LED in Series

[martinmcnally]

Please forgive my dumbness in the field of electronics. Looking for some advice or someone to point me in the right direction.

I have a circuit of LEDs and they must be in series as follows:

**broken link removed**

This all works great unless one of the LEDs fails and then they all stop working by nature of the series circuit.

So my question is can I simply by pass each of the LEDs with a resistor to overcome that problem?

**broken link removed**

Is this the right direction I should be going in?

Many thanks
Martin

 

[Veraxis]

Doing this would probably work, but, in my opinion, may not be the best solution. If one of the LEDs went out, the rest of the lights would just go dim, which would make it a lot easier to find the burnt out LED, but, that said, it would also increase the power consumption of the circuit to some degree, depending on the resistors you used. If you rigged the LEDs in parallel, the brightness of the LEDs would not change at all if one went out, but, again, the power consumption would be higher than just rigging them up in series. depending on what the application is, I would be inclined to go with running them in parallel if easy maintenance is your main concern.

 

[ronv]

How about a zener diode with a breakdown voltage above the Vf of the LED?

 

[ronsimpson]

Do not run LEDs in parallel they do not share current well.
The failure rate on LEDs is very very low.

 

[carbonzit]

Do not run LEDs in parallel they do not share current well.
The failure rate on LEDs is very very low.

Maybe you should tell that to all those multi-LED flashlight manufacturers, whose products you see everywhere these days. I don't think they have many problems with LEDs in parallel.

This is starting to look like another of those oft-repeated urban legends that people believe is true just because they're heard it so many times.

Show me a peer-reviewed paper that explains why this is a bad practice and I might believe you. Otherwise, I think this advice can be safely ignored.

 

[Diver300]

Maybe you should tell that to all those multi-LED flashlight manufacturers, whose products you see everywhere these days. I don't think they have many problems with LEDs in parallel.

This is starting to look like another of those oft-repeated urban legends that people believe is true just because they're heard it so many times.

Show me a peer-reviewed paper that explains why this is a bad practice and I might believe you. Otherwise, I think this advice can be safely ignored.

You are not going to find peer-reviewed published papers. This is engineering not science.

A lot of equipment is not well engineered. I have seen commercial products with LEDs that have failed within a few days because they paralleled LEDs and did not work out how the current would be shared. On other products the brightness LEDs was different because the current shared unequally.

It all depends on the slope of the I/V graph of the LEDs, in effect the resistance of the LEDs. Some power LEDs can be run at quite high current, and they have significant resistance, so that they are safe to run at voltages quite a lot higher than the voltage that they first conduct at.

For example, https://www.farnell.com/datasheets/457476.pdf is designed to run at 150 mA, and the typical graph (figure 3) shows about 3.4 V, and maybe 100 mA at 3.2 V

The forward voltage will change with temperature. Figure 9 shows that the voltage will fall by about 0.05 V if the temperature rises by 40 deg C. If you put two in parallel, the hotter one will have a lower voltage and it will take more current than the colder one. That can lead to temperature imbalance and thermal runaway, but it very much depends on what heatsink there is.

There is then the problem of any voltage drop in the wires between the LEDs. Obviously a small voltage drop can lead to a large difference in current.

Finally, there is the problem of forward voltage variation within batches of LEDs. Those ones show the typical voltage is 3.4 V but the maximum is 3.8 V at 150 mA. A quick look at figure 3 shows that if you feed one of those LEDs that has forward voltage of 3.4 V with 3.8 V, it will take 300mA. I'll guess that it will last for a second or two at that voltage.

If you look at lower power LEDs, the problem is much worse. https://www.farnell.com/datasheets/80864.pdf That is one of the few low power LEDs where the manufacturer has bothered to include a V/I graph.

It is very steep, in that the current shown in figure 2 goes from 10 to 20 mA over about 0.1 V. There is also a huge possible variation of forward voltages, as they quote 1.6 V to 2.4 V as the acceptable range. I realise that the actual variation will be a lot less, but without knowing what it is in practice, you can't go and assume that the variation will be less than 10% of what the manufacturer says it might be.

Now I accept that a torch, if well engineered, can parallel the LEDs. The heatsink characteristics will be known. The voltage drops will be considered, and the LEDs may be sorted before manufacture to make sure that the forward voltages are close together within any one torch.

I also realise that you could easily take a few LEDs, especially if they came from the same batch, and if you don't run them near their limit, and put them in parallel and it would work. That does not mean that you could do it every time, and it certainly does not mean that if you replaced one of the LEDs with one from another batch that it would continue to work. If you are making one-off projects, it is your risk and it's only a few LEDs, and that's fine. You are just trusting that the LED currents will be near enough the same. That is not engineering, it is luck if it happens to work.

This forum has a lot of readers who are learning the basic rules. If you stick to putting LEDs in series, the current in each LED will be the same whatever. That is a very simple rule, and a far greater proportion of readers can follow that one rule than can, or want to, consider variations of temperature, variations within batches and voltage drops that are needed to safely run LEDs in parallel.

The circuit diagram for LEDs in parallel does not show any of the considerations that are vital for getting the circuit to work reliably in practice. That is another reason why, even in the commercial environment, a designer would put LEDs in series. The designer can be sure that if the circuit is followed, the current in each LED will be the same. If the design has them in parallel, the circuit diagram has to be accompanied by pages of other rules about temperature equalisation, interconnection resistance and LED choice.

I've made lots of circuits with LEDs, but I have never been in a situation where the cost of working out whether they are safe to go in parallel is less than the cost of putting them in series. If I were a designer for a product that were to be made in thousands, I would do all the calculations and tests, and I might go for parallel LEDs. Even then, the increase in time taken to get the product to market might make that approach stupid.

 

[carbonzit]

You are not going to find peer-reviewed published papers. This is engineering not science.

[snip]

Well, thanks for all that.

With all that said, here's my take on it: I think that if one were designing a product to be sold by the thousands, one would certainly want to study what you've said, and the things you pointed to, very carefully.

However, if one is a hobbyist playing with LEDs, one could probably be a lot less rigorous and still get acceptable results. (Or not, but that's part of being a hobbyist, isn't it?)

Regarding strings of LEDs, it does seem that putting them in series is better, but if you have too many diodes to do this with all of them, or if you don't want the whole damn project to go dark if one of them fails, then it seems the commonly-accepted practice is to put series strings of LEDs in parallel. At least this seems to be the norm with the multi-LED flashlights that have more than, say, a dozen diodes. So this seems to be an acceptably safe practice. Provided, of course, that one keeps in mind the things you've brought up here.

For myself, if I bought all the LEDs in one batch, I probably wouldn't worry too much about running strings in parallel. But it would be a good thing to keep one's eye on.

 

[Diver300]

Well if you have LEDs in parallel, and one fails by shorting out, the whole lot goes dark. (And it is hard to find which one is shorted). If one fails by going open circuit, the current in the others will increase, and they will all fail one by one.

As others have said, LEDs are very reliable when treated nicely. I have no idea if they ever fail short circuit. I know diodes and transistors do.

Good designs are all about treating components nicely.

After my long post, I put ten surface mount LEDs in parallel and connected the lot to a 200 mA source. There is no visible difference in the brightness. They were straight off a the same reel, and they are close together on veroboard, so close to idea conditions. At lower currents, if I heat one with a soldering iron, all the others go out, but they all even out when I remove the heat source.

 

[Diver300]

Series strings of LEDs, each with their own current control or resistor, then put in parallel, is certainly standard practice.

 

[ronv]

Martin,

What is your voltage to the series string of leds? Do you have a current source or resistor in series with the string?
What is the Vf spec. of your leds?

 

[Veraxis]

Well if you have LEDs in parallel, and one fails by shorting out, the whole lot goes dark. (And it is hard to find which one is shorted). If one fails by going open circuit, the current in the others will increase, and they will all fail one by one.

Actually, that is not true. If one LED goes out by open circuit, the current in the others will not increase, as the total resistance is also increased proportionally.

 

[Diver300]

Actually, that is not true. If one LED goes out by open circuit, the current in the others will not increase, as the total resistance is also increased proportionally.

It depends on the supply circuit characteristics. A constant voltage supply feeding LEDs in parallel will result in the current in each LED staying the same if one goes open circuit. If a constant current supply is used, the current in the remaining LEDs will increase if one fails.

 

[Veraxis]

It depends on the supply circuit characteristics. A constant voltage supply feeding LEDs in parallel will result in the current in each LED staying the same if one goes open circuit. If a constant current supply is used, the current in the remaining LEDs will increase if one fails.

point well taken. I had not thought of that. I suppose that is an important distinction to make in this case.