DEC H7822 power supply

Peter Coghlan cctalk at
Thu May 12 14:14:13 CDT 2022

Wayne S wrote:
>> On May 12, 2022, at 09:56, Peter Coghlan via cctalk <cctalk at> wrote:
>> 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.
> I’m really trying to understand what you’re getting at here.
> In practical terms, The assumption that there is a sum of a DC and AC
> component is incorrect.

Why? What evidence do you have that the assumption is incorrect?

If I am trying to calculate the height a sea harbour wall needs to be, can I
not consider the height of the tide and the height of the waves separately
and add them together, just to make it easier to work it out even though I
know that it would be physically impossible to distinguish the water that
makes up the tide from the water that makes up the waves?

(It's not even that bad in the case of electric current because instruments
exist which can measure the DC component of a current and the AC component
of a current separately or together.)

> There is no DC component in an AC wave. A wave is either AC meaning
> reversing polarity or it is DC meaning there is no reversal of polarity.

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

How about we take a 3V doorbell transformer and make a circuit consisting
of the secondary of the transformer, a 6V battery and a 9V light bulb all
connected in series?  How would the current in this circuit be described?
A varying current? An AC current? A DC current?

How about we use a 6V doorbell transformer and a 3V battery instead?  Would
that change how the current is described?

I would say that in both cases, the current flowing has an AC component and
it also has a DC component.

> And a rectifier does not sum anything.

I'm not saying the rectifier sums anything.  What I am saying is that the
current that goes through the rectifier can be considered as the sum of
two component currents.  It's just a way of looking at it, like considering
the tide and the waves separately in the harbour wall calculation.

> Are you maybe using “Rectifier” as the whole circuit?  A Rectifier
> can be a single component, like a diode used to pass one half of an
> alternating current or it can be a few components like 4 diodes connected
> as a bridge to pass both halfs.

Strictly speaking, a circuit involves a closed path for a current to flow in.
However, "circuit" also gets used to describe a substantial group of components
that needs to be connected to something else to form a fully closed path.  I
don't think a rectifier would be enough to be considered a whole circuit. 

> This may help to visualize.
> Do an experiment. If you have an Oscope take a small battery maybe 9 volt and
> wire the negative lead to the neg scope input. Touch the positive lead to the
> scope positive and watch the deflection. You should see a quick trace  rise
> and the trace should remain above the zero point.
> That is what pure DC looks like. No ripple at all. Power Supplies try to make
> that from your wall power input. 
> Next, untouch the positive and watch the trace fall. At no point does it fall
> below the zero line.
> NoW touch and untouch the positive as fast as you can and watch the trace.
> That is pulsating DC although with irregular frequency. That would be the
> output from the rectifier component.
> Reverse the leads and do it again.
> This will show the equivalent of a negative polarity.
> If you have a variac, set it to 9 volts or so, plug in a light or something to
> provide a load and using your oscilloscope probes touch across the light and
> you should see a sine wave with regular frequency.  Notice the negative going
> half. That is AC.
> It’s important to use a variac for safety ( or an isolation transformer very
> carefully) as wall voltages can hurt you.

A variac does not provide isolation.  If it is not wired correctly, it is
possible to end up scooping 9V from the live end of the mains supply instead
of from the neutral end.  If there is a bad connection or a break in the
winding near or at the neutral end, it is possible to end up with the whole
mains supply at the output.  I don't want to connect the earth lead of my
scope probe to the neutral output of a variac either.  If I were doing this,
I would prefer to use a transformer designed to provide an isolated fixed low
voltage output.

> This is a very good way to visually explain AC and DC.

It can be used to visualise AC and DC voltages but it is not quite as good
for currents.

How do I get myself into these quagmires?

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.

I would be delighted if someone else who is familiar with this power supply
or power supplies like it could take a look at my findings and say whether
I figured it out correctly or not.

If anybody else is working on a fault in this power supply or a similar power
supply, I would be happy to answer any questions they have about how I think
it works or how to disassemble it or what failed in mine or anything else
about it that I figured out along the way.

Other than that, I suspect many of the other mailing list members are starting
to find the going a bit heavy by now.

Peter Coghlan.

>> 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
>> component.
>> 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.)
>> Regards,
>> Peter Coghlan.

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