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Old 07-20-2014, 11:09 AM   #1
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Current limiting Resistor


Hi there,

I recently built an aquarium LED light which went pretty well, however i read that I need to include a current limiting resistor into to the circuits.

I basically have used a constant voltage supply outputting at 12 volts and then ran 4 sets of series strings running in parallel with each other.

I am using a combination of cool white, royal blue and standard blue 3 Watt LEDS. I know that each LED will receive 4 volts each. So I did not include a resistor as this is about right for the LED as the specs of each essentially the same. However since building the light I have read that you always should add a resistor with any LEDs, however I can't see why this needed as I don't need the voltage dropping at all? I also read this was to limit the current, however I can't see how this works and obvious it seems more efficient not to include resistors?

Could anyone clear this up for please as I don't want to blow anything.

Oh the the specs for the LEDs are here :

http://stores.shop.ebay.co.uk/Future-Eden


The type of driver is like this

http://www.ebay.co.uk/itm/LED-Strip-...item33833ae82e


Many thanks in advance for anyone takes the time to reply.

Best regards

Jack

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Last edited by Jacks2High; 07-20-2014 at 11:36 AM.
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Old 07-20-2014, 04:29 PM   #2
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Current limiting Resistor


Quote:
Originally Posted by Jacks2High View Post
Hi there,

I recently built an aquarium LED light which went pretty well, however i read that I need to include a current limiting resistor into to the circuits.

I basically have used a constant voltage supply outputting at 12 volts and then ran 4 sets of series strings running in parallel with each other.

I am using a combination of cool white, royal blue and standard blue 3 Watt LEDS. I know that each LED will receive 4 volts each. So I did not include a resistor as this is about right for the LED as the specs of each essentially the same. However since building the light I have read that you always should add a resistor with any LEDs, however I can't see why this needed as I don't need the voltage dropping at all? I also read this was to limit the current, however I can't see how this works and obvious it seems more efficient not to include resistors?

Could anyone clear this up for please as I don't want to blow anything.

Oh the the specs for the LEDs are here :

http://stores.shop.ebay.co.uk/Future-Eden


The type of driver is like this

http://www.ebay.co.uk/itm/LED-Strip-...item33833ae82e


Many thanks in advance for anyone takes the time to reply.

Best regards

Jack
Anyone???

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Old 07-20-2014, 05:11 PM   #3
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Current limiting Resistor


Need to confirm one thing....

How many LED's in series?
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Old 07-20-2014, 05:43 PM   #4
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Current limiting Resistor


It is never a good thing to run LEDS flat out for long periods,
It's a sure recipe for a a short life span.

Are you sure 4 volts is right for those leds ?
Most leds I have seen are around 3.5v max.

Try running less leds in each string then include a current
limiting resistor, reduce the current to approx 3/4 of max
this way you still get good light output and a much longer life.
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Old 07-20-2014, 05:52 PM   #5
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Current limiting Resistor


Quote:
Originally Posted by dmxtothemax View Post
It is never a good thing to run LEDS flat out for long periods,
It's a sure recipe for a a short life span.

Are you sure 4 volts is right for those leds ?
Most leds I have seen are around 3.5v max.

Try running less leds in each string then include a current
limiting resistor, reduce the current to approx 3/4 of max
this way you still get good light output and a much longer life.
Hi there,

Thanks for this.

If you look on the spec on the listing that is

http://cgi.ebay.co.uk/ws/eBayISAPI.d...e=STRK:MESE:IT

I like the idea of just adding a resistor but not sure how to work out the resistor value? I have used those online resistance calculators before but not sure if they are accurate.

Basically what I would like to know is if I had 3 LEDs in series (as per the spec for the cool whites above) what resistance value would I need for it safe with a current limited to 700 (max 750mA) according to the spec.

Sorry for all the questions?

Many thanks

J
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Old 07-20-2014, 08:46 PM   #6
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Current limiting Resistor


Use a one ohm resistor five watts power rating.

For 3 x LED's on a 12v supply.

Expect the resistor to be a little warm.

And allow for that when you mount the resistor.
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Old 07-20-2014, 10:16 PM   #7
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Current limiting Resistor


Since the box you have is sold as an LED driver maybe it has current limiting built in. I don't suppose it came with documentation?

If you need a resistor, size it so that V=IR where V is the voltage it has to drop (supply voltage minus sum of LED voltages) and I is the amount of current you want going through the LEDs.

As said above, install it with the understanding that it will generate heat.
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Old 07-20-2014, 10:43 PM   #8
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Current limiting Resistor


The resistance across an LED will change in relation to the temperature of the LED Junction...

As the junction warms up, the resistance value drops causing more current to flow across the junction.. The increase in current then causes the junction to heat up even more and further decreases the resistance which causes even more current to flow.. bla bla bla.. (see where this is going?)

Eventually, your LED becomes a flash bulb and goes supernova when it pops.

By installing a resistor in series with the the led, the resistor acts as a governing device and only allows a certain amount of current to flow. The excess energy it "governs" is converted to heat and dissipated.. (kind of wasteful but it works)

The specs on your led's vary via the link you provided.. However, if you provide a 1 ohm resistor rated for 1 watt in series with each string of 3 LEDS, you'll be pretty safe.

Also, I should point out that if you look at a graph that plots the input power as compared to output lumens, you will find that LED chips are most efficient at around 70% of their rated current..

Constant voltage drivers are NOT very good for led systems. They waste a lot of energy and can cause rapid cascade failures without the current limiting resistor in place.

Constant current drivers are BY FAR the best way to go.. (Meanwell is a good quality example) They greatly simplify installation, use energy much more efficiently, and pretty much eliminate any risk of cascade failure caused by thermal runaway.

Hope that helps,
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Old 07-20-2014, 10:45 PM   #9
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Current limiting Resistor


Oh.. I should point out one other thing...

If you're going to spend the extra money on the "name brand" chips, just go to fasttech and get some CREE XML leds.. by far the best bang for your buck..
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Old 07-21-2014, 05:04 AM   #10
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Current limiting Resistor


This is all very good thanks guys, really appreciate your time and effort. Think I understand it now.

Think I will run a test with some 1OHM resistor rated at 10W just be safe (overkill but I can reuse them elsewhere)

Also I saw this which appears to regulate voltage and current via two pots. I think this would work as well - probably a better solution? Does anyone have any experience of some of these little modules?



Many thanks again for your help it really is so valuable
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Old 07-21-2014, 07:19 AM   #11
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Current limiting Resistor


Quote:
Originally Posted by Jacks2High View Post
This is all very good thanks guys, really appreciate your time and effort. Think I understand it now.

Think I will run a test with some 1OHM resistor rated at 10W just be safe (overkill but I can reuse them elsewhere)
10 watt is drastic overkill, and they're kind of big. Shouldn't need anything more than 1W

If you're running a 12 v supply and you're using 3W led's then

3 / 12 = 250mA
12 /.25 = 48 ohm

So the math says 48 ohms and I would raise that to about 50

(At 10 ohms you will supply 14 watts and blow the led)

Last edited by Bob Sanders; 07-21-2014 at 07:29 AM.
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Old 07-21-2014, 07:39 AM   #12
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Current limiting Resistor


Quote:
Originally Posted by Jacks2High View Post
This is all very good thanks guys, really appreciate your time and effort. Think I understand it now.

Think I will run a test with some 1OHM resistor rated at 10W just be safe (overkill but I can reuse them elsewhere)

Also I saw this which appears to regulate voltage and current via two pots. I think this would work as well - probably a better solution? Does anyone have any experience of some of these little modules?



Many thanks again for your help it really is so valuable

If you can regulate the current that is all you need.
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Old 07-21-2014, 08:16 AM   #13
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Current limiting Resistor


Quote:
Originally Posted by Bob Sanders View Post
10 watt is drastic overkill, and they're kind of big. Shouldn't need anything more than 1W

If you're running a 12 v supply and you're using 3W led's then

3 / 12 = 250mA
12 /.25 = 48 ohm

So the math says 48 ohms and I would raise that to about 50

(At 10 ohms you will supply 14 watts and blow the led)
NO... Your math is correct but your formulas are totally wrong.. 1 Ohm at 1 watt is all he needs..
For some extra heavy duty measure, you could get a 2 watt (bigger) resistor.. But keep it at 1 ohm..
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Old 07-21-2014, 08:26 AM   #14
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Current limiting Resistor


This is what you want to drive all those led's.
http://www.ebay.com/itm/Mean-Well-MW...item4184265616

This is a wet location 48 volt driver that puts out a regulated 700ma.. It won't burn up your leds, its made by Meanwell (+1), and it is very simple to hook up so it eliminates a bunch of junctions and extra wires.

Hope that helps,
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Old 07-21-2014, 08:36 AM   #15
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Current limiting Resistor


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Originally Posted by PaulBob View Post
This is what you want to drive all those led's.
http://www.ebay.com/itm/Mean-Well-MW...item4184265616

This is a wet location 48 volt driver that puts out a regulated 700ma.. It won't burn up your leds, its made by Meanwell (+1), and it is very simple to hook up so it eliminates a bunch of junctions and extra wires.

Hope that helps,
Great stuff, thanks for this. Really appreciate you taking the time for looking for me. I think I might just get one of these. Looks ideal!

Thanks again

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