Way to step down voltage to LED.

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divzr0
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Way to step down voltage to LED.

Post by divzr0 » Sat May 06, 2006 2:43 pm

Hey All-

This is probably a pretty basic question for most of you but here it goes...

I'm working on a side project of mine where I need LEDs to indicate if a 12V supply is avilable. The problem is 12V leds are relatively expensive compared to their lower voltage counterparts. What would the cheapest/easiest way be to step down the voltage to the led to still provide an accurate 12V supply available light.

Thanks!

Mark

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Post by JPKNHTP » Sat May 06, 2006 3:13 pm

-JPKNHTP
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philba
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Post by philba » Sat May 06, 2006 4:08 pm

A 12V LED is simply an LED with a series resistor built in.

use ohms law to determine the resistor value. a typical LED has a 2V forward drop (Vf) so you subtract that from the supply voltage. 12-2 is 10V. Typical LEDs are spec'd for 20 mA (.02A) forward current (If). So V=IR, rearrange - R = V/I = 10/.02 = 500 ohms. closest standard 5% value is 510 but a 470 would probably be ok. of course, you should, if possible, check the LEDs datasheet for Vf and If. If you don't have one, the above values will most likely work fine.

you then need to calculate the wattage of the resistor - V*I = 10 * .02 = .2W use a 1/4 watt resistor.

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jollyrgr
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Post by jollyrgr » Sat May 06, 2006 4:28 pm

You don't "Step Down" voltage for an LED; it is a current device. You limit the current through it with a series resistor, just like philba says. This is if you just want a pilot light to say something is on.

If you want to indicate that the voltage present is 12 volts, and not something like 5V or 14.2V, this gets a little more complex. Do you need an ON light or a "12 volts present" light?
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Chris Smith
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Post by Chris Smith » Sat May 06, 2006 4:47 pm

If and only if,.... your looking for a 12 volt trigger light using the LED, then you use the Zener in a crowbar circuit, complete with threshold settings.

The LED can run using 5v 12v or 100 volts, as it only requires a current limitation and not a voltage regulation.

But even that is only ....â€

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Post by divzr0 » Sat May 06, 2006 5:26 pm

What I'm basically looking for is a 12V present light. Its simply for debugging purposes. The device that will be powered by a 12V lawn mower battery requires 12V, so it would be nice to have an external interface that says hey "12V supply is avaible this isn't what the problem is, go look somewhere else"

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Chris Smith
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Post by Chris Smith » Sat May 06, 2006 5:53 pm

A standard LED and dropping resistor is all you need.

Use the standard V-led Formula.

http://www.liquidcrystaltechnologies.co ... sistor.htm

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Post by divzr0 » Sat May 06, 2006 6:30 pm

Thank you very much.

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Post by jollyrgr » Tue May 09, 2006 5:48 pm

Now that you got your answer, let me throw a wrench into the works.

An LED can run off a watch battery. It will show "12 volts" is present from a nearly dead battery. So while it may show voltage present, it may not show that enough battery CURRENT is present to perform the required duty. In other words if you have a "dead" battery, the LED may light but any high powered starter motor or similar may not even click.
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philba
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Post by philba » Tue May 09, 2006 5:56 pm

I think people are making this way too hard.

The led/resistor is a voltage-present indicator (idiot light). If he want's to test for 12V (not 11, not 13...) he should just get a cheap DMM. Anyone here probably already has a DMM, anyway.

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