How to determine resistor needed to drop 30v to 24v?

Question

How do you determine the size resistor needed to drop a 30v power source to 24v. The amperage is unknown.

Answer 1

A resistor drops voltage in direct proportion to the current through it, so with an unknown amperage, you cannot determine the proper resistor.  However...

If you want an easy way to drop the 30v to 24v, you can do it with diodes instead.

If the voltage you are talking about is DC, just get some high current silicon rectifiers, and put them in series (all forwad biased).  Each diode will drop 0.6 to 0.7 Volts, so if you were to use 9 of them in series, you will get very close to 24v for a whole range of amperage.  

You can do this with AC voltage too, but you have to make 2 sets of these series diodes and hook them up in anti-parallel (the cathode of one string to the anode of the other and vice versa).  This will give you the desired voltage drop for each half of the waveform.

You just have to make sure that the diodes can handle the maximum current you will be drawing.

Answer 2

You need to know the current. Use ohm's law... calculate resistance from voltage (30-24)/current.

Answer 3

You need to use a potential divider which is two resistors otherwise your voltage will vary greatly as the load changes.

Using resistors anyway is only suitable for small loads. You will still need to consider the load when you size them.

Whatever you do, you need to know the current requirement.

Answer 4

i assume that you have a 30v battery and an appliance that is rated for 24 volts. unless you have a current rating for that appliance you would not know what the resistance of the appliance is (R=V/I). if you do however know the resistance of your appliance, then you simply use a resistor that is exactly 1/5 the resistance of your appliance in series with your item to drop1/5 or 6 volts of your battery across it. given your information, thats the best i can do. i'm just trolling for best answer. give it up.

Answer 5

You can't use a resistor to drop voltage when the current is unknown, or can change. It doesn't work.

You need more elaborate methods, like a voltage regulator, or a DC-DC converter. But while these will work over a range of currents, you need to know the maximum current needed before you can design or buy these.

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