DEC H7822 power supply

Peter Coghlan cctalk at
Thu May 12 10:37:01 CDT 2022

Toby Thain via cctalk wrote:
> On 2022-05-11 7:02 p.m., Peter Coghlan via cctalk wrote:
>> Given the normal usage that has evolved for the terms DC and AC rather than
>> their dictionary definitions, I would suggest that the current that gets
>> passed by a rectifier has both a DC component and an AC component.  When
> It does not, due to unidirectionality.

Consider the current through the rectifier as the sum of a "DC" current plus
an "AC" current.  The "DC" current has a steady positive value and the "AC"
current varies above and below zero with a magnitude less than or equal to
that of the "DC" current.

When the two are summed, the result is a varying current which does not go
below zero.  This is what I mean by it having a DC component and an AC

This sort of analysis often used in electronic engineering to break down
more complex entities into simpler ones which can be analysed separately
with greater ease.  Didn't somebody have a theorem or an axiom or something
that says this is a valid way to do it? I forget who.  It was a long time
ago.  Kirchoff maybe? No, it wasn't him, maybe he was the one that said
the sum of currents into and out of nodes is zero and stuff like that...
It is a particularly suitable strategy for the case in hand because a
transformer will not pass "DC" from primary to secondary so the effect of
this component can be ignored (except that it could cause the transformer
core to saturate which must be allowed for.  This is only relevant to the
designer of the power supply but if I don't mention it, someone will
surely tell me that I should have.)

(I am starting to regret making the effort to accurately describe this
unusual and confusing (to me anyway) circuit for the benefit of others who
might find themselves struggling with this power supply as I was and might
find some hints on how it operates to be helpful.)

Peter Coghlan.

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