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tim89
 Post subject: electrical
PostPosted: Sat Jul 17, 2010 10:50 am 
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Hi,
I need to connect a led rated at 2.2volts and 25 milli amps dc. I am instaling it to a vehicle battery rated somewhere between 12.4v to 14.4v. My question is what size resistor do I need and the formula please.
Thanks Tim89
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kellys_eye
 Post subject: Re: electrical
PostPosted: Sat Jul 17, 2010 12:49 pm 
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(Vsupply minus Vled) divided by Iled

therefore (14.4 - 2.2) / 0.025

= 12.2/0.025

= 488 ohms.

The nearest PRACTICAL value available would be 470 ohms. The colour code for this resistor is yellow, violet, brown (with an additional gold band signifying 5% tolerance)

Use a resistor rated at 1/4 watt or higher.

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tim89
 Post subject: Re: electrical
PostPosted: Sun Jul 18, 2010 8:28 am 
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kellys_eye wrote:
(Vsupply minus Vled) divided by Iled

therefore (14.4 - 2.2) / 0.025

= 12.2/0.025

= 488 ohms.

The nearest PRACTICAL value available would be 470 ohms. The colour code for this resistor is yellow, violet, brown (with an additional gold band signifying 5% tolerance)

Use a resistor rated at 1/4 watt or higher.


Big thank you. tim89
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Agile
 Post subject: Re: electrical
PostPosted: Mon Jul 19, 2010 8:26 pm 
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But if its for a vehicle application you can get them with an integral bezel and built in resistor.

I used to sell them at the electronics shows for 10p each with a black plastic bezel but more for the chrome eazels, perhaps 25p !

Tony
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