DEC H7822 power supply

[Wayne Sudol]

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...


-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Peter Coghlan via cctalk
Sent: Wednesday, May 11, 2022 4:02 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: DEC H7822 power supply

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.

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


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?

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.

Or maybe it could equally be said that a transformer can be used to convert pulsed DC to AC? 
Wayne: There is no negative component of Pulsed DC so no 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...)
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.  

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...) 

Regards,
Peter Coghlan.


Sent from my DEC Alphaserver 800

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

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 classiccmp.org> wrote:
>> 
>> The ripple on the rectified 5V and 12V supplies gets transformed into 
>> an isolated AC source for the 9V supply.
>