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ONE-AMP CURRENT INJECTOR
Have you ever been frustrated with your digital multimeter (DMM) when trying to measure resistors below one
ohm? Most meters will only display a low value resistance down to the nearest 0.1 ohm. This project shows you
how to build a DMM accessory that will enable you to measure accurately a resistance down to the nearest 0.0001
ohm. The unit injects a precise 1-amp of current into the resistance to be tested. You simply use your DMM to
measure the voltage drop produced by the injected current. With typical meters, resistance from 1.5 ohms to
0.0001 ohms can be measured with the aid of the current injector.
Two "C" battery cells power the injector circuit and features a power indication light which doubles as a wea
k
battery indicator. If the batteries need to be replaced during operation, the light will fail to turn on.
You should find may uses for the unit. I have used it to measure shunt resistors, circuit board trace resistance,
relay/switch contact resistance and motor/transformer winding resistance. You also can use it to determine a wire
g
au
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e b
y
measurin
g
its resistance
p
er unit len
g
th and to test diode rectifiers or transistors at 1-am
p
.
HOW IT WORKS
The Circuit
shows the schematic diagram for the One-Amp Current Injector circuit. The circuit divided the
o
p
eration of the unit into three main sections:
p
ower su
pp
l
y
, current Controller and batter
y
volta
g
e monitor.
Power Su
pp
l
y
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-vol
t
b
attery 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
discoura
g
es
p
rolon
g
ed measurement times, increasin
g
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
p
ass 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
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3One-Amp Current In
j
ector -- Circuit Description
2012-4-16http://www.discovercircuits.com/front/in
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ector.html
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