I have such a schematic based on the TL074 opamp and the TIP127 PNP darlington transistor to control the fan voltage (shown as motor near the D3 diode). VCC = 12V, VDIG = 5V generated by a LM7805. The problem is that this schematic works perfectly in the simulation, yet it resists to work on the breadboard until I touch the opamp's positive input. After that, the schematic works perfectly until I try to turn it off and on again - it turns off fluently (using the pot), yet it does not turn on until I touch that positive input.
I don't understand what is going on. My guess is that I should put a capacitor somewhere, but I don't understand where and - more imporantly - why to make it work.
I am using a 12V 0.3A fan as a load.
Answer
Here are my thoughts: -
- The TL074 has a common mode input range that does not include the negative rail (0 volts on your circuit) so, when you power it up the non-inverting input is breaking the rules and the op-amp cannot be expected to function correctly. Typically, anything between 0 V and 3 volt on the input will likely cause problems.
- The TL074 has an output swing that does not reach the power rails so it's very unlikely that Q1 will switch off once it is on. This is probably what is mentioned in the question "until I try to turn it off".
- Adding an extra level of voltage gain to an op-amp is usually a recipe for disaster (it turns into an oscillator) if not done carefully. Q1 is a common emitter hence it has plenty of voltage gain from base to collector. In this circuit, that gain could be in the realm of about 50 to the junction of R2 and R3. If you look at figure 9 in the TL074 data sheet you will see that increasing gain by ten is probably enough to make the phase margin zero (it becomes an oscillator).
Problems all round really.
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