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Analog Voltage regulator

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marovacka

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Hello everybody!

I have built the circuit which scheme is shown in 1. Figure.
I have done measurement (with 50kHz of sampling frequency) of transient responses and part of captured data you can see on the 2. figure. On 3. figure is shown detail where is see what is my problem. When the system is in zero voltage state and the transient starts from this state to arbitrary other state, this transient is delayed. Can anybody help me?
Thank you
 

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That is because the cap is mounted wrong.

In beginning (moment when voltage applies) all current to the motor goes through opamp output through the cap. Remember that any opamp can only deliver a limited amount of current from its output.

Also be aware that exept the cap, the circuit does just the same as any voltage folower. That is - if you can look away from the transistor stage and threat it like a part of the opamp.
 
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In beginning (moment when voltage applies) all current to the motor goes through opamp output through the cap.

Ok, very good understanding :) Thank you very much ... Has LM358 overcurrent protection and due to that is away from normal operation for a moment? And how can i solve it? I know that opamp is connected like voltage folower, I can connect it also like noninverting amplifier with large resistors and then put a feedback cap. Will this solve this problem?
 
What is the purpose of the capacitor? It's not needed for circuit stability.
 
If you need that cap, then there will be a transient delay from 0V to charge that cap to the 0.7V base-emitter voltage of the transistor. The problem is that at 0V input, small offset voltages can cause the loop to try to go negative which opens the feedback loop and sends the op amp to 0V.

To avoid the delay you can bias the input with a small voltage (say 0.1V) to keep the loop closed and pre-charge the cap.
 
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