Voltage reference stabilizes current sink
Produce a stable current for long-term component testing.
Suded Emmanuel, Emmanuel’s Controls, Auckland, New Zealand; Edited by Martin Rowe and Fran Granville -- EDN, February 18, 2010
Analog circuits for long-term testing of passive components, such as 0.1%-tolerant resistors or high-intensity white LEDs, often require a constant current. Using two op amps and a voltage reference, you can develop a circuit that provides a constant-current sink with a variable setting of 0 mA to 0.99A. The circuit in Figure 1 sinks a stable current through the load. The load current is insensitive to power-supply-voltage variations. IC1 is a voltage reference that gives a stable 5V dc. It requires 500 µA of current from the power supply. IC2 is a National Semiconductor LM324 quad op-amp. Voltage follower IC2A buffers the reference voltage from the rest of the circuit, which increases stability.
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Resistor R2 and potentiometer R3 form a variable voltage divider that reduces the 5V reference voltage to a value between 0 and 3.26V. Unity-
gain amplifier IC2D drives the base of Q1, a Darlington power transistor that has a current gain of 750, through R4. R4 and C5 form a lowpass filter that prevents oscillation. You can drive Q1 with a small base current. C4 connects between the collector and the base of Q1, adding further stability.
Operating as an emitter follower, Q1 can drive an active or a passive load, such as a resistor or a high-brightness LED. Q1’s emitter connects to R5, a 3.3Ω, 5W grounded power resistor. The voltage at IC2D’s Pin 14 sets the voltage across R5, which fixes Q1’s emitter current. Because of Q1’s high gain, the current in the load is effectively Q1’s emitter current.
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I have to disagree with Rick W. The first op amp is there to prevent loading on the shunt reference. Why not use a reference with a voltage output; then R2 could be connected directly. And maybe something a little more 21st century than LM324?
Nick Allen - 2010-4-3 08:32:00 PST -
No kidding! Supply a stable voltage to a current sink and get a stable current! Well, almost. Depends on what's meant by stable. Hfe changes with temperature?...OK, it's high enough to mostly ignore base current. And there's that 3.3 watts heating up R5. Heat sink it and Q1.
Nick Allen - 2010-2-3 13:13:00 PST -
Save one op amp (IC2a) by connecting R2 directly to R1/IC1. R2 & R3 go to the (+) input of IC2b, which is a high-impedance input, so there is no need for a buffer.
Rick W - 2010-21-2 04:55:00 PST
