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Power Supply Section

Because much of the unit's electronic circuits function better at a higher voltage than at the 3-volts supplied by the battery, a simple flyback voltage converter is created. A C-MOS version of the classic 555 timer (A1) is wired as an astable oscillator. When pin 7 of the timer output turns on, a current flows into the inductor (L1) from the 3-volt battery supply. When pin 7 turns off, the energy stored in the inductor emerges at a short voltage pulse, higher in Amplitude than the supply. The circuit routes the voltage pulses produced at pin 7 to a filter capacitor (C3) through a diode (D1.) With the component values chosen, the voltage produced at C3 ranges from about 9 to 12 volts. The large 100FF capacitor (C1) is placed across the 3-volt supply to help dampen transients caused by the heavy DC current drawn from the battery during operation. The circuit uses a momentary pushbutton POWER switch (S1) to insure that the system draws power from the battery only when you activate S1. This momentary operation also discourages prolonged measurement times, increasing the number of measurements that can be taken.

Current Controller Section

The circuit uses a power MOSFET (Q1) to control the current passed through the unknown resistance. By controlling the voltage applied to the gate terminal of the FET, it can be made to behave as a variable resistor. You can, therefore, vary the current as needed. One half of the IC LM393 (A2b) supplies the FET gate control voltage. Although you normally use the IC as a voltage comparator, this circuit modifies it to operate as an operational Amplifier, with the aid of the resistor (R9) and the capacitor (C5.) The two components form a filter network to help stabilize the voltage fed to the FET. A regulated voltage, developed by the reference diode (D3) and the resistor (R5), is connected to the non-inverting side of A2b through a voltage divider network consisting of R6, R7 and R8. The potentiometer (R7) trims the voltage produced by the divider circuit and allows the output injector's current to be adjusted precisely to 1-amp.

The injector's output current, which passes through the unknown resistance and the FET (Q1), is also forced to pass through an internal current monitoring resistor (R10). The voltage developed across R10 is thus proportional to the injector's output current. The current monitoring voltage from R10 is fed to the non-inverting side of the control IC (A2b.) Thus the IC's output supplies the gate of the power FET with the necessary voltage to maintain the unit's output current at a fixed level. For protection, you wire the diode (D4) and capacitor (C6) across the output terminals. If you measure resistors with a large inductive components, they absorb any energy that may be sent back to the circuit.

Battery Voltage Monitor Section

With two fresh "C" cell batteries installed in the unit, a nominal 3-volts is supplied to the current control circuit. However, during operation the battery voltage will drop. The unit will continue to operate properly, providing the battery voltage remains above 2-volts. The battery voltage monitor's job is to activate the power indication LED (D2) as long as the battery voltage remains above 2-volts. If the LED fails to turn on, it would be an indication that the batteries need to be replaced.

The battery monitor uses the second half of the dual comparator IC LM393 (A2a.) It is wired as a classic voltage comparator. A regulated voltage from the reference diode (D3) is compared to a voltage developed by the voltage divider (R3 and R4) that is connected to the battery. As long as the voltage from the divider is greater than the reference voltage, the output of the comparator remains low, turning on the power indication LED.


You need a good digital multimeter to accurately calibrate the unit. Before connecting the injector's output to the digital multimeter's leads make sure the meter is set to measure DC Amps, (usually 2-amp scale), and not DC milliamps. Warning: If you have mistakenly set the multimeter in the milliamp/microamp scales, the one Amp of current from the unit could blow the meter's protection fuses. With everything ready, push and hold the current injector's POWER switch. Now adjust the trim pot (R7) until you read 1.000 Amp of current on the multimeter. That's it. You're done. Once you have calibrated the units, you can attach the box lid using four screws.

Some multi-meters cannot measure the current directly or don't have a 2-amp scale. If this happens to be the case, you will need to measure the current indirectly with the use of an accurate resistor of known value. A 1% resistor, ranging from 0.1 to 1-ohm, that can handle 1-amp would be ideal. Attach the current injector clips across the calibration resistor. Then, connect the digital multimeter leads across the resistor and set the meter to measure the appropriate DC voltage. With everything connected, press and hold the POWER pushbutton on the top of the unit's box. Note the voltage reading and adjust the current control pot (R7) on the circuit board for a voltage reading equal to the resistor value.


With fresh batteries installed, the FET (Q1) can dissipate a maximum of 2.5 watts. The transistor dissipation will be maximum when the unit is measuring low value resistances. Although you installed a small heat sink onto the transistor, it could still get hot if the unit is in operation for longer that a few minutes. I recommend that you use the system only in a momentary manner. Connect it up to the resistor to be measured, attach your digital volt meter, then take a quick measurement. With fresh alkaline batteries, you should be able to make hundreds of measurements before the batteries have to be replaced. Also, under weak battery conditions the power indication LED may turn on when the unit is not connected to a resistor but will fail to turn on when a resistor is connected. This is caused by a drop in the battery voltage under a loaded condition. The best way to make sure the unit's battery is still good is by activating the unit while the two test clips are shorted together. If the light fails to turn on, replace the batteries.



2"C" cells (alkaline recommended)


A1 -- TLC555CP C-MOS timer
A2 -- LM393N Dual comparator

A3 -- lM385BZ-1.2 voltage reference


C1 -- 100Ff 10v, aluminum electrolytic
C2 -- 1000rf 50v ceramic or mylar
C3 -- 10Ff 25v, aluminum electrolytic

C4, C% -- 0.1Ff50v, ceramic or mylar
C6 -- 0.4Ff50V, mylar


R1 -- 82K 1/4W 5% carbon
R2 -- 100W 1/4W 5% carbon
R3 -- 10K 1/4W 5% carbon
R4 -- 15K 1/4W 5% carbon
R5 -- 3.3K 1/4W 5% carbon

R6 -- 18K 1/4W 5% carbon
R7 -- 500W multi turn trim pot, Digi-Key CEG52
R8 -- 1.3K 1/4W 5% carbon
R9 --  100K 1/4W 5% carbon
R10 -- 01.W, 1 watt, Digi-Key SC1A0.1


L1 -- 4.7FH choke, Digi-Key TK4320


Q1 -- IRF521 or IRFZ12, 50v, 0.3W Power FET


D1 -- 1N4148 general purpose diode
D2 -- LED, Red, super-bright T 1: style

D3 -- 4001


PC board
LED lens/holder -- Digi-Key L3000
Pushbutton switch, SPST, Normally open, Radio Shack 275-1566
Battery holder, 2 "C" cell, Radio Shack 270-385

Enclosure Box, Radio Shack 270-222
Transistor Heat Sink, Digi-KEy HS111
Test Lead Wire, 36", 20ga, red/black, Radio Shack 278-553, 278-554
2 Alligator Clips, red/black, Radio Shack 270-356

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