Why is my op amp not turning on my transistor?

[likephysics]

I am trying to drive a NPN transistor with a LM324 op amp. The darn transistor does not turn on when driven by an op amp.
This is basically a current limit circuit. Instead of using a large value sense resistor, I used a small resistor(0.1 ohm) and used an op amp to sense and increase the voltage to turn ON the NPN(2N2222) transistor when the current limit is reached.
The op amp I am using is LM324. From the datasheet, it's more than capable of souring enough output current to turn ON the 2222. Not sure why it's not working.
pdf attached.

 

[schip666!]

The circuit, having your transistor in some kind of feedback loop, makes me a little cross-eyed. But if the voltages are as indicated your 2N2222 is not turning on because the base voltage is 1.61v and the emitter voltage is 16.2v, so no base current is flowing...

If you want to shut off the TIP31 when the current and op-amp output goes over some threshold, you might try connecting the 2222 emitter to ground and use up a resistor divider on the base to set the turn-on level.

 

[es1]

The gain of the LM324AB diff amp circuit is wrong. The B of the 2N2222 has to be above the voltage on R_load for the 2N2222 to steal current from R1.

Right now the gain is basically 1V/A (the R is 1V/10A and the diff amp gain is 10V/1V). Since V_in is 22.5 and R_load is 10ohm there will never be more than 2.25V at the base of the 2N2222 which is not enough to turn it on.

Try increasing R5&R6. With your measured values and with those resistors at 200K (so now it would be 20V/A) the 2N2222 will definitely turn on.

I am not sure what limits you are trying to set so I cannot recommend proper R values.

 

[yungman]

At 1.6A, the emitter of the 2N2222 is at 16V, you have to have the base drive of at least about 16.7V to turn it on. Your op-amp gain is 10. At 1.6A, only 0.16V drop across the sensing resistor and the output of the op-amp is 1.6V. You are not even close. You might turn the 2N2222 into a zener diode since the emitter voltage is at 16V.

This is not a good way to design a limiter circuit. Even if you make it work, it is likely that you can get into occilation because it become a closed loop circuit and the op-amp try to balance the current limit. You don't need this kind of headack.

 

[es1]

I think the circuit will be stable for DC loads, which is what is shown in the schematic.

It's not really a current limit circuit though (as in if I(R_load) > X turn off). I think it's trying to be more like a fixed current source. i.e. find the V(R_load) to make I(R_load)=X.

 

[yungman]

Anything with closed loop is prone to instability. Not enough info to look at stability. If it is used to drive a pure resistor as show, it is simplier. Just put a cap across R5 to create a dominant pole and likely it would be fine. But if it is some inductive or capacitive load, you really have to be more careful.

I did a lot of closed loop feedback design. You always have to look out. How many times you see DC to DC switching regulator instability. People don't even know and just said it is noisy when the load is too light! That is instability at it's finest. A lot of the closed loop circuits are only conditional stable and if you just follow the application notes and build the DC to DC converter circuits, good luck. We had so much problems and I got to fix those!

 

[Bob S]

The circuit is a constant current source, with Q1 being the pass transistor, and the 2N2222 transistor being the current limiter transistor, monitoring the voltage across the series sense resistor R_s, controlling the base voltage of Q1. This would be a simple current limiting circuit, if the base of the 2N2222 were attached to the top end of the series sense resistor R_s. The LM324 is just a fully differential amplifier with a gain of 10 (look at R4 R5 R2 R6).

The circuit design problem in my opinion is that the top end of R6 should not be grounded, which references the differential amplifier output to ground, but should instead be attached to the top end of the series sense resistor R_s. In this case, the op amp will just be amplifying the voltage across the series sense resistor R_s, and turn on the 2N2222 if the voltage across R_s exceeds about 0.07 volts.

The circuit will function the same if
a) the differential op amp circuit is eliminated,
b) the base of the 2N2222 were attached to the top end of the series sense resistor R_s and
c) R_s were increased from 0.1 to 1.0 ohms.

Bob S

 

[likephysics]

The circuit, having your transistor in some kind of feedback loop, makes me a little cross-eyed. But if the voltages are as indicated your 2N2222 is not turning on because the base voltage is 1.61v and the emitter voltage is 16.2v, so no base current is flowing...

If you want to shut off the TIP31 when the current and op-amp output goes over some threshold, you might try connecting the 2222 emitter to ground and use up a resistor divider on the base to set the turn-on level.

Thanks Schip666. That was it. I knew it was something simple. It works now.