[Milsurplus] ammo can power supply ideas

[WA5CAB at cs.com]

Jay,

The short answer is that what would happen if you series the transformer 
primary windings for 230 VAC but only apply 115 VAC depends entirely upon 
whether the regulator is series, shunt (highly unlikely) or switching.  

Ignoring the shunt type (no one in their right mind would use one for 
anything over the 40-50 mA that VR tubes handle), a series regulator takes a 
higher input voltage than that desired as the output and sets its resistance to 
drop the input voltage to the desired output voltage.  All of the voltage 
drop from input to output appears as heat.  If the input voltage drops below 
the desired output voltage, the regulator turns itself hard on, reducing its 
resistance to a minimum and the output voltage goes on down as the input 
goes down.  I'll may a not too wild guess that the designer will have chosen 
the transformer such that with rated input voltage (115 or 230, or later 120 
or 240), the moderate load DC output voltage of the rectifier and filter will 
have been around 35 VDC.  If you wire the primary for 230 but only apply 
115 (or 240 and only apply 120), the rectifier will output around 17.5 volts.  
The regulator, being turned hard on trying to get the output up to 24 VDC 
(or 28 or whatever you have it set to), will output around 16.5-17 volts.  So 
you will blow up the radio if it is set for 12-14 VDC.  Actually, depending 
upon how the reference circuit is designed it might be more technically 
correct to say that with insufficient input voltage, the regulator is trying to 
get the output up to the input, but the results are the same.  But anyway, 
wiring the input of a 24-28 VDC regulated power supply for 230/240 and only 
inputting 115/120 will not give you 12-14 VDC regulated output.  

If you change component values in the regulator circuit at the same time as 
you change the primary wiring to drop the unregulated DC in half, you could 
drop the regulated output in half.  But the output current rating would be 
only what it was at 24 out, not double that as you might assume.  

The way that you build a 115/230 VAC input, 12/24 VDC output power supply 
(like the PP-1104) is that you have two primaries for the input voltage 
choice, and two secondaries for the output voltage choice.  That gives you a 
12-14 VDC output rating of twice the current of the 24-28 VDC rating.  
TANSTAAFL.

If the regulator is a switching one, there are other things to consider but 
the transformer comments still apply.  You cannot double the output current 
rating at half the voltage simply by switching the primary.

In a message dated 6/2/2009 9:02:18 PM Central Daylight Time, 
jcoward5452 at aol.com writes: 
> Greatings Folks,
>   About 2 months ago I was at the Dump tossing some scrap metal into the 
> big bin,when a fellow recycler came over to me and offered me an ammo 
> can (7.62mm) and said I looked like I could use it.(Looked like?) 
> Well,of course I can use just about anything for free,so I accepted and 
> thanked him for his gift.Well, at about the same time my VRC-7 project 
> was coming together and I had a +24V DC Power/Mate Corp .supply to 
> operate the radio and I thought the ammo can would make a great 
> enclosure for the p.s. Well, time ran out and the West Coast MRCG event 
> was upon me and I did not get this idea into the hardware 
> stage.Interestingly enough there was a power supply in an ammo can at 
> the meet so my idea was not original but now validated.
>   Here's what I have in mind since I have a bit of junque box parts to 
> put to good use:Supply will be mounted on a plywood base on standoffs 
> with an operating panal above,all made up like a sandwich.I will use 
> velcro to stick the module to the ammo can floor.(I don't want to drill 
> any holes in the can).The panal will have the usual on/off switch,fuse 
> holder,pilot lamp, output connector and a voltmeter that a coworker 
> gave me yesturday( mil aviation type,full scale 30 volts,working).This 
> should look quite Mil-ish when done and thus hide itself in a display 
> environment by looking like it belongs there.
>   Now, here's the question to the group having never tried this but 
> maybe someone has:
>   I would like to make this a more versitle supply by having two output 
> voltages available: +12 V and +24 V.My thinking is that there are two 
> 115V AC primary windings so that in parralel one can operate on 115V AC 
> or in series one can utilize 230V AC and get the +24 V DC out.If I 
> connect in series and just apply 115V AC will the supply deliver +12V 
> DC?  Does the regulator pcb need to be modified to do this? Has anyone 
> tried this and what sort of trouble will I get myself into?I can accept 
> ratings changes and all that.Just don't want to fry the supply trying 
> to find out.
> Thanks,
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480