Confused with resistance and LEDs

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steve70
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Confused with resistance and LEDs

Postby steve70 » Fri Feb 12, 2016 7:27 pm

Sorry for asking stupid questions but I'm confused and not sure why I'm doing what I'm doing. :D
I bought some 1.8mm LED's from Kytes Lights. They are 3V and they say that if you are using 12V supply then you need to use 680 Ohm Resistors.
I've tested my LED's using a couple of 1.5V AAA batteries and they are fine but before I wire the resistors in place and plug them in to a 12V supply I wanted to actually see what voltage my 12V power supply that I found at work today was actually producing 12V.
Well it seems to be producing 15.3V so then I put one of my 680 Ohm resistors on the end of the positive supply and it still read 15.3V!

Now I don't want to plug my lights into it for fear of blowing them all up. :(
What's going on?

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Flashbang
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Re: Confused with resistance and LEDs

Postby Flashbang » Fri Feb 12, 2016 7:54 pm

Hi
It would seem your 'found' power supply isn't regulated.
Regulated means the rated output voltage is constant regardless of the load placed upon the supply, up to its maximum.

Your volts measured are those found on an unregulated supply with no loading. The volts dropping as the load (current) increases.

Assuming the power supply provides a DC output, I would use a series resistor of 1K0 (1000 Ohms) per lit LED. This will give an average lit LED current of 10 milliamperes (0.01Amp) per 3volt rated LED. So 50 lit LEDs would draw approx ½ Amp (500ma) assuming the suppy is then running at as near 12 volts DC as possible, but a slight increase in output volts won't do any harm with a 1K0 resistor.
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Broken? It was working correctly when I left it.

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End2end
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Re: Confused with resistance and LEDs

Postby End2end » Fri Feb 12, 2016 8:19 pm

As Flashbang mentions, I ALWAYS use a 1KOhm resistor on every single one of my LED's and they are run on a 12v circuit.
I have in the past installed, glued down and painted over the wires of one of my LED's only then to realise it didn't work. (OK I forgot to add a resistor to this one so it blew as soon as power was applied :oops: ) :lol:
Minor modelling catastrophe :shock: . So with that im mind may I suggest getting yourself one of these and checking each LED you buy.
Yes I am known to open the post and sit and test 100 LED's :lol:
Image
http://www.ebay.co.uk/itm/Mini-Handy-LE ... xyd8NSa6BH
Cheap at only £1.43, runs on a 9v battery and can test multiple value LED's. You probably won't blow any even in the higest rated holes... But don't quote me on that!
Also tests multi-colour LED's :mrgreen:
Hope it helps.
Thanks
End2end
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steve70
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Re: Confused with resistance and LEDs

Postby steve70 » Fri Feb 12, 2016 9:23 pm

Thanks again for the advice guys.

I've just conducted another test and connected an LED with 680 Ohm resistor to this power supply and hey presto, it didn't blow.
I then measured the voltage across it and it was 3.06V. :o

So I've thought about it and, being a plumber/heating engineer have remembered that voltage is the same as water pressure and current is flow and it makes sense :)

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Peterm
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Re: Confused with resistance and LEDs

Postby Peterm » Fri Feb 12, 2016 11:11 pm

If you want the led's to last and not be too bright, I'd go with the 1K resistors suggested by Flashbang rather the 680 ohm.
Pete.

steve70
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Re: Confused with resistance and LEDs

Postby steve70 » Sat Feb 13, 2016 12:05 am

Thanks.. These are 'warm white' LED's and are just fine on a 3V battery but I will check them with 12V supply.

EDIT - I think I might go for 1K's :)
Now which ones do I go for? There are loads of 1000 Ohm Resistors on Rapid Electronics!

Suzie
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Re: Confused with resistance and LEDs

Postby Suzie » Sat Feb 13, 2016 12:17 am

Use the resistor that gives you the brightness you need. 680R is a good start and only use a higher value if it is too bright.

steve70
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Re: Confused with resistance and LEDs

Postby steve70 » Sat Feb 13, 2016 12:25 am


Gordon H
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Re: Confused with resistance and LEDs

Postby Gordon H » Sat Feb 13, 2016 9:57 am

There are a number of ways to calculate what rating of resistor is needed.
If your supply is 12V, and the LED has a forward voltage of 3V, your resistor will be dropping 9V of the 12V available.
Assuming you stick with the 680 Ohm resistor value, the resistor will be passing 9V / 680 Ohms = 13.2mA approx.
Its power dissipation is therefore 13.2mA x 9V = 0.12W approx. (Watts = Amps x Volts)
Therefore you need a resistor which has a power rating of at least 0.12W - and preferable somewhat more to spare. It is a bit close for a 1/8W (0.125W) type, so the next option up would be a 1/4W (0.25W) type. You might also find some 0.4W types of similar size. The tolerance is not particularly critical, 5% is fine.
In this instance, you might select this one from Rapid:
http://www.rapidonline.com/electronic-components/truohm-cr-025-680r-carbon-film-resistor-680r-cr25-0-25w-pack-of-100-62-0366
However, they do come in packs of 100, so you will get plenty of spares!

steve70
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Re: Confused with resistance and LEDs

Postby steve70 » Sat Feb 13, 2016 10:47 am

Thanks for this Gordon. Very useful.
So it's the Carbon Film type I should be looking at rather than metal film or other types ?

Suzie
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Re: Confused with resistance and LEDs

Postby Suzie » Sat Feb 13, 2016 11:57 am

Go for Metal Film, 1% and 0.25W. The metal film ones are in my experience more reliable than Carbon, the minimal extra cost is worth the peace of mind that it will work! If temperature of the resistor is a problem you might like to use the 0.4W ones which are bigger and should not get so hot.

Suzie x

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Re: Confused with resistance and LEDs

Postby Gordon H » Sat Feb 13, 2016 1:50 pm

Suzie wrote:Go for Metal Film, 1% and 0.25W. The metal film ones are in my experience more reliable than Carbon, the minimal extra cost is worth the peace of mind that it will work! If temperature of the resistor is a problem you might like to use the 0.4W ones which are bigger and should not get so hot.

Personally I have never had any failures of Rapid's carbon films types - and I have used quite a few thousand over the years, so I wouldn't worry about reliability too much.

Suzie
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Re: Confused with resistance and LEDs

Postby Suzie » Sat Feb 13, 2016 2:24 pm

I have only had a couple of carbon ones fail, but no failures of Metal Film. They would probably be OK in more competent hands, the failures were of a mechanical nature (a poke with a finger got them going again) - I don't think they liked my soldering! I have used a lot more Metal Film than Carbon, I guess they are just made a little differently.

steve70
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Re: Confused with resistance and LEDs

Postby steve70 » Sat Feb 13, 2016 6:15 pm

And there was me thinking a resistor was a resistor.
So metal or carbon....?
There's only one way to find out......... :lol:


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