[Milsurplus] LV Supply...

[ersmar at comcast.net]

Rick:

    I'd ignore your second adn third alternatives.  Both waste too much power and your dropping resistors will be massive.

     My recommendations would be variac first (assuming you already own one) then electronic regulation.  For the latter, check out Astron's schematic for their 70A supply: http://www.wa8dbw.ifip.com/rs-70a.gif .  Essentially, their supply starts with a hefty 26 VDC from the HD windings of their transformer (voltage shown on the schematic for my RS-35 supply.)  They use the paralleled pass transistors (2n3771's) to regulate the output voltage, obtaining their feedback (output sample) voltage through the voltage divider R8/R5/R7 and LM723 regulator.  You can ignore the crowbar circuit (ca 2N3906, 1N5232B, etc.) if you'd like.

     But maybe this would cost almost as much as a new Astron.

73 de
Gene Smar  AD3F


 -------------- Original message ----------------------
From: Rick Brashear <rickbras at airmail.net>
> I would like to ask a little advice about my low voltage supply.  I have 
> a capable transformer with very good filtering capacitors and a high 
> current diode bridge.  My problem is, the transformer puts out about 16 
> vac and by the time I get it rectified and run through the filter 
> capacitors I am facing about 22.5 no load dc volts.  I am using it for a 
> few different projects and experiments one of which is to operate a 12 
> volt dynamotor.  However, the supply is so sturdy (capable of 80 plus 
> amps) that even with the load of the dynamotor the voltage is still too 
> high, about 16.5 vdc.  I'm not sure as to the best method to reduce the 
> voltage to prevent damage to any of the old 12 vdc stuff I want to use 
> it with.  My possibilities are, among others I'm sure:
> 
> *    Use a variac in the primary of the transformer to decrease the line 
> voltage.
> 
> *    Use high wattage zener diode regulation.
> 
> *    Use dropping resistors to attain the proper voltage (too much wattage)
> 
> *    Electronic regulation
> 
> Any and all help will be greatly appreciated. Thanks,
> Rick
> 
> 
> 
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