G8MNY  > TECH     30.05.25 09:00z 65 Lines 2595 Bytes #89 (0) @ WW
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Subj: Regulating a Plug Top PS
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From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                              (Corrections Nov 19)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Small odd PS are availble at rallies etc. these are either lighweight Switch
Mode regulated types, unsuitable if you want low QRM, or the heavier
transformer type with unregulated output (e.g. says 12V but 19V off load). If
there is room, here is what is needed to regulate one of these, & if of course
you can open box!
                                    ÚÄ´<ÃÄ¿D1
                        +19V        ³     ³
                 Br ÚÄÄÄÄÄÄÄÂÄÄÄÂÄÄÄÁ¿NPNÚÁÄÄÄÄÂÄÄÄÄÄÂÄÄ>+13V
L<ÄÄÂÄÄÄthÄÄ¿ || __/+\_     ³   1K    \ /e     ³     ³
   m22  fuse )||( ~\-/~³   +³   ³    ÄÄÂÄÄ     ³1m  _³_
   neon      )||(   ³  ³   ===  ÀÄÄÄÄÄÄ´      ===   /_\D2
N<ÄÄÁÄÄÄÄÄÄÄÙ || ÀÄÄ)ÄÄÙ Cap³4m7      _³_    C2³16v  ³
                    ³       ³25V      /_\'14V  ³     ³
                    ÀÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄ>-ve

Also if there is room & it safe to do so, I often add a mains neon & series
high value mains resistor (e.g. 2x 100k) to warn the unit is on. This MUST be
done safely on the mains side of the box with the neon light visable through a
small hole, not with the neon poking out!

ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³  ___±±±___~~~~~~~~|³
³N³   ±±±   ³    C2 |³
³E³   ±±±   ³      N|³
³O³   ±±±   ³Br    P|====DC
³N³   ±±±   ³      N|³
³ ³___±±±___³  Cap  |³
³     ±±±  _________|³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

To keep the regulator circuit simple I used a high gain darlington TAB
transistor mounted on a folded ally plate on the secondary side of the box. The
NPN emitter followers the zener 14V voltage to give about 13V regulated.

I could just as easily used a 12V low drop out regulator (uplifted to 13.8V)
instead & gained current limiting!

The 2 diodes (1N4001) are protection features, D1 ensures the output can't be
more than +0.7V higher than the input, & D2 stops reverse polarity harm
(external fuse?).
 
HEATING
As with all small PS heat can be a problem, derate the PSU if needed, or
improve ventalation with bigger holes etc.


Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP



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