What size resistor do I need?

Hi, I have a Wilson Sleek antenna booster that runs on car battery power, 12VDC and draws 2.5A.  I would like to power it away from the vehicle using a 19.2VDC Craftsman drill battery that measures 21.7VDC at the terminals, fully charged.  What size resistor installed in series with the Sleek will allow it to safely operate from the Craftsman battery?

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How many amps does it draw ? 2.5A

How many volts do you want to supply it with ? 19.2 V

What supply does it need ? 12.

Resistance = (19.2 -12)/2.5 Ohms. ~~ 3Ohms.

Power rating = I^2 x R = 2.5^2 x 3 = 18.75 - call it 20W.
plentyblackcoffee (author)  steveastrouk5 years ago
OK, thanks, I understand a bit better.

So I bought a 3ohm/20W resistor.

Reading the battery alone, I get 21VDC, but placing the resistor in the loop I don't see any change, it still reads 21VDC. I expected a drop in voltage.

Any thoughts? I just don't want to fry the antenna booster with too much voltage.
The BATTERY voltage shouldn't change much, measure the voltage on your LOAD.

19.5 - 12 = 7.5 Volts

7.5 / 2.5 = 3.o Ohms

7.5 × 2.5 = 18.75 ⇒ 20 Watts
plentyblackcoffee (author) 6 years ago
Hi, thanks for the answers. Any chance you would show the method used in arriving atbthe answer?

The tech at Wilson said the Sleek uses 20watts. I was not sure if that played a role in the problem, but now that I see that resistors are rated to include power, maybe you could factor it in. [just to be complete, he said it requires 2.5amps, 20 watts, from a 12VDC source, normally a vehicle cigarette lighter. That's all he said, but if we need more info,, let me know.]