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Electronics => Electronics => Topic started by: newInRobotics on May 27, 2011, 09:56:34 AM

Title: Switching voltage regulator
Post by: newInRobotics on May 27, 2011, 09:56:34 AM
Would a circuit attached below work?

I got myself thinking about switching voltage regulators and came up with a simple circuit.

Short explanation:

Would such circuit work?
Title: Re: Switching voltage regulator
Post by: waltr on May 27, 2011, 10:48:23 AM
That is not a switcher as the Op-amp and transistor are linear devices. What will happen is the transistor's base current will be driven to the point where the transistor's output voltage is equal to the op-amps Vref.
Except that the wrong transistor is being used. Remember that an NPN transistor passes current from collector to emitter when current flows from base to emitter.
The transistor should be an PNP with its emitter connected to the 12V. Then the op-amp sinks current out of the base to turn on the transistor.

This circuit is really just a linear voltage regulator using a pass transistor not a switcher. You could use a biased zener diode on the transistors base instead of a Vregulator and op-amp.

Or you can just put a PNP transistor across the linear regulator to increase the regulator's current. See the application section of linear regulator's data sheet that typically shows how to use a pass transistor. Try looking at the data sheet for an LM7805 regulator.

The other part that is missing is an output filter cap.

Title: Re: Switching voltage regulator
Post by: newInRobotics on May 27, 2011, 10:55:08 AM
That is not a switcher as the Op-amp and transistor are linear devices. What will happen is the transistor's base current will be driven to the point where the transistor's output voltage is equal to the op-amps Vref.

Hmmm, I do understand that transistor is linear, but how come that Op-Amp is? I have always thought that Op-Amp used as comparator gives either LOW or HIGH output and nothing in between.
Title: Re: Switching voltage regulator
Post by: waltr on May 27, 2011, 11:07:48 AM
It is connected with negative feed-back. An Op-amp changes its output so that the two inputs are the same.
This works as a comparator if the output is not feed-back to the inverting input. And many times one adds a small amount of positive feed-back when using an op-amp as a comparator.

Get out your proto-board and the parts then wire it up and play until you understand how it works.