# how to drive 5mm LEDs?



## coolwaters (Sep 3, 2008)

im trying to light up about 50 of these RED 5mm LEDs. standard 2v 20mA LEDs.

i couldnt find a constant current regulator that goes that low. lowest i seen was 300mA.

what do i need to make something like this? my power supply is 12v.
i dont like using resistors because they dont help save the LEDs and just waste power and create heat.

is there a diagram on how to make a circuit that gives out 20mA?


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## TigerhawkT3 (Sep 3, 2008)

If you want to power 50 LEDs (even red ones) with 12V, you'll either need to wire them in some series-parallel configuration or use one heck of a boost circuit.

How stable is that 12V? Is it exactly 12V, or more or less? If it's a stable 12.0V, you might be able to just DD 6 LEDs in series, and use 8 parallel strings (2 left over). If it's a little more than 12V, you could try 7S7P.

An alternative would be to wire them all in parallel and hook them up to a 1A regulator, but it's not a very ideal alternative.

What is this for? Do you want to use 5mm LEDs in particular for some reason (like if you already have them around and just want to put them to use)? If you just want a lot of red light off a 12V PS, you could try red XR-Es. Five or six of those emitters and a cheap "Kennan" circuit from KD, and you'd be in business.


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## coolwaters (Sep 3, 2008)

its for a car so the 12v power source isnt going to be 100% stable.

cant i make a circuit board that has a wide range of power input? i have one for a high power LED that accepts 5-18v of power.

the regulator for a high power LED 3-18v at 700mA didnt look too complicated. so why is the 20mA harder?

if i use one of my 5-18v 700mA regulator and wire about 35 in parallel will each LED get 20mA?


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## LukeA (Sep 3, 2008)

coolwaters said:


> its for a car so the 12v power source isnt going to be 100% stable.
> 
> cant i make a circuit board that has a wide range of power input? i have one for a high power LED that accepts 5-18v of power.
> 
> ...



But the question has been raised: Why use the 5mm LEDs when there are red Crees that can handle the current of the pre-existing regulators?


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## Steve K (Sep 3, 2008)

coolwaters said:


> im trying to light up about 50 of these RED 5mm LEDs. standard 2v 20mA LEDs.
> 
> i couldnt find a constant current regulator that goes that low. lowest i seen was 300mA.
> 
> ...



If you can handle soldering a small circuit together, I'd recommend building a current regulator from the LM317 adjustable voltage regulator. Details are in National Semiconductor's applications note:

http://www.national.com/an/AN/AN-181.pdf

The schematic for the regulator is on page 4. 
You'll need to attach a modest heatsink to the LM317 to keep it from getting hot and shutting off. 

Battery power in cars is extremely nasty, so you may want to add some protection...
One item is a series diode, which will prevent negative voltage spikes from getting to the circuit.
A second item is a 15v zener diode downstream from the series diode. This will clamp the positive voltage spikes to a manageable level. I'd suggest a 3 watt zener, just in case.
A third item would be a moderate sized capacitor, to minimize the effects of other noise on the battery power. Maybe 100uF at 35v?

There are a lot of other ways to build a simple current regulator, but this one is nearly foolproof. 

good luck,
Steve K.


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## frenzee (Sep 4, 2008)

National's LM5000 (datasheet) is a generic switch mode boost regulator and can drive up to 20 white or 30 red 5mms from a 12V supply, so maybe you could use two of these.


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## TigerhawkT3 (Sep 4, 2008)

Steve K said:


> If you can handle soldering a small circuit together, I'd recommend building a current regulator from the LM317 adjustable voltage regulator. Details are in National Semiconductor's applications note:
> 
> http://www.national.com/an/AN/AN-181.pdf
> 
> ...


I don't think the 317 is suited to this application. OP already said he doesn't want to use resistors because of inefficiency, and with a Vin of only 12V, the 317 will be quite inefficient, even compared to resistors. For 5 red LEDs (total Vf 10V) and a 13V battery, efficiency would be slightly over 75%.

Also, the LEDs would have to be arranged something like 5S10P, so there would have to be a 317 for each string. If you used a single 317 for that 5S10P string at a regulated 200mA, like my "wire them all in parallel and hook them up to a 1A regulator" idea above, it might work for some amount of time, but like I said, it'd be far from ideal.

The LM317 can handle Vin-Vout of up to 36V (max Vout 37V). Unless the car's battery spikes up to 40V or so, it'd be fine without a diode.

frenzee, that LM5000 looks like a good choice. Any ideas where to get it? Maybe ask National for a sample?


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## Christexan (Sep 4, 2008)

Instead of the LM317 (1.5v dropout roughly, depending on load), use an LM108x (LM1085 for example), at worst case it's about a 0.5V dropout, and less at lower currents. Works just like the LM317T, but with much better efficiency (lower dropout means less heat lost in the regulator, and the ability to closer match the V(source) to V(output), for even more efficiency. 
It's still just a fancy resistor circuit, but much much more efficient than the LM317 circuit, with all the benefits. 

So what's the drawback? Cost... an LM317T at mouser.com is something like $0.32... an LM1085 is something like $0.97 (from memory last time I looked a while back)... so if $0.65 breaks your budget... otherwise there isn't any real drawback in our current discussion, plug-and-play replacement for LM317.


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## Oznog (Sep 4, 2008)

Well, depending on Vf you will put 3-5 in series with a sharing resistor and make a bunch of these series strings (each with its own sharing resistor). Must be clear here- _each series string must have a sharing resistor._ Even if the whole thing has a constant-current source. 

The resistor must be large enough to compensate for the temp coefficient of forward voltage but doesn't need to be large enough to compensate for the 11v-14.6v difference in source voltage if you have a regulated source. It is still advantageous to have a source regulator instead of larger resistors, it is more efficient and is stable when the source voltage changes.

Calculate the heat of a linear reg. Say you have Vf=3v LEDs. To ensure they turn on when the engine is off with the dropout voltage of a reg and drop across the sharing resistors, you can put 3 in series at most. So 16 strings, 48 LEDs total. 
Total reg current is 320mA. 
Once the resistor is included, the strings are say 9.6V.
With the engine running the voltage is 14.6V.
(14.6V-9.6V)*320mA=1.6W

Even though these are LEDs, you will need to heatsink this regulator!

Building a buck converter is not hard. I am normally partial to the Supertex HV9910, however, that uses an external transistor and since your current is LOW I'd recommend something like a PT4105 with an internal transistor.

Ah wait... here we go:
http://www.dealextreme.com/details.dx/sku.13553
http://www.dealextreme.com/details.dx/sku.13555
http://www.dealextreme.com/details.dx/sku.13557

Are all PT4105-based boards at a bargain price. 300mA-350mA will match your demands perfectly and you can't beat the price!

There are some obscure differences between these 3, something to do with Vout I think. Buck converters require special compensation when Vout>0.5*Vin so that's probably single LED vs 3x LED versions.

Be aware these solutions operate in constant-current mode, thus "trying it out" with 1 series string instead of 16x strings in parallel will force all the current through one string and blow it out. In fact any LED failure in the array may result in the destruction of the entire array. If one LED goes "open", it will result in that string's share of the current being transferred to the other strings which may overheat them (well, 1 out of 16 is not too serious though, if we had only 4 strings I'd be more worried). Any short in an LED will overwhelm the capability of the sharing resistor and results in almost all the array's current going through that one string which will destroy the entire string and eventually result in an open.


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## coolwaters (Sep 8, 2008)

i like the LM5000 idea but dont even know how to make it...

the LEDs im getting is 1.8-2.4v, 20mA and im planning to wire at least 20-30 of them.

i find it really easy to just get a constant power source and just wire the LEDs in series. but i cant seem to find anything like 20mA at 5v-36v....


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## LED Boatguy (Sep 8, 2008)

New guy here. 

I used to make arrays of white LEDs ranging from 3 to 66 pieces. What I did was solder up 3 in series (you can do more for red) and then parallel them. For the 66 units, I just used a CC linear power supply with a sense resistor set to 450ma, which gave each series (22 in this case) ~20ma. Works like a champ and have hundreds of hours on them without a failure.

Also, find out the REAL Vf of your LEDs and make sure your series Vf is below 12v minus your regulator's dropout (say 1.5V or .5V for LDO) minus .7V for the series diode.

Automotive electricity is NASTY, so be sure to add the protective stuff listed in a post above. Lastly, 5mm LEDs are very prone to ESD damage. If you don't have one, get yourself a good quality grounding strap and use it every time you're near those things.

Good luck.


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## Budman231 (Oct 15, 2008)

I did an array of 48 5mm LEDs powered by a 12v battery. I wired them 3 in series (3.4vf each) x 16 rows in parallel with a single 1/4 watt resistor on the negetive side. Works great...!


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## chainrash (Jun 24, 2016)

This post is pretty helping in finding how to power and drive 5mm LEDs. Also they include a resistor calculator on their site!


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