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Old 01-07-2009   #1
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Electronics Wizards!

Right, wiring up some new buttons and switches in the 126 and some have 2.2vdc LED's in. What components do I need to step the voltage down?
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Old 02-07-2009   #2
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Re: Electronics Wizards!

a simple resistor.... and ohms law to work out what value resistor you will need.

have a go at this.... (dont forget that LEDS only work one way round, and a resistor value close to the required will be fine, it does not have to be EXACT)

A bright orange LED might have these specs: voltage drop of 2.2V and a current of 75 milliamps (ie 0.075 amps).

From Ohms Law above, ohms = voltage drop (across the resistor) divided by amps
If we are supply 12V, and we only want 2.2V at the LED, we want the resistor to drop (12V – 2.2V =) 9.8V.

So, ohms of the required resistor = 9.8V divided by 0.075 amps (the required current flow through the LED)

9.8 divided by 0.075 = 131 ohms.

Therefore a 131 ohm resistor will limit the current flow to 0.075 amps through the LED.

The other spec of a resistor is its required power dissipation in watts.
Watts = amps x volts, so that’s 0.075 amps x the voltage drop across the resistor, which is 9.8V.
Watts = 0.075 x 9.8V = 0.7 watts.

So we’ve worked out we need a resistor with 131 ohms of resistance and a power handling of 0.7 watts. The nearest off-the-shelf design to this is a spec of 120 ohms and 1 watt.

(However, with a running car voltage that is higher than the nominal 12V, and with the fact that you probably don't want a dashboard monitoring LED to be super-bright, a 620 ohm, 1 watt resistor will drop LED current and still result in a LED bright enough to be easily seen.)

Taken from this article

http://autospeed.com/cms/A_110894/printArticle.html

hope it helps
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Last edited by billysollux; 02-07-2009 at 11:18.
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