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.