[R-390] Power supply fun

[Scott, Barry (Clyde B)]

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