Any Optical LED experts?

[Perry Holbrook]

I'm working on a design for a back-lit shadow box. I plan to light it using LEDs and have just bought a pack of 50 on ebay to play with. As I understand it, the LED can be wired in series with individual resistors or in parellel with 1 resistor. My questions are:

Since I will be designing as I go, I don't really know how many LEDs I'll be using, so can I use a adjustable resistor (reastat (sp)) and adjust the resistance as I add or remove LEDs?

What happens if the resistance is not properly matched to the number of LEDs?

Would such a set up make the lighting able to be dimmed?

If it helps, I plan to use a 12VDC supply. The LEDs are 50,000 whatever units, voltage is 3.2 - 3.6, 20 ma.

Thanks for any input, Perry

[Dev Emch]

LEDs are very sensitive and are low current devices that can be fried like a slab of chicken if your not careful. The load resistor's sole perpose in life is to act as a current limiter. Since the amps to drive are in the mA range, a pretty good estimate is to ignore the LED completely and compute the amps based on the voltage drop across the resistor. Since this is mainly a DC application and since there are no frequency dependent signals, there are no active load issues to deal with. So good old ohms law will work just fine. Crude but Effective. Voltage(volts) = Resistance(ohms) * Current (amps).

Now if I place these in parallel, the idea is to maintain a constant voltage. Thus, your LED amp load will be equal to the number of LEDs times the LED amperage. The limiter resistor to this parallel network can be in series with the network. So size the resistor based on the maxium LED load of ALL the LEDs being used.

If you wire the LEDs in series, then you have a voltage drop across each LED and the becme sequentially dimmer. Also, you will loose all LEDs downstream of an LED burn out. So parallel does make more sense.

Another way to do parallel is to give each LED is own current limiter resistor since resistors are cheap. Make sure the restistor is down stream of the LED to eliminate voltage drop on the LED. So your scheme would be a ladder network of LED + limiter resistors.

Hope this makes sense.

As a cute aside. The government hunts down folks growing pot in their basements by examing homes with excessive spikes in power consumption. The logic is that indoor grow lights are being used. Well, NASA in their infinite wisdom to feed crews on long duration Mars runs are experimenting with new ways of growning food on orbit. The newest scheme is to use LEDs that emit plant grow light color spectra. Thus the plant gets the light for growing but none of the extra heat and with LEDs power consumption profile. I can see a huge market for these on EBAY.

[Ian Abraham]

My maths (and Dev's formula) says that you could wire 3 of your LEDs in series with approx 75 ohm resistor to limit current.
Hook that to 12 volts, you should get about 20ma thru the circuit and all will be well.
Wire up as many sets as you need and hook each seperate set up in parrallel.
You can use 1 resistor per LED but thats wastes 3 times the power warming resistors
Best not to wire individual LED in parralell because if one gets warm it's voltage will drop, it will draw more than it's share, get hotter still, and eventually thermal runaway and burn out.

If you hook your array of LEDs to a variable power supply with a max voltage of 12v then you will have a dimmer. The LEDS will start to glow about 10v and be max at 12 volts.

If the resistance you use is too high, you wont get the full 20ma, the leds will still glow, just not bright, but they wont be harmed by the experience.

If the resistance is too low you will get more than 20ma thru the LED, it will get hot and bothered and the magic smoke will escape.

I've attached a pic of some LED arrays that I'm talking about, i'm building them to go in a PC case.

Cheers

Ian

[Perry Holbrook]

I've been using the following link in my calculations so far. My question is, if I use a reastat in the parrallel set up, can I adjust as I add or remove LEDs and will it also serve as a dimmer?

http://metku.net/index.html?sect=vie...calc/index_eng

Perry

(I'll be away from the PC for a couple of days, will reply on my return)

[Ian Abraham]

Use the option 2 'leds in series'. Just wire up as many sets as you need. 3 leds + 1 resistor should be right. Their calcs tell you to use a 68 ohm resistor, sounds about right.

Then yes you could put a rheostat in the 12v feed to the whole circuit. Make sure you size the fixed current limit resistors so the whole thing runs off 12 volts OK. Then the rheostat can be used to cut that back. Dont rely on the rheostat alone to limit current, if it accidently gets turned to zero ohms your LEDs will smoke without the fixed resistors.

You need a power rheostat that will handle the current your LEDS are going to draw, but it's not going to be excessive. Otherwise you need some more complex circuity with a power regulator IC in there, but still simple enough to wire up.

Send me a note if you want a simple circuit drawn up

Cheers

Ian

[Ken Garlock]

Yet Another Opinion.

Start with you 12V supply. Use a 7805 regulator to drop down to 5v. The regulator is rated at one amp. It is a 3 terminal device, input(12v), ground(negative), and output(5v). I hope that your 12V supply is truly a real 12V supply and not some 'wall wart.'

With 5V available you can put a 50 LED load on the 1 amp supply. Each LED will need a current/voltage limiting resistor of around 70 ohms.

70 Ohms because:
LED wants 3.6v at .020 amp. We need to drop the voltage by 1.4v to make the LED happy. Hence, a series resistor for each LED. Mr Ohm tells us that 1.4V drop at .020 amp is a a 70 Ohm resistor, or nearest higher standard value. The resistor will only dissipate .02X.02X70 watts, or .028 watts. That is not enough to be of concern, a 1/4 watt(.25) resistor will work.

Please note that an LED are first and foremost a DIODE, and hence has a polarity to them. Put them in reverse and they will act like a short circuit and very quickly release their smoke. This is bad because it is widely known that all electronics run on smoke; let the smoke out and the device and it ceases to the work.

I am not an engineer, I don't play one on TV, and I don't give a hoot about the NEC.

[Dick Strauss]

Perry,
Let's make this simple...string 4 LEDs in series and wire all of the strings in parallel. That way each LED gets just under 3V and your total current with 12 strings (4 LEDs each) is ~240mA. Your power supply is most likely in the 250mA range for 12V.

If you check third lights (upper brake light) in autos like older Chevy Blazers, they are wired this way using a cars 12V system.

[Ian Abraham]

Hi Ken

Your circuit will work fine, but it's not very efficient.
If he wires them series / parrallel then he can run the 50 LEDs off ~ 330 milliamps (and a wall wart can supply that)

OR

going one step further you can user a variable regulator IC instead of the fixed 5 volt one and get the dimming action that way

Kinda OT but a while back I was working for an electrical company and they were asked to supply a light for a local kinetic sculpture. It had to last for at least 5 years and live about 135 feet up a flexible fibreglass pole. A large crane or helicopter was needed to take the pole down if the light needed changing. They wired up about 700 leds onto a football shaped frame and powered them series / parrallel from 24 volt supply.
The light is still working, although the question of why you would want to put a light on top of a 135ft flexible pole still hasn't been answered
If you are interested in strange sculptures you can see it here.

http://www.windwand.co.nz/ourwindwand_main.htm
http://www.faylooney.com/north-island/windwand.html

Cheers

Ian

[Ian Abraham]

Perry,
Let's make this simple...string 4 LEDs in series and wire all of the strings in parallel. That way each LED gets just under 3V and your total current with 12 strings (4 LEDs each) is ~240mA. Your power supply is most likely in the 250mA range for 12V.

If you check third lights (upper brake light) in autos like older Chevy Blazers, they are wired this way using a cars 12V system.

Safer to use 3 LEDS and a resistor though.
Otherwise he needs 12.8 volts for the circuit to light up and it smokes if he goes over 14.4 With the resistors in there it's just so much safer and easier to control the brightness.

Cheers

Ian

[Ian Abraham]

Perry

I've attached some sample wiring diagrams to give you some ideas.

The first one is using 3 leds in series / parrallel. It uses a simple potentiometer to reduce the current / brightness from the 12v max. Only problem is you will need a wire wound power pot, it may have to dissipate several watts at 1/2 brightness if you have a lot of LEDs.

Second diagram replaces the pot with a variable regulator IC. They are probably cheaper and easier to find than a power pot anyway. This is the data sheet for the LM317 chip.
http://www.national.com/ds/LM/LM117.pdf
It's a little plastic package chip with 3 pins, bolt it to a piece of alloy to remove the heat if you are using a lot of LEDs.

Cheers

Ian

[Cliff Rohrabacher]

fried like a slab of chicken

That's a unique expression.

The Air Force has a lime green led array that strobes at some specific speed, which when pointed at a person causes spasms.

[Perry Holbrook]

Thanks so much Ian. Before I go off and ruin something, a few questions:

The 1.2k ohm resistor is just hanging there, does it connect to the negative leg?

There are 2 LED's in series with the 220 ohm resistor, if one of the LED's burns out, the other one is also out, right?

How many of the series strings of 220 ohm + LED + LED can be wired in parallel? What is the limit?

Again, thanks so much, I owe you.

Perry

[Ben Grunow]

The regulators are coming and they are all amped up! Resistance is futile. Watch out or they will fill you with LED! (seemed funny to me at the time?)

[Vaughn McMillan]

...(seemed funny to me at the time?)

Worked for me.

- Vaughn

[Ian Abraham]

The 1.2k ohm resistor is just hanging there, does it connect to the negative leg?

There are 2 LED's in series with the 220 ohm resistor, if one of the LED's burns out, the other one is also out, right?

How many of the series strings of 220 ohm + LED + LED can be wired in parallel? What is the limit?

Perry

If you view the picture full size you will see where the 1.2k resistor goes, if your PC shrinks the picture to fit on the screen then some lines vanish. Print it out, it should be clearer.

Yes.. if one LED burns out the other will not light, but LEDs dont burn out unless they are badly overdriven. You could use 1 resistor (of a slightly higher value) and 1 led if you want, it's just not as efficient.

The regulator chip should be good for 1000mA as long as you heat sink it, so if each led chain draws 20 ma, then you should be good to power 50 sets (100 LEDs) If you need more light than that we can always beef up the circuit (Just use the 3amp version of the chip)

Cheers

Ian