I am in the process of building a constant current load. The specs are 0-2 amps with an input voltage not to exceed 25 volts. Before getting into the discussion, here is the circuit:
The problem is that I am not getting the full 2 amp load as I increase R3. Here is a table that shows what is happening:
Pin 3 Pin 5 Pin 6 Pin 7
Volts Volts Volts Volts
0.00 0.00 0.05 2.30
0.50 0.20 0.20 2.90
1.00 0.40 0.40 3.13
1.50 0.60 0.53 3.25
2.00 0.80 0.54 3.25
2.50 1.00 0.51 3.23
3.00 1.20 0.52 3.23
3.50 1.40 0.53 3.23
4.00 1.60 0.53 3.23
4.50 1.80 0.53 3.23
5.00 2.00 0.53 3.23
The pins refer to the LM324. What I would expect to happen is that as the non-inverting input (pin 5) increases, the Op-amp would do its thing and adjust the output so that the inverting input (pin 6) matches the non-inverting input. However we seem to hit a brick wall around .53 volts, and consequently the current maxes out at .53 amps.
Here is some additional info:
The fan runs all the time, as soon as we power up the circuit.
The voltage drop across R6 is 2.53 volts which indicates a 25 mA current when there should be NONE!
LOAD is 11.90 volt, 2.2Ah LiPo
Could it be that Q2 and Q3 are installed backwards?
All comments and suggestions welcome.
The problem is that I am not getting the full 2 amp load as I increase R3. Here is a table that shows what is happening:
Pin 3 Pin 5 Pin 6 Pin 7
Volts Volts Volts Volts
0.00 0.00 0.05 2.30
0.50 0.20 0.20 2.90
1.00 0.40 0.40 3.13
1.50 0.60 0.53 3.25
2.00 0.80 0.54 3.25
2.50 1.00 0.51 3.23
3.00 1.20 0.52 3.23
3.50 1.40 0.53 3.23
4.00 1.60 0.53 3.23
4.50 1.80 0.53 3.23
5.00 2.00 0.53 3.23
The pins refer to the LM324. What I would expect to happen is that as the non-inverting input (pin 5) increases, the Op-amp would do its thing and adjust the output so that the inverting input (pin 6) matches the non-inverting input. However we seem to hit a brick wall around .53 volts, and consequently the current maxes out at .53 amps.
Here is some additional info:
The fan runs all the time, as soon as we power up the circuit.
The voltage drop across R6 is 2.53 volts which indicates a 25 mA current when there should be NONE!
LOAD is 11.90 volt, 2.2Ah LiPo
Could it be that Q2 and Q3 are installed backwards?
All comments and suggestions welcome.
Last edited: