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Andy

Tue Nov 05 2013, 04:09AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Hi I'm think about using these

and was wondering how much power would they dissipate as heat, if wired in series(4 of them)?
They are 3.6 forward voltage, does that mean I need >14.4 for 4 of them to work, or will the light be less.

Any extra info about LEDs would be helpful, complete newbie :)

Thanks

Conundrum

Tue Nov 05 2013, 06:15AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Hi, LEDs need constant current and also thermal runaway protection.
Which diodes are you using?

Kind regards, -Andre

Andy

Tue Nov 05 2013, 06:45AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874

Hi , the LEDs are
High Power LEDs - Single Color Blue 455-465nm 200lm @ 3.15A Luminus Devices
High Power LEDs - Single Color Red 619-623nm 475lm @ 3.15A Luminus Devices

A grow light company say they use LEDs at 450nm and 630nm, these were the closest, and what they were selling about 90watts covers 1m3.

With constant current would a resistor make it, if the power supply can supply more amps than the resistor limits it to?

This might not sound like the brightest idea, but was thinking mains to a battery 12 or 24volt, was just going to test 1m3 to see if the thing will work.

Thanks

Sulaiman

Tue Nov 05 2013, 08:15PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

For info. on LED try typing LED into Wikipedia search box.......

I find that it is always easier to drive LEDs in series with series resistance or constant current,
the main problems are that an open circuit anywhere in the chain causes failure and
possibly high voltage on LEDs concerns.
BUT, if you have a spare/safe/reliable power supply that gives say 5V at up to 20A
then one resistor per LED is good.
For a typical PC psu you could 'use up' all available power by also putting three in series with a resistor for +12v, -12v etc...you may already have an old PC PSU.
For 5 V supply, R = (5 - 3.25)/3.15 = 0.5555 Ohms, 5.5 W, use 0.56 Ohm 10 W per LED.
Due to all sorts of variations the actual current will be as designed,
but close enough.
For 12V 3 series LED plus 0.71 Ohms, 7.1 W, e.g. 2x (1.5 Ohm 7 Watt) in parallel.
Inefficient but cheap, easy, reliable and an extra source of heat?

1 Watt of green light = 683 lumens, so 200 lumen is about 0.3 Watts of light, the rest is heat...etc.
e.g. 3.25V x 3.15A = 10.2375 W, -0.3 W = 9.9375 W = 10 W of heat.

P.S. 10 W is a lot of heat to remove continuously, especially if the ambient temperature is high,
LEDs don't live long if hot. the most important part of the project is heat management I suspect.

2Spoons

Wed Nov 06 2013, 10:48PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Multiple lower power leds are easier to power and heat manage than single high power leds. Since you are doing area illumination i don't see any need to use high power leds. There are plenty of efficient switchmode LED drivers around for currents up to ~1A - RECOM make some nice ones

BigBad

Thu Nov 07 2013, 01:01AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

Last time I drove a bunch of LEDS (about 60 LEDs or something), I chained them up in groups and made a constant current source with a transistor on the end of each chain and ran it off a 12 volt wall wart. I used one of the LEDs instead of a zener on each chain; that one ran a bit darker due to the voltage drop through the transistor, but it still basically worked.

IRC it was this circuit:

where the load is a chain of LEDs.

Andy

Fri Nov 08 2013, 06:40AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Thanks you lot for the replies,
I've being trying to find a source of LEDs that will output the same power as

,at a cheaper price, so far not much luck, theirs these two

, but the second one works out to about 0.13lm . I'll need 35,000 of them.

Is there a cheaper way rather than buy the above 90watt source?

Shrad

Fri Nov 08 2013, 10:48AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

perhaps have a look at cannabis growing forums?

I know this is not legal but if you just read their homemade LED growlight threads, you wouldn't do any harm, no?

they have done a bunch of research since at least the first 1W leds years, years ago...

Andy

Sat Nov 09 2013, 04:10AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Hi Sulaiman
Reading up it says 50-90lm is a watt of light, 683lm seems abit high?. There are these

that might workout, a 24volt battery bank might not fluctuate to much would a 1.7ohm resistor be all thats needed with one of the strips in series?

Hi Shrad
Thinking about it, just not wanting to get to much traffic to a site like that, if I do start to grow indoors.

Hi BigBad
I was hoping for something really easy, not necessary efficient, like just a resistor, but thanks I might have to go that way.

Hi 2Spoons
The above links have a driver to go with them, 4.2amp 24volt ac-dc, but it will make the price reach close to buying from

,will keep searching.

BigBad

Sat Nov 09 2013, 05:51PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

The resistors do work, but they're not temperature compensated, and they're not very efficient; using the LED/transistor like that gives you automatic current compensation. It's not a very complicated circuit, with care you could probably run it straight off rectified, smoothed AC with no other regulation.

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