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1-18volt variable power supply

Kevin Weddle

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I deleted the 1-25volt variable power supply because it doesn't work. This one is better.

1) Why did you make a heater since the 36V input is much too high for your 18V power supply?
To make 18V then an input of 24V maximum can be used and it will be much cooler.
2) Why did you use such a high voltage zener diode? A high voltage zener diode has its voltage increase a lot when it gets warm and it does not regulate very well. The datasheet for a family of zener diodes shows that a 5.6V to 6.8V zener diode has almost no voltage change with temperature changes and it regulates the best.
3) Nobody makes a 34V zener diode. 33V and 36V are the nearest standard voltages.
4) Why do you use TWO inverting opamps when a single non-inverting opamp can do the same thing?
5) Why does the second opamp have a voltage gain of 100 times so it amplifies its own noise 100 times?
6) What limits the current if the output is overloaded or is shorted to ground?
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Thank you audioguru. I'd like to change this design a little. The feedback opamp needs to have a gain closer to 1. Probably 5kom resistors will do and provide adequate feedback.

If your main opamp is non-inverting then the second opamp (the feedback opamp) is not needed.
Then the feedback goes from the output to the (-) input of the single opamp.

You can add two resistors to the feedback so the circuit can amplify a very good 5.6V zener diode up to 18V.

Or you can replace the zener diode with a 78L05 5V little regulator.
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Why does the opamp in the 30v stabilized power supply circuit need a gain of at least 3. The voltage follower opamp doesn't seem to do much. But it looks good!

No opamp in the circuit has a gain of 3. Also there is no voltage follower opamp.

Opamp U1 has a voltage gain of 2 so that the 5.6V zener diode is amplified to make the 11.2V reference.
I told you before why the 5.6V zener diode performs much better than one with a higher voltage.

Opamp U2, the driver transistor and the output transistors are a power amplifier with a voltage gain of 30V/11.2V= 2.679 times. The output transistors have series emitter resistors to balance them which reduces their voltage gain to a little less than 1 so U2 has a little more gain.  The negative feedback is around all of them, not just around the opamp.

Opamp U3 has a voltage gain of typically 220,000 to regulate the output current from a few mA to 3A.
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Why use U1 to generate a regulated 11.2 volts? 20 volts regulated can be reduced with a potentiometer to get 18 volts.

Where is 20V regulated going to come from? A 20V zener diode has poor voltage regulation and its voltage increases when its temperature increases as it warms up.

As I said before, a 5.6V zener diode has very good voltage regulation and its voltage does not change when the temperature changes. U1 is a constant current source for the zener diode to make its voltage regulation even better.

Kevin, maybe you need to learn some basics of electronics.
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The design I posted should have the same current as this one. But the direct feedback reminds me of a self resonating oscillator. If it doesn't produce harmonics, does it make this a better design even then?

You did not post a schematic in this reply so we do not know what you are talking about.

This thread was for a power supply, not about an oscillator anyway.
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