[R-390] Power supply fun

Roger L Ruszkowski [email protected]
Wed, 13 Mar 2002 09:48:11 -0800


Scott,

It's this learning part of the field that I do like so much.
This is the part that's fun, You feel so good when you get it  right.
A lot like how I feel when I stop banging my head against the wall.

Roger.



                                                                                                                       
                    "Scott, Barry                                                                                      
                    (Clyde B)"               To:     "R390 (E-mail)" <[email protected]>                           
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                    >                        Subject:     [R-390] Power supply fun                                     
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                    03/13/2002 09:00                                                                                   
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Some of you may remember my post a while ago about filtering on a simple
12V power supply.  It is a gounded centertap full wave configuration and I
had a 20mfd cap for a filter.  When I applied the load, the voltage dropped
(naturally as I realize now) and it was suggested 20mfd is insufficient.

Yesterday, I picked up a 1000mfd cap.  Before I replaced the 20mfd cap, I
did a little snooping with the scope.

Under no load, I was getting a nice flat 12VDC.  Under load, the deep
"valleys" would appear (as a result of the cap discharging through the
load).  This accounted for the 9VDC (approx.) reading I was seeing with the
DVM.

Replaced the 20mfd cap with the 1000mfd cap.

Under no load, still the nice flat 12VDC.  Under load, however, I now get a
fairly flat line with just a hint of a ripple (maybe 1/2 volt).

It's funny, but you can read about these things, but they don't sink in
until you actually do the "labwork".  Next time, I'll be a whole lot less
likely to make the same dumb mistake.  I think what I was basing my
original value on is the fact that the same rectifier configuration in the
R390A only uses the 30mfd and 45mfd caps.  What I wasn't taking into
consideration, though, is the chokes and the overall filtering design.  It
makes quite a difference!

I realize this is pretty "ho-hum" for some (most?) on the list, but, like I
said, sometimes you have to actually do the experiment before it sinks in.
This was fun...

73,

Barry(III) - N4BUQ


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