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I've always had trouble understanding potentials and current flow between secondary transformer windings and this example is fairly typical.

The schematic shown is a straightforward linear regulated power supply circuit from Heathkit (IP-28) with two secondary windings, one of which creates a reference voltage for the control element (Q5). Looking at the schematic, I'm struggling to see why current would flow through D6 and into Q5 at all. What would entice current to flow between two otherwise isolated secondary windings?

enter image description here

Original schematic here.

Assuming the bottom of the secondary winding was ground (it isn't in the schematic as far as I can tell), you could say that the negative rail was 4 (or two) diode drops below the reference voltage. But, that assumes you know that the negative rail sits below the reference voltage; what establishes these potentials?

It could be diodes D9 and D10, but again how are the potentials established. I'm guessing no current flows through D9 and D10, so this just adds to the confusion for me.

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  • \$\begingroup\$ Why would you guess no current in D9 and D10? \$\endgroup\$ – user28910 Jun 9 '17 at 15:04
  • \$\begingroup\$ Because, and I don't quite understand why this is so, but according to the schematic the reference secondary is at a constant 32V whereas the positive rail is max 30V. So the diodes are reversed biased, right? \$\endgroup\$ – Buck8pe Jun 9 '17 at 15:13
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You're just misreading the schematic. The winding isn't isolated - the blue (5) wire is connected directly to the + Red output (along with the sense line for that output, white-red) when the standby switch is closed. When it is open, the circuit is in standby.

Pin 3 of the voltage control potentiometer is connected to the "ground" of the isolated winding, and since the blue winding is connected to the red output, which is not galvanically isolated from the transistor (though it does connect through a number of components), there you have it. You need two connections for current to flow between two isolated windings, and you have them. They are no longer isolated.

The dotted lines with "J" just mean jumpers, and are there to indicate some sort of wire jumper is used, but it's otherwise the same as a solid line connecting those two things. I suspect that's what's causing you problems. If you ignore the jumper connections, then it does indeed appear to be an impossible circuit (and is - it won't work!).

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