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I am a complete noob on electronics, please answer in basic terms, what I know about resistors is in this question nothing more, unable to make any reliable calculations therefor I require a specific answer I can apply :)

I have an AC device with output of 20V and 4.5A from 220V input all consistant. I need to reduce the Volts only and not the A for two applications 1. Reduce to 15V and maintain 4.5A (required to run very sensitive 15V electric miniture motor) and 2. Reduce to 5V and maintain 4.5A (required to run single LED with 1. above). Which resistors must I install for example 6000k 200ohm carbon or metal film? I don't own a volt or multimeter.

Many thanks

Krunal Desai
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Gerhard van der Merwe
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1 Answers1

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Which resistors must I install for example 6000k 200ohm carbon or metal film?

You don't use resistors for this - you use a switching voltage regulator, probably a buck voltage regulator. It will have round about 90% efficiency and will therefore only burn a little energy in doing what you want.

If you tried to use a resistor to drop from 20 volts to 15 volts at 4.5 amps, the power dissipated would be 5 volts x 4.5 amps = 22.5 watts and you would have an unregulated output.

If you tried to use a resistor to drop from 20 volts to 5 volts at 4.5 amps, the power dissipated would be 15 volts x 4.5 amps = 67.5 watts.

Use a buck voltage regulator. You can buy them from ebay or other internet vendors. However, if you want to build one you are looking at something like this: -

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The above circuit can be made to output 15 volts or 5 volts by correctly selecting the resistors going to the VFB pin

Andy aka
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