteries that you

purchase.

http://www.batterystuff.com/kb/articles/battery-articles/battery-basics.html

This DIY kit automates assessing the Ah (ampere hour) and by extension an RC (reserve

capacity) estimate (roughly 2x Ah).

We all are better informed in knowing how long a battery will ‘last’ under a given load. When

new, the battery rating (e.g. 60Ah = C or 100% rating) gives this information. Usually, these

ratings are done at 20Hr or 60/20 (C/20) = 3A current load in this example.

C/10 is also used from time to time and I recommend this loading as it delivers a faster result.

You should know that a C/10 rating will be a bit less than a C/20 rating due to the battery’s

Peukert index. Similarly a C/5 will also be less than a C/10 rating. This has to do with the

chemistry in the battery.

The kit permits a maximum of 16 hours of logging (memory limited) so C/16 is the closest to

C/20 that is possible with it. 16 samples per hour are taken.

The kit can use up to 120Watts of incandescent 12V lamps as loads as it is fused at 10A and

12V x 10A = 120W. This can make a LOT of heat and I prefer to limit the current to about 8A or

almost 100W. Thus the maximum suggested battery size works out to 8A x 16hrs = 128Ah. The

2 x 50W lamps (as suggested) provide these figures. Take 125Ah as the realistic limit to

prevent loss of data at the end of the sampling period. Remember the 2 digit display shows

‘Hi’ after 99Ah is passed, up to 127Ah is data logged for upload. With more development, a 4

Digit display can be used and a (1 or 1/10th) scaling factor uploaded along with the Ah rating

to permit a larger 0-255 Ah of rating with a 0.0 to 25.5Ah, 1/10th scaling option. This is

suitable for larger stationary battery and gen-set applications. The sample rate will have to

drop to 8x per hour to accommodate the increased hours to accumulate the Ah rating at 8A

maximum suggested loading, 10A absolute maximum.

The 127Ah battery capacity covers most Automotive, all motorbike, small UPS and some

marine batteries. Using a lower wattage load such as a single 36 watt, 12V lamp can permit a

minimum load calculated like this: 36/12 * 1/8 = 3/8A. This can be stepped up in 1/8th steps up

to the max 3A by the kit. Thus a single 36W lamp can test a battery up to 3A x 16hrs = 48Ah or

as small as 3 x10 x1/8 = 3.75Ah @ a C/10 rate.

How It Works

The standard for rating a 12V lead acid battery requires a controlled discharge down to 10.5 V

under load. The cumulative Amperes x Hrs. gives the Ah rating.

The Kit uses a PIC 16F690 microcontroller that has a number of features built in which make

this job simple. It offers a 10bit analog to digital converter (ADC) so it can measure voltage.

Also, there are quite a few input/output pins for connecting input switches and driving small

loads like LEDs and transistor switches. Two other important features are the EUSART module

for easy RS232 communications and the ECCP module for pulse width modulation (PWM).

The program for the 16F690 is written in assembly language which is a bit cryptic but puts you

in direct control of its hardware capabilities. The written code is compiled by the assembly

compiler into hexadecimal values which is stored in the 16F690 code memory. It understands

and executes the hexadecimal – machine language.

Measuring voltage is important to determine if the battery is charged and when the discharge

is complete (@10.5V). We measure current indirectly by measuring the voltage drop across a

known resistance through which the load current passes. The 4 ‘block’ power resistors can

dissipate 4 x 5W = 20W all together and they net 0.1ohm resistance. This ‘current sense’

resistor produces 0.1 x 8 = 0.8V across the resistor for an 8 Amp current draw from the

battery. This voltage can be measured by the 16F690 and we derive the current! At 8 A the

heat generated by the resistance is 0.1 * 8 * 8 = 6.4Watts, well under the rated limit of 20W.

Keeping the resistors from getting hot also keeps their values from thermal ‘drift’ and

improves accuracy.

If you note the Block Diagram at the end of the document, the function is clear. Voltage and

current are sensed by the microcontroller via a smoothing filter as sampled from the battery

and the current sense resistor.

Over-riding the ‘LO’ display

By long pressing Key1 (leftmost from 2 digit display), the ‘Lo’ voltage display can be

disregarded and a discharge forced. Thus a test can be conducted on a battery that is below

12.5V. Note that if the battery fails to pass the first 225 seconds under load before falling

under 10.5V the load will be halted and no data will be written to the log, thus the last logged

battery data remains intact. The display will not flash as per a ‘normal’ discharge test end.

The smoothing filter is necessary since the signal driving the MOSFET switch is a pulse width

modulated signal which has a base 7812 Hz frequency or 128 millisecond period.

You can see that this works by increasing the Duty Cycle (or pulse area) of the maximum current pulse train to

‘average’ a desired current, e.g. 50%. This is advantageous in that less heat is generated and lost by this type

of switching. However, we cannot measure average current directly here; we use resistor and capacitor

combinations to smooth the pulses into an average value which the 16F690 ADC can read properly. The lamps

appear to glow normally rather than fluctuate between bright and dark because of the latency of the hot

filament; it cannot change temperature from 0 to 100% at 7800 times per second, it can barely flicker at once

per second. Nor can your eyes see a change faster than about 30 times per second before it blurs into a

smoothed version.

The LED display also works similarly but by swapping each digit on and off very fast so you ‘see’ both at the

same time. This is necessary since each digit ‘shares’ the same 9 control lines excepting the common cathode.

The unique control line controls the swapping of the visible digits. This ‘saves’ 8 control lines and allows for a

smaller microcontroller to be used.

The 16F690 is configured to output the 7812Hz PWM signal which determines the percentage duty cycle of the

lamp load to use for a desired average drain load. This average current is measured by the 16F690 ADC and

used to calculate the Ah rating by using a ‘clock’ timer that is setup within the processor to track time elapsed

down to millisecond accuracy. The battery voltage is monitored and logged 16 times per hour to the 16F690’s

internal EPROM storage (256 byte) for a maximum of 16 hours x 16 samples = 256. Data is stored to the

EPROM once per hour, 16 bytes at a time and at the end of discharge @ 10.5V loaded battery volts.

One of these memory bytes is reserved for the calibration constant which compensates for any wiring and

connection resistive losses between the 16f690 ADC and the battery. This constant is altered by button #0

clicks to ensure the voltage sampled by the 16F690 matches that on a calibration reference meter. In effect

Figure 7 - PWM example

this data byte represents the resistance (R.) of all the connections and wiring so that by applying OHM’s law of

V=IR we can offset the wiring voltage drop (V) for all currents (I).

Another approach is used for the ‘unloaded’ voltage errors introduced by the tolerance of the resistors in the

circuit which form a voltage divider sampled by the 16F690 ADC. Here we simply use a trimmer

potentiometer to correct for any errors by adjusting it until the voltage displayed is correct. Calibrating every

few months is worthwhile for maintaining accuracy as component values drift with age.

The RS232 output is a direct digital TTL signal (220 ohm impedance) from the 16F690 EUSART module that

transmits the data stored in its EPROM from the last battery test. The TTL signal to switch the relay is amplified

by a 2n3904 NPN transistor via a 10K & 1uF RC filter (prevents boot up transient response) and the

1n4001 diode across the relay’s 400 ohm coil traps the coil’s inductive back EMF. The 7 segments plus DP, 2

Digit display is likewise direct driven from the 16F690 outputs via a 220 x 8 array resistor for current limiting.

Higher current, common cathode display control lines are fed by 2n3904 NPN transistors which are driven by

the 16f690 pins configured as TTL outputs. The buttons are sampled by TTL pins configured as inputs.

The back to back Zener diodes will ‘fail short’ if 16V is exceeded by the battery (wrong type), crowbarring the

10A FUSE to blow. They need to be replaced if this happens, but the overall kit is protected.

Thus the 16F690 microcontroller handles all the controls, inputs, processing, outputs and displays to make this

circuit simple and reliable. Via its voltage monitoring it can detect that only an RS232 5V is connected thereby

repurposing the voltage calibration button to become a data upload button. So the same hardware can be

used for multiple purposes under different conditions via intelligent programming, thus economizing on

components, cost and size.

I’d be happy to answer any questions or perhaps make improvements to the firmware if it becomes required.

Please enjoy building and using this kit. It has saved me a lot of time in accurately assessing when to retire

batteries.

Sincerely,

Ancel Bhagwandeen UWI, MIT.

mosaicmerc@yahoo.com

April 2014.

PS: As of Feb 2015, version 1-1 is suited for better thermal load handling. MOSFET upgraded to IRF3205 and

should use a small TO220 heatsink (note Fig 10) plus radiant heat shields (soda can material). The TC4420

MOSFET driver is likewise shielded. Machine screws (6-32) substitute for the feet and permit attachment of

the DIY heat shields (note FIG 11 at end).

Kit Includes:

Part No. Qty. Description

24707 1 DISPLAY,CC,ORG,635nm,RHDP,

35975 10 DIODE,SIL REC,1N4001,1A,

36185 10 DIODE,ZENER,1N4744A,15V,1W

38607 10 SOCKET,IC,20PIN,SOLDERTAIL

51262 1 IC,7805T,TO-220FP,

69462 1 FUSE,AGC FAST ACTING,10A,

81509 1 CAPACITOR,MONO,1uF,50V,20%

108564 1 RESISTOR NET,16PIN,220 OHM,

326596 1 TO-220 Heatsink With 1 Hole

149948 2 SWITCH,PB,TACT,SPST,OFF-(ON)

178597 10 TRANSISTOR,2N3904,NPN

489766 2 CONN,TERM,QD,FEMALE,14-16AWG

51571 10 SOCKET,IC,8 PIN,390261-2,

153700 1 HEADER,.1"ST MALE,1RW,6PIN,

174432 1 RELAY,T7C TYPE,SPDT

70991 1 CLIP,ALLIGATOR CLIP,CRIMP,

659956 4 RESISTOR,PW5,AXAIL LEADS,0.10 OHM

102842 10 FUSEHOLDER,PC FUSE CLIP,3AG

253982 1 POTENTIOMETER,1K OHM,3362P-102LF,

330772 10 CAPACITOR,RADIAL,1uF,50V,

330799 10 CAPACITOR,RADIAL,10uF,50V,

332672 10 CAPACITOR,MONO,0.1UF,100V,10%,X7R

618089 1 MOSFET,IRF3205,TO-220N Chanel

691180 10 RESISTOR,CF,22 OHM,1/4 WATT,5%,

690700 10 RESISTOR,CF,220 OHM,1/4 WATT,5%

690865 10 RESISTOR,CF,1K OHM,1/4 WATT,5%,

691104 10 RESISTOR,CF,10K OHM,1/4 WATT,5%

693901 10 LED,GRN,GRN DIF,T1-3/4,

697522 10 LED,RED ORG,DIFFUS,T-1 3/4,

853927 10 TERMINAL,QUICK DISCONNECT,M

1291647 1 IC,TC4420CPA,DIP-8, 6A SNGL

2094493 2 HEADER,5mm,TERM BLOCK,3 POS,

2127523 1 IC,PIC16F690-I/P,PDIP-20,7K

--- 1 PCB

--- 1 INSTRUCTIONS

2094426 10 3/8” Pan head 6-32 screw

Note that no 6-32 nuts are supplied. 6-32 standoffs commonly found in PC cases (mobo mounts) are good

substitutes to use to secure the TO-220 heat-sink (suggested for >6A loads). There is an extra 6-32 hole

placed at the PCB corner nearest the 7 segment display that can be snipped off with a flush cutter and used

as a 6-32 nut to secure the To-220 heatsink to the MOSFET.

Figure 8: SCHEMATIC

Figure 9-VERSION 1-1 LAYOUT for improved Load handling

Figure 10; V1-1, ASSY: note screws for Heat shields and feet.

Figure 11: V1-1, with Brass or Alum. heat shields. Soda can aluminium can be used.Cut a 1/8” notch in the heat shield

for the 6-32 screw slot.

Operational Block Diagram

_

USER

INPUT

) Switches (

Voltage

Current as a

voltage

value

SMOOTH

RC - ADC

FILTERS

16 F690

Controller

Relay

MOSFET LOAD

CURRENT SWITCH

) PWM (

12 V

INCANDESCENT

LAMP LOADS

Current Sense

Resistance

LEAD ACID

12 V Battery

EXTERNAL

LED

Displays

EUSART RS232

t o TTL PC

) internal (

5 V

Regulator

ICSP

Header