Need help figureing out resistor Value Needed

Jody_Green · Post by **Jody_Green** » Monday 1st Dec 2008, 6:30

Hello i hope this is the right place but what i am trying to figure out is i have a 15 volt 7 amp power supply and i am wanting to run a Cobra 29 off of it. what i am trying to figure out is in order to get my voltage down to 13.8 what size resistor would i need to use on the out put of this PS to get 13.8V without sacrificing much amperage... Thanks

timrim · Post by **timrim** » Monday 1st Dec 2008, 10:25

You would need a monster resistor because the current draw of the radio on transmit may be approx 2 amps. You can run the radio off 15 volts, but if you need to get it down, a better way would be to use 3 diodes in series with the positive lead, the diodes will need a rating of at least double of your maximum expected current draw, so three 4 amp or larger diodes will bring the output down. You can also open up the power supply and see if there is a variable inside.

Jody_Green · Post by **Jody_Green** » Monday 1st Dec 2008, 10:33

timrim wrote:You would need a monster resistor because the current draw of the radio on transmit may be approx 2 amps. You can run the radio off 15 volts, but if you need to get it down, a better way would be to use 3 diodes in series with the positive lead, the diodes will need a rating of at least double of your maximum expected current draw, so three 4 amp or larger diodes will bring the output down. You can also open up the power supply and see if there is a variable inside.

yea i opened it and there is no variable..... someone tole me these would work. http://www.radioshack.com/product/index ... b=features

just now i wonder how many amps i could draw through them

timrim · Post by **timrim** » Monday 1st Dec 2008, 10:38

3 of those will work ok, they may get warm on transmit, but should not be a problem. I suggested double the max expected current just to have a hefty margin for error.

They are rated at 3 amps, so you can pull up to that through them without frying them.

James Unit 198 · Post by **James Unit 198** » Tuesday 2nd Dec 2008, 21:50

This is a case of danger.

The diodes will need to be 8 amps or more and you will need a constant load or it will not work.
You will be best off getting the right power supply.

James.

timrim · Post by **timrim** » Tuesday 2nd Dec 2008, 22:08

Explain please?

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