Author Topic: 7812 regulator question  (Read 1974 times)

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Offline Chris WilsonTopic starter

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7812 regulator question
« on: August 13, 2015, 03:44:08 pm »
I have a circuit where the CMOS and other bits and bobs run off a 15V DC supply feeding a 7812 regulator. The 15 or so volts DC comes off a winding on a toroidal 240V AC transformer, and a full wave bridge. This transformer also supplies a 100 V DC circuit via a  full wave bridge and smoothing. I want to run the transformer off a Variac, to make the device, an RF amp, give less power by reducing the 100  V DC. But reducing the 100V DC down low enough  to reduce the power sufficiently means the 15 V drops low enough so it will not drive the 7812. What will happen if I feed the low voltage circuits from another regulated DC supply, so the 7812 is effectively back fed, tapping in *after* the 7812? Will it damage anything? If I put a hefty diode after the output of the 7812, would that work, if the circuit doesn't mind the forward voltage drop of the diode? I would like to put a DC in socket on the amp to enable me to test on low power, feeding the low voltage stuff remotely, when required.

Need to see the circuit? it's attached, many thanks.

Thanks. I know i could just try it, but the 7812 is pretty buried and replacing it is not trivial.
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                 Chris Wilson.
 

Offline unitedatoms

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Re: 7812 regulator question
« Reply #1 on: August 13, 2015, 03:54:08 pm »
If freqdoubler has low output impedance, has no DC blocking cap, and is powered independently, and timing of powering up powering down is not simultaneous, then CMOS ICs will have their embedded diodes fried.

Say, you have freqdoubler AC signal +/-1V. The ICs are unpowered, yet to be powered in few msec, effectively their inputs are still at ground potential.  The diodes inside ICs will work like a short to filtering cap, the voltage starts to ramp slowly being rectified, the ICs will heat and may fail.
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Offline edavid

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Re: 7812 regulator question
« Reply #2 on: August 13, 2015, 04:01:26 pm »
No, you can't apply voltage to the output of an unpowered 7812.  Diode or-ing should work OK.

 

Offline Chris WilsonTopic starter

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Re: 7812 regulator question
« Reply #3 on: August 15, 2015, 06:23:11 pm »
OK, understood. Thank you both for the replies, appreciated, have a good weekend!
Best regards,

                 Chris Wilson.
 


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