Led and voltage

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Daniel Quinn
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Led and voltage

Unread post by Daniel Quinn »

With my definitive amps, I want to fit a led so I know there on.

I've got several 5mm leds that say they run on 3v.

The voltage of my rails is 58v.

I used the resistor the doc uses on his but did not illuminate them. Any advise or alternatives appreciated.

valvesRus
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Re: Led and voltage

Unread post by valvesRus »

If you know how much current the LED draws then Ohms law will tell you the resistance needed to drop 55v.

If something like this would do I have a spare one (blue) you can have. It's 220v - 240v

https://www.ebay.co.uk/itm/5Pcs-220V-24 ... SwFahf3k~w

PS have you got the LED the right way round ?

Geoff.R.G
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Re: Led and voltage

Unread post by Geoff.R.G »

LEDs are current devices so lets us assume 20mA.
Thus voltage divided by current gives resistance 58/0.02= 2,900 so a 3KΩ 1/4W resistor should do nicely.

Daniel Quinn
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Re: Led and voltage

Unread post by Daniel Quinn »

Thanks, you given me somethings to think about and try.

Geoff.R.G
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Re: Led and voltage

Unread post by Geoff.R.G »

Daniel Quinn wrote: Mon Feb 01, 2021 9:22 pm Thanks, you given me somethings to think about and try.
No problem.
I was actually waiting for someone to tell me that I have done the maths wrong. Indeed I have if maximum brightness is the aim but my opinion is that the 3V of the LED can be ignored unless there is some specific reason to take it into account. As it happens the difference is 150Ω* and simple rounding to a standard value makes more difference.

*2,750Ω at 55V against 2,900Ω at 58V

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