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Novice help required - reducing voltage


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OK, so i'm a complete novice at this electronics stuff and need a little help with something here.

Here's what i want to do:
I have a 2-way radio that i wish to power off a motorcycle battery [for use whilst riding so i can talk to other bikers], instead of using the Nicd battery that came with the radio.
But - i have conflicting info on how to do this.

The facts:
Motorcycle battery = 12V [or as close as you get with a vehicle battery]
2-way radio requires max 4.5V 500mA [Nicd battery supplied = 3.6V]

The solution:
Run wire off 12V battery terminals to 2-way radio terminals and reduce voltage using resistor.

The problem:
OK, so this is the part i have a problem with. I am given conflicting info from two people on how to work this out.

opinion 1
I must first subtract the voltage required by the radio from the source, and use the remainder to find the resistor value required - ie:
12V - 4.5 = 7.5 / .5 = 15ohm

... and power rating:
7.5 * .5 = 3.75W

requiring 15ohm 3.75W resistor to power 2-way radio from motorcycle battery

opinion 2
No need to subtract the voltage required by the radio and the resistor required as follows:
12V / .5 = 24ohm

... and power rating:
12 * .5 = 6W

requiring 24ohm 6W resistor to power 2-way radio from motorcycle battery


Can some kind person please shed some light on this?


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Hi Detrjo,
Welcome to our forum. ;D
You can't use a resistor to reduce voltage for a product unless the product's current draw is always the same. Surely your 2-way radio draws a high current when it transmits or plays voices loudly, and a low current the rest of the time. Therefore if you used a resistor to drop the voltage then the voltage will vary all over the place.

You can't simply use a voltage regulator IC because the 2-way radio has a rechargeable battery. A charger circuit for the battery will not be able to provide enough current safely to charge the battery and power the radio so you have a problem with your idea.

A safe way to power the 2-way radio is if you remove the rechargeable battery and use an LM317 adjustable voltage regulator IC to drop the motorcycle's 12V down to 3.6V. ;D

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Guest SM2GXN

Hi Detrjo!

Why not use an 3-pin voltage regulator called 7805 and at the outgoing pin a diode in series to reduce the output voltage to about 4,5 volts, less components  ;D.


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