DEC H7822 power supply

Peter Coghlan cctalk at
Thu May 12 09:00:20 CDT 2022

> Wayne: AC DC  terminology has been well documented since the 1800's. 
> Don't try to reinvent the terms or no one will know what you are talking about.
> I answered a few things below...

They didn't have switch mode power supplies in the 1800s.  Terms like AC and
DC as understood back then can be used to describe what goes in to and comes
out of this power supply but they are not ideal for describing what's going
on in the middle of it.

It seems like an unusual sort of design to me.  I haven't come across the
idea of extracting ripple from one output and using it to create another
output before.  It took me a long time to figure out what was going on.
Maybe it is a common thing to do and I haven't seen it because of my
lack of experience with this sort of thing?  Could anyone who is more
familiar with switch mode power supplies comment?  Anyway, I was just
trying to describe what I've seen. 

> Wayne: Alternating Current  is a continuously varying sine wave. The polarity
> does reverse over time.  Perhaps Alternating Current isn't a good term and
> should be Alternating voltage instead but AC is the terminology and it
> describes the form of the wave. Is basically says that there is a zero to
> positive component and a zero to negative component of the voltage as 
> measured over time in 2 half waves

Ok. I doubt we will find many true sine waves inside this power supply though.

> Wayne:  There is no AC component.  The output from a rectifier is pulsed DC,
> either half wave or full wave. An additional circuit after the rectifier
> provides the smoothing to provide nearly pure DC. Nearly pure meaning the
> voltage remains constant and does not drop much when measured over time.
> The most pure DC source is a battery.  Transformers work on AC or Pulsed DC.

It depends on your definition of AC component.  How about we say the output
of a rectifier consists of a steady component plus a varying component?
I guess you could call it pulsed DC if you want.  Whatever it is, when you
put it through a transformer, what you get out the other side will have a
varying component and no steady component.  The output of the transformer
is probably vaguely sinusoidal because the performance of the core is likely
to fall off as frequency rises.  It could be described as AC.  It could not
be described as pulsed DC.  

> Wayne: There is no negative component of Pulsed DC so no AC.

As before, how about we call it a steady component plus a varying component?

> Wayne.  Lotta good text on this out there. Basically a variable current
> induced through a wire generates a magnetic field. Any wire placed next
> to it, has that field induced in it as well.  The catch is that the field
> has to pulse or alternate to keep generating the field and being induced
> into the other wire. It's the movement IE the up and down motion of the 
> voltage that causes the field to be induced in the other wire.

In a transformer, it is the variation of the current in one wire which
creates a varying magnetic field which induces a current in the other wire.

> Wayne:  I just got an Dec AlphaStation 200. Look like its running NT though. 

It's probably ok but check the power supply for leaky capacitors!
Have a look around for batteries that might leak while it was/is in
storage too!

> >
> > “ 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 ?

No, it shouldn't.  What goes into the transformer could be described as
pulsed DC.  What comes out of the transformer cannot be described as pulsed DC.
It has a varying component and no steady component, therefore I described it
as AC.

Peter Coghlan.

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