LEDs in parallel

[abenn]

I want to run 4 LEDs together as a single strobe light driven by a single driver. For the purpose of this discussion, assume the driver has the capacity to take the combined load.

Normally each LED would have a resister in series to limit the current. Apart from mechanical considerations (layout, component count), is there any reason why I shouldn't just parallel all four pairs of leads together and then give the assembly one common resistor?

All I can think of is, "what if one LED fails?". But I'm sure that wouldn't have a catastrophic effect on the current going through the remaining 3 LEDs (we're talking about a 5v supply), provided the failed one is replaced reasonably promptly.

[ShokWaveRider]

I want to run 4 LEDs together as a single strobe light driven by a single driver. For the purpose of this discussion, assume the driver has the capacity to take the combined load.

Normally each LED would have a resister in series to limit the current. Apart from mechanical considerations (layout, component count), is there any reason why I shouldn't just parallel all four pairs of leads together and then give the assembly one common resistor?

All I can think of is, "what if one LED fails?". But I'm sure that wouldn't have a catastrophic effect on the current going through the remaining 3 LEDs (we're talking about a 5v supply), provided the failed one is replaced reasonably promptly.

Most LEDS can handle 20ma or so before blowing. So 1 LED would require 250 ohms min for 5v. 4 would need approximately 125 Ohms. Most LEDS work well on 6 - 10ma So You could get away with one 250 Ohm resistor. This assumes all LEDS are identical. If any blow the others should get dimmer not brighter as LEDS do not often short, so the resistor should not get snarfed not that it would at 6 or 3ma. :)

I would use 4 separate 1/8th watt 500 ohm resistors. they are small and can be soldered in such a way that they are no larger than the led and their leads.

SWR

[Kenny Fong]

You certainly can use only ONE resister in the joint line of 4 parallel LEDs.
Just select a resister with higher Watt......
Given the 5v supply voltage, the resister can be 30om......

[fredhs]

Hi,

provided the LEDs are absolutely identical (same batch etc.) you might get away with it, although you have to accept the risk that the LED with the lowest voltage receives the total current, burns out, the LED with the next lowest voltage receives the total current, burns out, ...

Don't do it unless space and/or weight constraints dictate the above approach. What color LEDs? Unless you use high intensity white or certain blue LEDs, you could put 2 in series (with one resistor in series) and run them off your 5 V supply, i.e. you would only need 2 resistors, and the whole setup would be perfectly safe.

Regards,

Fred

[abenn]

Thanks, guys. I'm using 3mm high intensity white LEDs which were all purchased together so I assume they're from the same batch.

But I think I'll keep things basic by giving them each their own resistor; as shokwaverider has said, 1/8 watt resistors hardly make a bump in the shrink-wrap.

Memo to self: Remember to double-check the current capability of the driver :)

By the way, shokwaverider, I thought I should need a smaller value resistance (maybe higher watts) with 4 in parallel, since I'm trying to pass 4 times the current of a single LED. Did you mean to say 100 ohm (or so) rather than 1k (compared with 500 ohm for each individual LED)?

[fredhs]

... I'm using 3mm high intensity white LEDs which were all purchased together so I assume they're from the same batch. ...


Wild guess: Vf at around 3 point some Volts. If 5 V is all that's available, you should definitely go for 4 LEDs and 1 resistor each.

Fred

[ShokWaveRider]

Thanks, guys. I'm using 3mm high intensity white LEDs which were all purchased together so I assume they're from the same batch.

But I think I'll keep things basic by giving them each their own resistor; as shokwaverider has said, 1/8 watt resistors hardly make a bump in the shrink-wrap.

Memo to self: Remember to double-check the current capability of the driver :)

By the way, shokwaverider, I thought I should need a smaller value resistance (maybe higher watts) with 4 in parallel, since I'm trying to pass 4 times the current of a single LED. Did you mean to say 100 ohm (or so) rather than 1k (compared with 500 ohm for each individual LED)?

Yeah sorry, I was thinking of something else I was designing with LEDs when I wrote that. I changed it to the correct ones. I would see what is the least ma you can get away with using the trial and error method. Even 250 ohms give 20ma. (Very Bright). I run most of my LEDS at 6ma. Again it depends on the type. White burn brighter at less ma as do Blue. at least the ones I have do. I use 1.8mm LEDS for everything as they are small and easy to work with. Here take a Look below..... LEDS never photograph well but you will get the idea. The last 2 in the first photo are bright Blues, these are running at approx 3ma. The other LEDS are ones I use Also.

Here is what the 1.8mm leds look in another application running at 6ma. I configured them as a boost gauge in a car.

Boost Gauge Video (http://www.impconcepts.com/vwpassatgauges/boostgaugevideo.avi)
SWR

[abenn]

OK, so my electronics theory isn't all that rusty ;)

Thanks a lot for those pictures. That boost gauge looks real neat. I've never come across the 1.8mm LEDs but, from looking at the picture, it seems like the shoulders on them prevent you from grouping them any closer than you can the 3mm ones. Am I right?

[ShokWaveRider]

OK, so my electronics theory isn't all that rusty ;)

Thanks a lot for those pictures. That boost gauge looks real neat. I've never come across the 1.8mm LEDs but, from looking at the picture, it seems like the shoulders on them prevent you from grouping them any closer than you can the 3mm ones. Am I right?

About the same. But they sit flat to each other. The I use a 0..67" drill for the dome and it sits snug. You can also get them with longer domes but I do not use those. I place them in my gauges quite par apart but on a nice surface they look sooooo Good. See below. I only showed the gauge so you could see how bright they are.

SWR