detrjo Posted July 18, 2005 Report Share Posted July 18, 2005 HiOK, 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 1I 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 = 3.75Wrequiring 15ohm 3.75W resistor to power 2-way radio from motorcycle batteryopinion 2No need to subtract the voltage required by the radio and the resistor required as follows:12V / .5 = 24ohm... and power rating:12 * .5 = 6Wrequiring 24ohm 6W resistor to power 2-way radio from motorcycle battery--------Can some kind person please shed some light on this?Thanksdetrjo Quote Link to comment Share on other sites More sharing options...
audioguru Posted July 19, 2005 Report Share Posted July 19, 2005 Hi Detrjo,Welcome to our forum. ;DYou 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 Quote Link to comment Share on other sites More sharing options...
Guest SM2GXN Posted July 19, 2005 Report Share Posted July 19, 2005 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.Bjorn Quote Link to comment Share on other sites More sharing options...
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