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40-120V DC to [email protected] Voltage Regulator.

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New Member
Problems with high voltages?

Gasson wrote in a pm ...
I need to build a high voltage regulator that will provide me with [email protected] and input voltage can varry from 40V to 120Volts. I know this is possible with two transistors building a buck regulator but I seem to have a problem. When I start the regulator from 0V and run it upto 120V it work perfectly but when I switch 120V immediately on then the transistor blows.
Buck regulator? I assume this is a conventional step-down series regulator?

This circuit is dropping between 35 and 115 v across the transistors? is it possible that when you switch on one of the transistors sees more (possibly 120v ?) than it can cope with, resulting in destruction of the collector or base-emitter junction? Are the transistors rated high enough to avoid this?
Or is there a current surge, charging a HUGE capacitor? (slowly increasing the voltage rather than 'banging' on would minimise both these effects).

The above is guesswork as there is no circuit diagram to look at.
Would it be possible to have two stages of regulation, the first dropping to maybe 30v. This could ease the stress on power-up.


Active Member
Use Varistors to protect the Base-Emitter junction from blowing out. Also don't forget to use godd large Heat-Sinks for your power transistors.

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