DEC H7822 power supply

Peter Coghlan cctalk at
Thu May 12 17:32:39 CDT 2022

>> 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?
> Using this analogy if you measured 10 waves at 10 feet high you would need
> a wall 100 feet high, correct? Or 20 waves at 7 feet it would need to be
> 140 feet. 
> Not valid at all.

The idea is to measure the maximum height of the waves on a windy day by
examining how far up they make splash marks on a vertical pole stuck in the
seabed for the purpose.  Then consult a tide table to see how high the tide
was at the time the wave measurement was made.  Subtracting the latter figure
from the former figure gives the height of the waves above the tide.  (This
is the AC component.  The tide is the DC component.)  Adding the height of
the highest expected tide to the figure calculated for the maximum height of
the waves would give a figure for the minimum height needed for the harbour

>> (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.)
> I have no idea what you are talking about. What instruments are you referring
> to? I’d like to read about them.

A suitable moving coil ammeter will register the average value of a current
flowing through it and will indicate which direction it is flowing in.  This
is the DC component of the current.  A suitable moving iron ammeter will
(crudely) register the RMS value of the AC current flowing through it.  I'm
not certain but as far as I recall, it will also register the actual value
of DC current.  There are probably lots of fancy computerised oscilloscope
like instruments that can be used to measure all sorts of oddball current
waveforms given suitable current probes.

>> 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?
> The current would be described by ohm’s law. Incidentally, if the primary
> of a xformer was actually connected it would probably blow up the battery.

Maybe I should have stated that I was assuming that power is applied to the
primary for the test?  Perhaps I should have also stated that I am assuming
that the light bulb is selected such that the current it draws at it's rated
voltage will not exceed the capability of either the transformer or the
battery?  I don't expect anything to blow up assuming we are understanding
each other correctly which may not be a valid assumption.

> You would describe it as Open. If not, its just a 6 volt DC circuit.

I don't understand.

>> How about we use a 6V doorbell transformer and a 3V battery instead?  Would
>> that change how the current is described?
> No it wouldn’t.  
>> I would say that in both cases, the current flowing has an AC component and
>> it also has a DC component.
> As stated and not rectified it’s not a valid circuit.
> In ps circuits there is a fuse on the secondary side of the circuit usually
> after the rectifier-filter circuit to prevent any rectifier shorts from
> damaging other components.

The transformer secondary, battery and light bulb are all connected in series
ie in a circle, end to end.  The battery is not going to be bothered by the
AC current flowing through it and the presence of the light bulb will limit
the current to a safe value.

If the battery and the transformer secondary and the light bulb were connected
in parallel ie the two terminals of the battery were connected directly to the
two terminals of the transformer secondary and the two terminals of the light
bulb, that would be a different matter.  Either the transformer or the battery
or both could be damaged in this case.  This is not what I was suggesting.

> Well, the previous example seemed to imply that the output from a rectifier
> was pure dc.

Pretty much everything I have said is to try to illustrate that what flows
through the rectifier has a DC component and an AC component.

> Its not, it’s pulsed DC

Pulsed DC sounds like something that has a DC component and an AC component.

> so i thought you might be referring
> to the rectifier and associated filter and possibly regulator components as
> the whole rectifier”
> Sometimes in talking about electronics the specific function of a portion
> of connected components is cited. IE filter circuit,  Detector circuit,
> rectifier circuit. It’s just shorthand speak.

Sorry, I didn't follow.

I'm going to bow out at this point because I don't there is anything
further I can add to this and I am clearly failing to make myself

Peter Coghlan.

More information about the cctalk mailing list