DEC H7822 power supply

Peter Coghlan cctalk at
Wed May 11 03:12:37 CDT 2022

Christian Corti via cctalk wrote:
>On Tue, 10 May 2022, Peter Coghlan wrote:
> > Ahead of the 78M09 is another TO220 package marked D45H8 which seems to
> > be a transistor.  Then I am completely lost.  I can't find the rectifier
> That TO220 part would be the rectifier. It either has two legs (simple 
> diode) or three (two-wave rectifier with presumably common cathode).

I thought that too at first.  However, it is definately a transistor.

I've made a bit more progress figuring out the circuit.  There is a
large torroid with three windings on it on the output side.  I thought
it was some sort of triple wound smoothing choke for the 5V, 12V and 9V
supplies but it is not.  It turns out that this is actually the source of
the 9V supply.  It is effectively a current transformer with two primaries.
The ripple on the rectified 5V and 12V supplies gets transformed into an
isolated AC source for the 9V supply.  This AC goes to a simple rectifier
which looks like a 1N4001 but isn't, a small choke and a smoothing capacitor.
The resulting DC goes to the emitter of the D45H8 whose collector feeds the
input of the 78(M)09.  I haven't figured out how the base of the D45H8 is
controlled yet.


> Christian

More information about the cctalk mailing list