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How I determine the resistor I have to use with a group of led connected in parallel? Answered

I know a formula like
Resistance = (Source Voltage - LED Voltage) / (LED Current / 1000)
but can I apply it to leds in parallel?

Discussions

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bwrussell

4 years ago

Lots of (good) warnings but lets talk possible solutions.

If your dealing with more than a few LEDs the best way to wire them is a series/parallel array. You put strings of series wired LEDs in parallel. Each string gets its own resistor. You can use THIS simple tool to help you build a robust array.

If you really need all the LEDs in parallel the you need to use your formula (which is Ohm's Law btw) but put a resistor in series with each LED.

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radiosoundbwrussell

Answer 2 years ago

Right....short on solutions for my question. I registered with Instructables in order to get some opinions about my Chinese flashlight, but have not received any answers, maybe since I don't yet know how to use Instructables.

The Chinese marketed a flashlight with multiple straw hat led's in parallel, many of which have failed. I'm wondering if that direct paralleling (no resistors) with applied voltage of 4.5 volts was for higher brilliance though reckless, because many of the led's have failed. I want opinions about what led's I can use to replace those, with ot without resistors.

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radiosoundradiosound

Answer 2 years ago

I'm (Radiosound) "replying" to my own question here, since I've been asked to publish or report, but really don't have anything new to address my original question about parallel leds. Though I do believe the answer(s) by bwrussell are good ones and could be done, in my case of a chinese flashlight with multiple (23 total) leds it would be more work than the thing is worth because of throwaway technology and cheap chinese imports. So I decided to simply replace one independent group (7) led's with super bright leds bought on eBay just to see how long they would last compared to the original chinese installed ones. That section of the flashlight is now working well with all the new super bright led's and appears to be relatively dependable in spite of the overvoltage. If those replacement led's do start to burn out however I can always go to the resistor solution that I'm sure is technically sound, or buy another flashlight. Thanks again bwrussell!

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bwrussellradiosound

Answer 2 years ago

Wiring solely in parallel requires the lowest voltage of any configuration. The trade off is that you burn more amps, lowering battery life. It does give the added advantage that if one burns out the rest keep working. The reason they burn out though is because of the lack of resistor. They most certainly are overvolting those LEDs, typically white LEDs are closer 3-3.5V, to 1) save cost and 2) they will appear brighter, for a shirt time before burning out.

You have three options to fix it

1) Just slap any LEDs in there and keep replacing them when they burn out.

2) Find a LED that has a forward voltage of 4.5 or higher and replace all of the LEDs with it. This is still a little rough on them but they should last longer.

3) rewire. Whether you simply add resistors to each leg to cut the forward voltage at the LEDs to around 3 or create an array on series and parallel (with resistors) doesn't matter. This is the most robust solution.

Optionally you could replace the entire system with a single Hugh powered LED.

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steveastrouk

4 years ago

So the answer is basically "No, you can't apply the formula to parallel LEDs"

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iceng

4 years ago

Steve and Seandogue are counseling you with wise advice !

Perhaps you saw the first schematic where one resistor feeds 8 LEDs,

this only works if the LEDs turn on ONLY one at a time.

The last two pictures show what Rally happens when you PARALLEL LEDs,

They do not light evenly some are brighter others are darker BAH....

4017x8x1R.gifLed-Box3.jpgLed-Box1.jpg
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steveastroukiceng

Answer 4 years ago

We had a cheap camping LED light, which failed in spectacular fashion entirely because it was constructed of parallel LEDs, and I put fresh batteries into it in the summer.

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icengsteveastrouk

Answer 4 years ago

Good Point !

IF the LEDs are far enough apart in their conduction curves,

One LED will hog all the current, over heat and burn up, usually taking itself out of the circuit which then forces its mate to repeat the same failure flash !

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steveastrouk

4 years ago

No, don't do LEDs in parallel, to reinforce Sean's comments. It CAN sometimes be done by manufacturers who are very cheap, and prepared to accept inevitable failures, and are also able to source large batch quantities of LEDs made from the same material.

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seandogue

4 years ago

It is generally not a good or sound idea to design an LED circuit that uses a single limit resistor for compensating parallel LEDS, since each diode (and led being a diode) has a unique forward voltage and associated forward current, and especially since most who use instrucables tend to operate LEDs at or very near to the maximum limits for forward current (ie 20mA for a max 20mA LED, etc.) .