DEC H7822 power supply

Christian Corti cc at
Fri May 13 02:36:08 CDT 2022

On Thu, 12 May 2022, Peter Coghlan wrote:
> Can we agree that there is such a thing as an "AC current" and there
> is such a thing as a "DC current" and the two of them can be added
> together?

Ouch... an alternating current current and direct current current ;-)
And you usually add the voltages, not the currents.

> A variac does not provide isolation.  If it is not wired correctly, it is

Good "variacs" provide galvanic isolation from the mains, i.e. they are 
not autotransformers.

> I just wanted to describe the obscure (to me) circuit I found in a DEC H7821
> power supply in case anyone else needs to know how it seems to work.

That indeed is really interesting. The only reason that I can imagine why 
they used the ripple voltage to generate the transceiver voltage is that 
they "forgot" to implement an extra secondary winding on the transformer 
(or they reused the transformer from another design).


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