# Constant Current Load on Battery

Discussion in 'General Electronics Chat' started by curiosity2222, May 18, 2012.

1. ### curiosity2222 Thread Starter New Member

May 18, 2012
2
0
Hello everyone,

If you apply a fixed resistor to the terminals of a battery, electrons will flow out of the negative terminal into the positive terminal, which eventually will drain the battery. How does a constant current load work in a similar scenario, using an op-amp and a npn transistor? I'd really appreciate it if someone could fully explain how the op-amp and npn transistor operate together to drain the battery.

2. ### crutschow Expert

Mar 14, 2008
13,498
3,374
A constant current circuit operates by monitoring the voltage across a small shunt resistor in series with the output current with an op amp. The op amp then adjusts the drive current of the transistor, also in series with the resistor, using negative feedback to keep the voltage across the resistor constant. This keeps the current through the circuit constant, independent of battery voltage, at least until the voltage gets so low that the circuit stops operating properly.

3. ### curiosity2222 Thread Starter New Member

May 18, 2012
2
0
Thank you very much for answering my question. I also have another question....the constant current load circuit will be powered by a variable dc power supply set at 12 VDC, is it necessary to use an LM7812 (fixed 12V) voltage regulator for my circuit, does the supply voltage from most dc power supplies (HP, Fluke, etc..) fluctuate over time?

4. ### steveb Senior Member

Jul 3, 2008
2,433
469
Most likely you don't need a voltage regulator because a constant current circuit will be relatively insensitive to voltage changes, as long as sufficient voltage is present. The amount of voltage variation (due to load changes) depends on the exact power supply you have. Some might be open loop (like a battery), and more likely would have a closed loop feedback circuit that maintains the voltage at the output terminal. Some power supplies have voltage sense terminals that can be connected closer to the actual load, which then allows the feedback to better maintain the voltage directly on the load. In other words, wire resistance and inductance will be compensated for.