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· Recent Transplant
2,681 Posts
Discussion Starter · #1 ·
I have a 5V blue LED that I'm wiring into my gauge panel for the high beam indicator. I managed to get red, green, and yellow 12V led's with built in resistors, but the blue is just by itself.

Tell me if I'm calculating this right:

5V forward voltage
30mA draw

12V power supply

so the forward voltage drop is 5V, 12V source minus 5V forward drop = 7V drop across the resistor.

7V / .030A = 233 ohms, so I need a 240 ohm resistor (I'm assuming they come in intervals of 10?)

Ugh it's been a while since what little physics I've had, someone double check this for me so I don't blow a fuse and/or LED.

· Registered
526 Posts
Im good with electronics but you did a math problem and lost me....

· Frosty Rider
387 Posts
A 100 ohm resistor will work just fine, the extra current wont hurt anything.

your math is right and if you really want to use 233 ohms, you can buy multiple resistors adding up to the desired value and wire them in series (end to end) if you can't find one that is exactly right.

· Registered
2 Posts
Your math is right and you can get a quarter watt resistor in 240. Although the LED would probably be fine with just a 100 ohm resistor, I wouldn't recommend it. The increased current flow through the LED would eventually cause it to fail sooner. Ideally, you should just go to radioshack and get a 5V regulator. that way you can connect whatever you want and not worry about loading it right. it's much more efficient to use the regulator instead of wasting all of that energy as heat through the resistor.

here's a part you can use:

here's an example data sheet:

The regulator package should have a simple circuit diagram on the back which will show how to connect it. Page 6 on the datasheet link also shows example circuits. The coupling capacitors would help, but aren't really needed.

· Member
90 Posts
Regulator!?! No, need! I the value of resistor you use is based on amount of forwarding bias voltage needed to turn on the LED and amount of current that it will draw.

Here is the formula for finding the resistor value:


For your LED: R=(12-5)/.03
So, R=233ohms. Just like you initially calculated! Good job. The closest available resistor value should work.

Just to add to the topic. If you want to use more than one resistor ( for like a brake light), use the following formula:

R=(12- (number of leds X Vled))/Iled

· HipsterKillerGarage
7,050 Posts
I would think this would have fallen under the "electrical" class in college, not "physics". ;)

Here's what you need, a good breadboard so you can put this stuff together, test it, and if it's right, drop it in the bike. :thumbup:
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