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Old 11th July 2007, 12:06 AM   #1
bst is offline bst  United States
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Default Diodes on power supply output for voltage drop?

I recently got a deal on several cased power supplies. Each is regulated at 13.8 volts and 4 amps. Can I simply put diodes on the supply and return terminals to drop the voltage? If I have two 5A diodes, each with a voltage drop of 0.7 volts, it would seem that I'd drop the voltage from 13.8 to 12.6; right where I want it for a regulated DC heater supply.

Is it this simple, or am I creating a problem for myself? Is a dropping resistor a better option, or should I tear these supplies apart and change the resistor which sets the regulator output?

Thanks for your help.
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Old 11th July 2007, 12:12 AM   #2
Colt45 is offline Colt45  Serbia
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It would work

keep in mind there will be pretty big surge when you first light the filaments, might pop the diodes.
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Old 11th July 2007, 12:17 AM   #3
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The answer is : YES

Diodes is a very good way to drop CC voltage .

Put the series diodes only in the positive rail , do not put them in the negative rail .

Regarding to eliminate any noise , put a parallel capacitor ( may
be ceramic 0.001 uf x 50 V ) , with each diode .

Regards ,

Carlos
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Old 11th July 2007, 12:29 AM   #4
bst is offline bst  United States
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Thanks for your replies! I'll have to go back to the surplus store and buy several more: at $3 apiece, it looks like they'll be a wonderful bargain.
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Old 11th July 2007, 01:41 AM   #5
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Quote:
Originally posted by Colt45
It would work

keep in mind there will be pretty big surge when you first light the filaments, might pop the diodes.
I doubt it. The surge rating on those diodes is quite high, even a IN400* has a 30 amp surge rating.
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Old 12th July 2007, 02:25 PM   #6
r221b is offline r221b  United States
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One thing to remember is that while using diodes to drop the voltage, when the voltage is DC (which it is in your case), the diodes will get very hot if you pass any substantial current through them. Diodes of this type are normally used to rectify AC current and are "on" only half of the time. That allows them to cool down between each conduction cycle. If you're going to run a few amps through your diodes, you might have to add some heat sinking to them. Or at least make sure they have plenty of ventillation around them. Good luck.

r221b
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Old 12th July 2007, 02:39 PM   #7
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While you're at the surplus store, pick up a few full-wave bridge rectifiers with 10-25A rating. Easy to heatsink with a single screw for mounting, and already insulated. And two series diodes or one by choosing the right terminals.

Even easier - if you look inside, you MIGHT find an adjustment pot...
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Old 12th July 2007, 03:54 PM   #8
alejo is offline alejo  Argentina
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Bst, I imagine the power supplies you buy is for halogen lamps. If is this case think the supplies dont use transformer, there use solid state its not safe. not isolate for the main AC in your house.
sorry for my bad English.
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