Got a question about driving LEDs? (Another work in progress)

[(*steve*)]

OK, lots of people ask questions about driving LEDs. So here's a list of common answers and pointers to documentation that is useful.

But not any more...

Thanks to a lot of feedback and valuable assistance by many members, this has achieved a maturity that I think deserves a place in our tutorial section.

This thread remains for any comments you might have, and also because many existing threads refer to it.

 

[janagyjr]

Oh, in that case it's brilliant, hehe. I was just wondering what else could be added to it. Though if you set out to speak generally on the subject it's golden. Might want to lock the topic.

 

[barathbushan]

Here's a calculator for the resistor to be attached in series with an LED

http://home.cogeco.ca/~rpaisley4/LEDcalc.html

Hope it helps !!

 

[(*steve*)]

Might want to lock the topic.

No, and for 2 reasons. Firstly, I am under no illusions that my statement is the final word on the topic, and secondly, sticky threads are great bait for spammers.

Here's a calculator for the resistor to be attached in series with an LED

http://home.cogeco.ca/~rpaisley4/LEDcalc.html

Hope it helps !!

Thanks, I've added that one.

 

[sohaibzafarsandhu@hotmail]

its agreat material realy helped me alot....thumbs up....

 

[chelsea_fan]

Great work indeed

 

[duke37]

Hi Steve
Great exposition!
What is the purpose of the capacitors in output of the switch mode current source?
What happens if there is no load, do the capacitors go pop?

 

[flashdom]

Few suggestion on improving the thread:

More info about practical issues such as what power supplies/resistors/capacitors/constant current psu's etc. are widely available rather that just pure maths because if for example you calculate the voltage needed and it's 13 volts then it might be hard for a new comer to figure out which psu to get.

Better explanation of diagrams. Like duke37 asked about the capacitor, I don't really know what it's for either but seen it in many diagrams b4. Also I looked for 0.1uF capacitor and I didn't see anything even close to this value, so more info would be nice.

More info on complex/unusual setups as this is where people have problems most of the time. Like 3x3W led's or 12W led's

Mostly more info and some examples on how to actually wire the led's and constant current circuits especially the ones with unusual requirements, such as the 3 led psu problem (~3x3+4.25=13.25V this is just over usual 12V power supply, is there a way to still use the 12V supply such as by wiring led's in parallel?).

Hope this helps with the brainstorming

 

[flashdom]

Also, this is a useful LM317 calculator, maybe it's worth linking to it from the main post...

http://diyaudioprojects.com/Technical/Voltage-Regulator/

 

[duke37]

Steve
I think the thread is pretty good as it stands. It is difficult to give one explanation to those of very different abilities. It is often necessary to explain in several ways before the penny drops, when it does, it is very satisfying.
One of the problems is to explain that there are often many ways of doing the same job and compromises can sometimes give more simplicity with almost the same performance.

 

[(*steve*)]

Flashdom, I hear what you say.

The problem I have is that we need to target this at the correct level so that we can help the most people. If we get too basic, things get very tedious and more experienced people may skip over the entire post.

In many cases I have provided links to more extensive (and generally basic) explanations.

In particular, I have sources the images from other pages which have both good images, AND good explanations. I also provide a link to them. When finding images from the web, I was deliberately conscious of the need to link to the source and therefore ensure that the source was a good one (not just one with a convenient image).

I understand that the issue of mixes strings of LEDs is probably only dealt with implicitly, and I may revisit it at a later stage. If you can find a good reference to it on the web somewhere it will ease my task somewhat,

I have added that LM317 calculator, and I am also considering pulling all the links to calculators that I have here into one section to make them easier to find.

Thanks

 

[fellfrosch]

While I am new to dc circuits, I do understand everything you said in this post. However the one thing i have searched and searched for is missing.
If i run 6 leds in a series/parallel circuit (2 x 3) The wizard says: In solution 0:

each 100 ohm resistor dissipates 40 mW
the wizard says the color code for 100 is brown black brown
the wizard thinks 1/4W resistors are fine for your application Help
together, all resistors dissipate 80 mW
together, the diodes dissipate 408 mW
total power dissipated by the array is 488 mW
the array draws current of 40 mA from the source.

So using a AC to DC adapter (I think they are referred to as a wall wart) that outputs 12v, I would need 40mA output current. My question is what is the max current the wall wart can put out? Or in dc circuits does that not matter, as long as its more than what is drawn from the components?

I have many 12v 500mA and 12v 1A wall warts, but i dont want to mess things up by supplying over 10x the current required to power the led circuit. But I've searched for about 4 hours now with no one saying if there is a limit to how much current is supplied.

 

[(*steve*)]

Normally I'd move your question to another thread, but this comes up so often that I'm going to answer it here, and then consider adding a note to the article.

The current rating of a power supply is what it is capable of supplying, not what it will actually supply. A 12V 1A supply WILL supply 12V, and is CAPABLE of supplying up to 1A.

Using a 12V 1A supply to power something that requires only 40mA is fine because the current required is less than the maximum the supply is capable of.

For constant current supplies, the rules are different, but these are pretty rare, and do not breed in captivity the same way that voltage regulated power supplies seem to.

 

[fellfrosch]

Normally I'd move your question to another thread, but this comes up so often that I'm going to answer it here, and then consider adding a note to the article.

The current rating of a power supply is what it is capable of supplying, not what it will actually supply. A 12V 1A supply WILL supply 12V, and is CAPABLE of supplying up to 1A.

Using a 12V 1A supply to power something that requires only 40mA is fine because the current required is less than the maximum the supply is capable of.

For constant current supplies, the rules are different, but these are pretty rare, and do not breed in captivity the same way that voltage regulated power supplies seem to.

That is exactly the information i was looking for. Thank you so much steve. Finally a 5 hour long search ends.

HOWEVER... I forgot to ask another question with the last one, but I'll just post it as a new topic. Thanks again steve!

 

[grandsoleil]

Strobing an LED Anchor Light

ORCA makes a tricolor and anchor light which has the ability to strobe the anchor light. I am told it is done by reversing the polarity of the 12v power to the light (it uses only three wires for all lights!).

Does anyone know how to make a circuit that will do the same for my existing LED Anchor light?

Thanks.

I don't want to get into the issue of the value of strobe masthead for emergency use only. It is just highly recommended by experienced Pacific sailors.

 

[tecbrochuelectro]

Thank you (*steve*) for the great explanations about the LEDs !
It will be very useful for me and my project to make a Pac man with LEDs for my Pac-Man machine.
You can see Pac Man machine in (do I have the right to shout link to my general forum, with only about 4 members ? ) :

[link removed]
http://www.forum.tecbrochuelectro.com/viewtopic.php?f=11&t=75
pictures of the Pac-Man arcade will be there today or tomorrow.

 

[eem2am]

Steve , thanks, but there are a lot of companies out there who are putting power LEDs in parallel......and these aren't specifically matched LEDs...they are just normal power LEDs (eg XPEWHT) of the same type but not actively made to have matching Vf at the LED foundry.

......so are you sure that you can't parallel say 10 parallel strings of XPEWHT LEDs? (with no equalising resistors or current mirrors)

Look at a LED police siren.......you see loads of blue LEDs flashing away....i garantee you that its a simple SINGLE buck converter thats used straight off the car battery , and no equalising resistors!!!!, and all those LEDs are in parallel!!!

..they're in parallel strings of two-in-series............i tell you it is the truth!

Parallel LEDs, as mad as it sounds, is in use all over!!!

 

[(*steve*)]

Parallel LEDs, as mad as it sounds, is in use all over!!!

Have you looked at LED stop lights on cars? You'll find that you can spot the older ones which have several dead LEDs in them. The cause? They've all been put in parallel.

It may be possible to get away with LEDs in parallel if you have them all very well thermally linked. Even so, it is not I who say not to do it, it is the manufacturers of these things.

Just because you see people saving a little money by omitting parts, it does not make it good engineering.

I would prefer to think that LEDs in the police lights were all in series and powered by a boost SMPS. (but I don't know).

If you can find a manufacturer of LEDs (via a spec sheet or applications note) that suggests wiring in parallel without any form of current sharing device then I'll gladly alter the sticky.

 

[CocaCola]

In addition to what Steve said, all you have to do is frequent a few 'car' forums to see just how poor quality a majority of the automotive LEDs replacements are... Just because it works (under ideal circumstances) does not make it proper or good engineering as Steve stated... Car forums are littered with people griping about their LED replacement bulb lasting 6 month or a year and wondering why it didn't last the advertised thousands upon thousands of hours, but actually had a life significantly shorter then the traditional bulb they replaced...

 

[raultiger]

Thanks for the guide!

Im still a little confused about one thing though. I need to run and LED off 120v wall power.
How can I do this?

Thanks