DEC H7822 power supply

[Wayne S]

> On May 12, 2022, at 09:56, Peter Coghlan via cctalk <cctalk at classiccmp.org> 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.
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.  And a rectifier does not sum anything.
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. 

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.

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


> 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.