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Fuse Tap Circuit Problem (easy?)

  1. Jul 25, 2010 #1
    Hello. I am a programmer and haven't done anything with hardware since my physics class a few years ago. Needless to say I've forgotten a few things and after a few hours of searching google and troubleshooting I'm unable to fix my problem.

    I am using a fuse tap (it allows me to use a fuse as a power source in my car with a wire coming out the end) with a 5 amp fuse in it. I am using a 12 volt battery in my car. I hooked an LED up with resistors in a parallel circuit. The problem is that the LED does not light up when power is being supplied.

    I'm assuming I've done a calculation wrong but I am not for sure about this. It is possible that the fuse tap I bought was bad but i doubt it. I have verified that the LED does work with a simple AA battery test so the LED is not dead.

    Here are my calculations: R = Resistance, V1 = my battery (12v), V2 = LED FW Supply (3.2V), Current = LED Amps (20mA)
    R = (V1 - V2) / Current
    R = (12 - 3.2) / 20mA
    R = (8.8) / .02 Amps
    R = 440 Ohms

    Therefore I bought two 220 Ohm resistors to be able to account for this. They are 1/8 Watt. I did a calculation for this part as well but I was very confused about what the watt rating of a resistor even does.

    P (in Watts) = V / R
    P = 12 / 440
    P = .0272 Watts

    Being as this watt rating is very low I am assuming that a 1/8 watt resistor will be able to handle this?

    Could the problem be that the Fuse of 5 Amps was never in any of the calculations? Did I miss something obvious here? I have attached a link to how my circuit looks like in diagram form. The wiring is 12 Guage Stranded Hookup Wire (type = AWM) and the LED is a two prong LED (positive and negative), both of which are hooked up to my wire as shown. I currently do not have a ground. This is my first electrical wiring I've done so if something obvious is wrong go easy on me :)

    Thanks a ton if you have taken time to read all this, and all help is appreciated!

    http://www.screencast.com/t/NzE3N2ViMWUt" [Broken]
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 25, 2010 #2
    circuit is not clear. from where you are taking -ve terminal.
  4. Jul 25, 2010 #3
    Edited the circuit diagram to show more of what is going on. The battery is connected to the fusebox, which is connected to my fuse tap, which is connected to my fuse, which is connected to my circuit.

    http://www.screencast.com/t/ZGUyMmM3NGEt" [Broken]
    Last edited by a moderator: May 4, 2017
  5. Jul 25, 2010 #4
    I don't understand the drawing, which shows a connection to +12V but no connection to Earth.
    I'll attach a drawing more like what I would expect...

    Attached Files:

  6. Jul 25, 2010 #5
    You calculations on the voltage are fine, but you don't have to be quite so accurate with the resistor. A single 470 ohm would have done.

    Your power calculations are a bit off though.

    [tex]Power = \frac{{{V^2}}}{R} = {I^2}R[/tex]

    Substituting, and remembering there is only 8.8 volts across the resistor not 12;

    Power = 8.8*8.8/440 = 0.176 watts.

    This is slightly more than the combined rating of your resistors (1/8 + 1/8 = 1/4 watts) so your resistors will burn out in time. I suggest a half watt 470 ohm.

    As to why you don't get the LED to light I suggest you check you connections.
    Is the LED the wrong way round?
    Is one end properly connected to chassis?
    Are all the other connections propely made?
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