Fuse keeps tripping.

[ChopperChris]

I have a motorcycle that I'm trying to put 6 LEDs on. The battery is a 9V wet cell and I'm trying to route the LEDs off the headlight circuit which uses a 10A fuse. I'm trying to use 20mA LEDs. Can anyone please tell me what I need to do to make this work? I was hoping to just route it off the positive end of the headlight bulb and connect the LEDs in series all the way to the negative end of the bulb. I figured there would be enough power to make both work. Thank you.

 

[jimthompson]

I assume that the LED's in question are Red. Typical Red LED's have a two volt drop at 20 ma so 6 of them would have a drop of 12 volts and would not work with 9 volts. Since the actual voltage drop is a function of temperature a resistor in series with three LED's will give the best LED life. The resistor value would be (9-6)/.02 = 150 ohms. Over current will cause rapid failure of the LED's.

 

[ChopperChris]

I'm sorry but I made a mistake. The motorcycle battery is 12V.

 

[electricuwe]

Since Chopperchris uses the LEDs on the headlight I assume he does not use red ones. But regardless of colour no LED can be used without current limiting by a resistor or an current control by some active circuit.

 

[Chuckles159]

Perhaps a more detailed explanation is needed for the person who posted this. A 20mA does not draw 20mA. The 20mA rating means that the LED can safely handle up to 20mA of current. To do this, a current limiting resistor must be wired in series with the LED. Mr. Thompson in his posting gave the correct formula for determining this resistance.
More generically, the formula is (Source Voltage - LED DROP)/(Desired LED Current).
The source is nominally 12V but really 13.6V. The voltage drop of a single LED is 1.6V. Therefore, if each led were wired with its own current limit resistor, the formula works out to (13.6 - 1.6)/(.02) ohms or about 600 ohms. 680 ohms is the nearest standard value higher (never go lower).However, to reduce current draw, I suggest wiring the LEDS into two groups of 3 in series. This would then require only 2 resistors of (13.6 - 4.8)/(.02) ohms or 440 ohms. 470 ohms is the nearest higher value. A 1/2W would be fine. I do not suggest putting all 6 in series and using a single resistor as the brightness would fluctuate too much with varying RPM of the bike.

 

[ChopperChris]

Thanks everyone for your help. Just for the record they are red LEDs. I was going to have them coming off the headlight wire and i was going to put them under the gas tank to give the effect that my engine is glowing. I was hoping just to tap into the headlight wire, have 6 LEDs in series, then reconnect it to the ground coming out of the headlight so the LED circuit and headlight were in parallel to each other. But the 10A fuse keeps blowing or I blow a LED.

 

[Chuckles159]

Yes, look at the previous posts. You need to have a resistor is series with the LEDs. If you connect them without a current limiting resistor, they will blow or the fuse will blow. Use a 470 ohm 1/2W resistor. Radio Shack will have these.