DEC H7822 power supply

Peter Coghlan cctalk at
Wed May 11 18:02:20 CDT 2022

It's hard to come up with suitable terms for this sort of stuff that
convey the meaning of what is going on.  I struggled with it for a while
and that was what I ended up with.

With the strict meaning of DC and AC being direct current and alternating
current and given that we are often talking about voltages rather than
currents, meanings are already getting stretched.  Besides, what's direct
about it?  Maybe it would be more accurate to use terms like steady voltage
and alternating voltage? Alternating doesn't seem like that good a term
either.  To me it suggests some sort of square wave switching very rapidly
between one extreme and another, not a nice lazy sine wave which is the
normally accepted meaning.

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
this mixture is fed into a transformer primary as in this case, the DC
component does not pass through from the primary to the secondary but the
AC component does.  Transformers only work on AC, right?

Or maybe it could equally be said that a transformer can be used to convert
pulsed DC to AC?

(The transformer has to be kinda special to avoid the core getting
saturated by the large DC current (What's a "direct current current"?)
flowing through the primaries...)

Anyway, the good news is that I think I have found the source of the
problem.  One of the capacitors used to filter the (DC?, pulsed DC?,
rippled DC?, biased AC?) supply to the 9V regulator is marked 330uF/25V.
It reads 6uF on the capacitance range on my multimeter.  This can't be
helping the cause.  It's not showing any signs of leakage but it's got
a brown sleeve and the same logo as the nasty, leaky SXF capacitors but
it is marked KME.  (I said there was only one capacitor in the filter
in a previous posting.  Originally I managed to spot a little 10uF/100V
capacitor but somehow failed to notice the chubbier 330uF/25V capacitor
completing a PI filter with a small choke...)

Peter Coghlan.

Sent from my DEC Alphaserver 800

Wayne S wrote:
> “ The ripple on the rectified 5V and 12V supplies gets transformed into an
> isolated AC source for the 9V supply.  ”
> Shouldn’t that be “pulsed DC” instead of “AC” as rectification
> changes AC to DC ?
> Sent from my iPhone
>> On May 11, 2022, at 01:36, Peter Coghlan via cctalk <cctalk at> wrote:
>> The ripple on the rectified 5V and 12V supplies gets transformed into an
>> isolated AC source for the 9V supply.  

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