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Unstable voltage regulator


marovacka
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Hello my friends
I try to build voltage regulator how you can see in attached figure. I removed resistors R1 and R2, so the Input/Output relationship is Vout=Vin. This scheme work properly if the circuit is not loaded or loaded with pure ohmic load with resistance higher than 10ohms. For inductive load such DC motor is unstable (output of the op amp and emitor voltage (Vout) oscilates). For pure ohmic load with resistance RL=4ohms it starts oscilate for voltages higher then 7volts. Has anybody idea how can i solve this problem?
Thank you very much

post-68915-14279144365558_thumb.jpg

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You have a couple of problems... first, if the the supply voltage to the LM358 is 12V, the base of the BDX33C needs to be at least a Vbe drop above the emitter voltage to get 12V at Vout... you can't get there with what you have. Second, put a feedback cap on the LM358, say .1

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Thank you indulis,
I tried to put feedback capacitor and it improved behavior, but 0.1uF was not enough so we tried 320uF and it is quite good.
I have done measurement (with 50kHz of sampling frequency) of transient responses and part of captured data you can see on 1. figure. On 2. figure is shown detail where is see what is my problem now. When the system is in zero voltage state and the transient has to be from this state to  arbitrary other, this transient is delayed.  Can anybody help me?
Thank you

PS: Scheme is on figure 3.

post-68915-14279144371392_thumb.jpg

post-68915-14279144371638_thumb.jpg

post-68915-14279144372522_thumb.jpg

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