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Old 23rd January 2003, 01:28 AM   #1
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Location: New York
Default A lighting a lot of LEDs....

Ok, maybe you guys can help me. I'm working on an art project that is lighting a lot of red LEDs....(standard T-1 3/4 5mm).

I have a power supply from a computer
Normal load current
output min. max
+5v 3.3a 20a
-5v 0 0.5a
+12v 1.3a 8a
-12v 0 0.5a

My questions are.... 1) how many could I power safely?
2) how would I hook them up?

I'm just looking for ideas of this.......Thanks.

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Old 23rd January 2003, 05:19 AM   #2
haldor is offline haldor  United States
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Location: Columbus, Ohio
You have several choices. You could just put them all in parallel with a single current limiting resistor. Problem is that unless the LEDs all have closely matched forward voltages you will get variations in brightness. Plus all in parallel with one current limiting resistor means the turn on current surge could blow LED's (typically LED turn on surge current should never be permitted to exceed 60 mA).

Another alternative would be to put them all in parallel with each LED having it's own current limiting resistor. This is probably the best solution if you are limited to lower supply voltages (5V or below). No turn on current surge issues and the LED brightness will be very closely matched. Negatives are this takes one resistor per LED and complicates the wiring.

If you have a higher supply voltage available, then the best method is to create strings of LED's in series with each string having it's own current limiting resistor. Then just parallel as many strings as it takes to get the total number of LED's you need. Putting a string of LED's in series will tend to average out variations in forward voltage and ensure even brightness while avoiding turn on surge issues.

Here is what you need to know to design this:

- Supply voltage.
- Forward voltage of LED.
- Desired LED current.
- Total number of LED's.

Let's say for example that you have the following data.

- Supply voltage = 12 V
- LED Forward voltage = 1.4V
- Desired LED current 20 mA
- Total number of LEDS = 30

I like to drop at least 3 V across the current limiting resistor to have some margin for variations in forward voltages (you might get a whole bunch that are at one end of the distribution).

6 LED's in series will result in an 8.4V drop across the LED string so that leaves us with 3.6 V to drop across the current limiting resistor, just about perfect.

Ohms law gives us the value of the current limiting resistor:
3.6 V / 0.02 A = 180 ohm at .072 Watt.

Finally 5 parallel strings of 6 LED's in series will draw 100 mA current from the +12 V supply. Not much of a power supply is required in this case.

Note: Switching power supplies do not normally work well without the load drawing at least 20% of rated current from the largest current supply (usually the +5 or +3.3 supply). I do not recommend you use a PC switching power supply for this project.

It will be much simpler and less risky to just use a 9 VDC, 300 mA to 500 mA unregulated wall transformer, (these typically put out around 14 VDC under a light load). If you go this route, adjust the current limit series resistance value to set the LED current flow for your particular transformer's loaded supply voltage.

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Old 23rd January 2003, 06:04 AM   #3
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Thank you.
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Old 23rd January 2003, 06:35 AM   #4
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Thanks for the help.

I'm just learning the laws of electricity, so bear with me.

I want to use at least 100 leds and a max of 200 for this project. I'm just afraid to wire them up and stick them to a power supply.

Any thoughts???

I'm going to try wiring up what you told me, I just want to be careful about this since I haven't used LEDs before. So any help is good. Thanks

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Old 23rd January 2003, 11:09 AM   #5
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Location: UK
I too am making a multi LED design[a sculpture] using about
700-800 leds.
these are a mix of standard and high brightness types.
I am using a 15v supply.
Like others have advised I have produced lots of series strings
connected in parallel.
Some of these strings use a limiting resister and some are simply
calculated so the voltage drops add up to the supply voltage
rating [with a safety margin] and do not need a resistor.
The power supply I use is a small desk switcher with good regulation.
I am also adding some protection circuitry to the input to ensure
the strings never get the chance to see over 15v, as the way this thing is built precludes any easy repairs!!

Have fun as there seems to be endless choices in the led market these days.

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Old 23rd January 2003, 11:55 AM   #6
DRC is offline DRC  United Kingdom
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The PSU you have is not really suitable for what you want to do - even with 200 LEDs at 20mA each, all in parellel (not a very neat solution), you would only be drawing 4A. This is **probably** not enough to meet the minimum current load for the switched mode supply you have - it might object in rather rude way !!! Of course you could include something to use up some more power (ugly) ....

Better to try to find a more suitable transformer (say 10V - 25V) and use strings of LEDs as described by other posters. This requires less series resistors (or current sources if you like) and will be ultra relable and safe (not to mention power saving). Transformers are so reliable you should be able to recycle something from some broken electronics ....

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Old 23rd January 2003, 12:18 PM   #7
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Default LED's in parallel ?

unless they each have a current limiting resistor its not a good idea. So says Agilent and they have more application notes than anyone in this area. Some of the reasons are obvious and some aren't. <p>You can statistically bin the LED's to get similar characteristics, but what guarantee do you have that the LED's on the perimeter of your design will be the same temperature as those in the center? The LED is a current driven device and the current will change as the temperature changes. (In a densitometer I built I drive a blue LED with a constant current source.) <p>I used some of the Agilent ultrabrights in one project -- wired in series -- to power the LED's I built a small boost switcher supply (I think that I used the National LM2574 adjustable regulator.) It works fine -- as long as each supply line to the LED array has a ferrite on it to keep the switching transients from propagating.
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Old 23rd January 2003, 05:34 PM   #8
haldor is offline haldor  United States
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200, 300 or 700 LED's are no different from my example. Just make strings of 6 or 7 LED's in series with a current limiting resistor in each string (this important to limit turn on surge current to less than 60 mA) and use a power supply that can supply enough current (usually 10 to 20 mA per string of LEDs).

You can get UL listed wall transformers that put out 12 VDC at 1200 mA very inexpensively ($10 or less). Figure out the current limiting resistor values needed to give you the desired current flow in each LED string and just wire up as many strings in parallel as it takes. This will be a safe and simple project to do.

A 12V, 1200 mA transformer could power 400+ LEDs at 20 mA or 800+ LEDs at 10 mA. If you need more than that I would use two transformers and wire the LEDs up as two independent systems.

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Old 23rd January 2003, 05:47 PM   #9
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I had not considered turn on surge currents.
The leds that are strung without a resister are 3.6v[being run at
about 3.2v] ultra-bright white/blue types.

How much of a problem do such currents present?
the application I am using them in will only be turned on a few times a day.

But for future designs I would prefer not to take chances
as I am planning designs with far far more leds than this first one.


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Old 23rd January 2003, 05:48 PM   #10
haldor is offline haldor  United States
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A quick search turned up this 12 VDC, 1A wall transformer for $5.


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