# Need electrical help

#### agentK

##### Senior member
Hi guys. I need the help of the resident electricians here.
I have an extra fender light lying around that i'd really like to install on my pc. It uses an ordinary automotive 12v - 5watt bulb. I know i can use LED's instead but: 1, it wont fit snug in the opening; 2, i don't care about heat as it is outside the pc; 3,it's free.

Here is my question:
The 12v brightness is a little too strong for me. I would like to attach a resistor to the positive rail to reduce it. I don't know how to compute this though. I know that V=IR. My problem is, the bulb doesn't state how much amperes it draws. Can anyone help me?

#### flyboy84

##### Golden Member
just try a bunch of resistors and see what level of brightness you like? :-D

#### GalvanizedYankee

##### Diamond Member
The bulb should list wattage on it's base. Divide watts by volts gives amps.
Divide volts by amps gives ohms. Adding the same resistence/ohms will drop the

...Galvanized

#### Aluvus

##### Platinum Member
Rated current is about 420 mA. A series resistor will do the trick, yes, but make sure you get something rated to handle a good amount of heat. An alternative would be a diode; forward-biased diodes provide a nice, consistent voltage drop and are easily available with current ratings in excess of an amp. Diodes aren't as cheap as the cheapest resistors, but power resistors aren't so cheap either.

The other concern of using a series resistor is that the resistance of the lamp will not be consistent. It will vary with temperature, and therefore the relationship of current and voltage will be non-linear. This makes any kind of predictive math from Ohm's Law just a bunch of guesswork.

The ideal approach would be to test on a variable-voltage supply until you find a supply voltage that works for you, and then slap in diodes until you hit something close.

#### agentK

##### Senior member
Thanks! Boy you guys are fast.
Follow up question if i may: i plan to use an SPDT relay on the light. I'll tap the trip switch to my HD LED so the thing blinks when reading. I figure a typical Bosch relay would do but i think they're overkill plus a bit more expensive. What replacement should i look at?

EDIT: Didn't read that in time Aluvus. Thanks for the tip on forward-biased diodes. I have a spare .5W Zener Diode. Would that do the trick?

#### Talcite

##### Senior member
a car fender light on your computer? post pics when you're done =p I'm intrigued.

#### agentK

##### Senior member
Originally posted by: Talcite
a car fender light on your computer? post pics when you're done =p I'm intrigued.

LOL. It's nothing fancy Talcite. They look like some of these: LINK
Just had spares lying around and some time to kill.