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D5

stops conducting,as do the secondary diodes, so no recovery problems exist.

The time constant of R 5 and C5 keepsthe MOSFET off for a while. The outputcapacitance of the MOSFET plus theparasitic capacitance of the primary res-onate with the primary inductance andthe voltage decreases. R 

5and C

5allow the

MOSFET to turn on when the voltage

has reached the minimum value. Thevalues are valid only for this case. Thecircuit of Figure 1 not only minimizes

turn-on losses, but also reduces electri-cal noise. Voltage regulation uses tradi-tional techniques, using a TL431. Theoptocoupler current adds to the shuntcurrent. Because the MOSFET turns onwhen current is zero, the gate resistormay be high, so parasitic capacitances

charge slowly, further reducing switch-ing noise. The circuit around Q

4is op-

tional; you can use it in most power sup-

plies. It kills the current glitch when Q3turns on. It is more effective than theusual RC circuit, and it allows a low duty cycle at low loads. Note that many of thecomponent values in Figure 1 are un-designated; you should determine thesevalues to fit the application.

When I was recently debugging adesign, I discovered that a shortcircuit existed from a ground

plane to a power plane.I did not have ac-cess to a milliohmmeter or an equivalenttester for locating this type of short cir-cuit. So, I logged onto the Internet to findan easily constructible milliohmmeter. I

found the answer in a manufacturer’sdata sheet, which outlined the basic four-wire method of making low-resistancemeasurements. The method uses a volt-age-reference IC as the input stage for acontrolled constant-current source. Aquick dig in the old component bucketrevealed a supply of LM317 variable-volt-

age regulators. These ICs provide 1.25Vbetween their VOUT

and VADJ

terminals, aconstant voltage to attack the constant-current problem. The other problem toattack was the output-voltage range of the constant-current source. The circuitI was working on used a 3.3V supply, soI had to limit the voltage to 3.3V. An

Simple circuit serves as milliohmmeter AM Hunt, Lancaster Hunt Systems Ltd, Shepperton, UK 

(continued from pg 74)

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LM317, configured as aconstant-current source,delivers an output voltage

equal to the in-put if the outputresistance is too high. Be-cause I wanted to use abench supply or a 9V bat-tery, the voltage would fry any 3.3V logic on theboard. Ideally, I wantedvoltage to be limited to1.5V. So,I came up with theconfiguration in Figure 1.

IC1

controls the base of the npn Darlington transis-

tor,Q1. The IC regulates thevoltage across the selected resistor to formthe constant-current source. The currentsource delivers either 10 or 100 mA, de-pending on which emitter resistor is in thecircuit.The purpose of S

1is to give longer

battery life. You can calibrate the currentsource by strapping a resistive load be-

tween test points A and B and measuringthe voltage across the resistor using aDVM (digital voltmeter). I used 5 and10 and set one S

2position for 10 mA

and the other for 100 mA. To measure asmall resistance, you attach test points Aand B across the resistance. You set the

DVM on a millivolt range.The DVM reads a voltagethat is proportional to the

resistance under test. If youcalibrate the circuit as sug-gested, then the reading is10/V on the 100-mArange and 100/V on the10-mA range.

To track down pc-boardshort circuits, attach theunit with test points A andB across the suspectedshorted signals.Attach oneDVM probe to test point A

and use the other to

probe the circuit. Con-stant voltage along a trace indicates thatno current is flowing and that the trace isnot the source of the short circuit. Look for high readings on the trace with thelow reading and low readings on the tracewith the high reading, to locate thesource of the short circuit.

LM317

IC1

S1A

S1B

VIN

  VOUT

P1

100

Q1

BD636

P2

10

R1

68

R2

6.8

VADJ

+

–

+

–

3V

BATTERY

9V

BATTERY

OR BENCH

SUPPLY

A

B

F igure 1

Make your own milliohmmeter, using a voltage-regulator IC and some resistors.