Need help finding resistance for 12V to get 300 mA.

Discussion in 'Electronic Basics' started by Ricky Spartacus, Dec 3, 2003.

1. Ricky SpartacusGuest

Assuming an ignition control module's trigger input can't exceed 300 mA,
what should resistor value to trigger this device with 12V. I've triggered
this device with 1.5V cell and works fine but not sure how to do the math
regarding the question above. I lost the website that has a current
calculator.
Ric

2. BrianGuest

Why not have your own program that can do that (plus a lot more), for less
than the price of a good technical book. See what I mean at
http://www.fncwired.com

R = E / I
P = E * I

R = resistor value
E = volts
I = current
P = Power rating needed for resistor

12 / .3 = 40 ohms
12 * .3 = 3.6 watts

Hope this helps,
Brian

4. Rich GriseGuest

Can you post the specs on that module? I can't imagine something
designed for a car that would be damaged with up to 12 volts at
the input. The spec _might_ be saying that the input won't load
your driver circuit more than 300 mA. I don't know if anybody's
ever seen a spec for an input where the drive to that input has
to be current-limited.

Thanks,
Rich