# how do i drop voltage of a power supply

Discussion in 'Power Electronics' started by daniel barter, Jun 6, 2016.

1. ### daniel barter

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Jun 6, 2016
Hi I am new to electronics so go easy on me. I am trying to drop to voltage of my power supply / battery charger from 29v 5a down to 25v 5a. Is there a easy way of doing this? can I just use a diode and resistor on output? any advice regards dan

2. ### Colin Mitchell

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Aug 31, 2014
Just use some 10 amp diodes.

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3. ### shrtrnd

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Jan 15, 2010
I'm going to stick my 2-cents worth in here.
I don't know your battery charger circuit, but some of them indicate a higher voltage when unloaded, than they do with a load. Just suggesting you verify that your power supply output with it's load, is still higher than you want, if you haven't done that already.

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4. ### daniel barter

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Jun 6, 2016
Thanks for your reply, how many 10 amp diodes would I need?

5. ### daniel barter

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Jun 6, 2016
I have already checked that and as the charger is designed for lion batteries it will not drop below 29v

6. ### hevans1944Hop - AC8NS

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Jun 21, 2012
The forward voltage drop across each diode will be approximately 0.6 V to 0.7 V at 5 A current. So to drop 29 V to 25 V you need to drop 4 V across the series-connected string of diodes. Do the math: 4 / 0.7 = 5.7 ==> six diodes, or maybe seven diodes if 4 / 0.6 = 6.6.

This is a "quick and dirty" way to drop power supply voltage. If you need a precision reduction, use a three-terminal voltage regulator with an external transistor to carry the 5 A. Many circuits available to search for and try out on the Internet.

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7. ### BobK

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Jan 5, 2010
And, no matter how you do it, you will need a heat sink since there is 20W to dissapate.

Bob

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