LED Lighting

I know a guy who just built an LED light fixture. It consists of nothing more than a single off the shelf adjustable current/voltage supply and three strings of LEDs connected in parallel.

He turned it on and it seems to work, but I would have thought the voltage drop in the individual strings might vary enough that one string would take the bulk of the current. And if one LED should open, the current in the remaining strings would go up 50% until the whole thing went dark.

Is that an acceptable practice for driving LEDs? Or is he likely to have problems in the long run? (They're all on a single large heatsink)

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Mark
Reply to
Mark Storkamp
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I would place a current limit resistor in line with each and every LED transducer (device). The price of one resistor to protect each one is worth it. It also equalizes what they get fed from 'the rail'. so single failures impose no change on any other alternate elements.

Well worth it, and the cheapest watchdog you'll find. Then, one can craft (or buy) individual "POL" circuits for each bank you mentioned.

Reply to
DecadentLinuxUserNumeroUno

LEDs are ohmic enough that you can usuallly drive them constant-voltage and get away with it. If the supply is run in constant-voltage mode, one string can open without affecting the other. In CC mode, it will have the problem that you note.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Surely makes more sense to put them in series with a current limited psu. Voltage drive does work ok with some LEDs.

NT

Reply to
meow2222

Doesn't the forward voltage drop as the temperature increases? (I know nothing about LED lighting.)

George H.

Reply to
George Herold

It's a common use as long as they're running at the same temperature. Once 4 or more LEDs are in series the forward voltages in each string start to match almost perfectly. Strings of two LEDs match well enough for use at reduced power.

So far I've never seen an LED go open-circuit. They usually short, causing the opposite failure where the string with n-1 LEDs gets all the current and unsolders itself. There are various tricks to avoid failure, but those failures are too rare to worry about for hobby use. Make sure it can't catch fire and call it done.

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Reply to
Kevin McMurtrie

That is not good practice. I have built several LED retrofits, and all of them are single strings of 1 W LEDs with a constant-current power supply.

Jon

Reply to
Jon Elson

I'm guessing that LEDs act like other diodes: negative TC at low currents and positive TC at high currents. I don't have any data sheets that confirm this.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

LED traffic signals frequently show open strings.

Reply to
John S

Most of the LED strings are composed of three LEDs in series with a resistor for 12V. The string is a number of such "modules" connected in parallel. So what exactly can go wrong with connecting many such strings in parallel? They have nominal voltage, not nominal current specification. CC is necessary for bare high-power LED dies.

Best regards, Piotr

Reply to
Piotr Wyderski

In this case, no resistors. The voltage is around 50-60V as I recall and the power supply is running as a current source.

Reply to
Mark Storkamp

That's the part I wasn't too sure about. Interesting to know that they will match so well.

I've seen many dead LED segments over the years, but I've never checked them to see if they opened or shorted. I do see quite a few tail lights with a string out. I assumed they opened, but I really don't know.

Reply to
Mark Storkamp

I'm pretty sure it's in CC mode. Somewhere around 50-60V. If exposed to a wide enough temperature swing (outdoor lighting) could the forward voltage drop vary too much to even use CV?

I get the overall impression from all the responses that it will work, but that there are better ways to do it.

Maybe he'll get away with it, but I don't think I'll have him make one for me.

Reply to
Mark Storkamp

If it is a series string, a shorted LED will not cause the string to go dead.

Reply to
John S

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