Overdriving/underdriving LEDs

[ALToronto]

What happens if I run too many LEDs on a constant current driver? Or too few?

I have lots of 5 watt LEDs, and drivers that are rated to run 7 pairs (7 parallel branches of two diodes wired in series). Voltage output of the driver is 27-36 vDC, current is 2100 mA. Each diode is rated for 15-17.5 v, 300 mA.

I need to set up two light fixtures at opposite ends of a room, and 7 pairs doesn't split nicely between them. What would happen if I put 8 pairs on this driver? Or if I set up two drivers, each with less than 7 pairs?

[Orchidgirl83]

I asked my electrician husband and here is what he says:

If you put an eighth diode, it will cause the circuit to trip, as that would take the driver over the maximum voltage. Splitting them into less than 7 pairs, should be safe.

[DelawareJim]

Overdriving the LEDs is not good for them. It shortens their life.

Cheers.
Jim

[ALToronto]

Originally Posted by DelawareJim

Overdriving the LEDs is not good for them. It shortens their life.

Cheers.
Jim

Thanks Jim, that's what I thought. So with a constant current driver, I would be overdriving them if there is too few diodes, correct? And underdriving (too many diodes) would not be harmful?

Is there a significant drop in brightness if I underdrive them? Is it a linear relationship?

Orchidgirl83, with all due respect, I think your husband is mistaken. The driver determines the circuit load, so it takes household power and outputs DC power at 300 mA, around 30 V. So the number of diodes downstream of the driver doesn't affect its output. It does affect the longevity and brightness of the diodes.

[DelawareJim]

Al, under driving them shouldn't harm the LEDs as that is basically what dimmers do. The only reason I know of that there are special dimmers for LEDs is that regular dimmers actually "leak" a little electricity which is just enough to fire up an LED, but not enough to light an incandescent or florescent bulb. When my electrician installed a new dimmer for an LED spotlight over the kitchen sink, he explained that the special LED dimmers actually open or "break" the circuit so they don't leak. That's it. Expensive lesson when the LED wouldn't completely shut off and flashed like a strobe light.

Yes, there is a drop in light output but I don't know how much or if it's linear.

Cheers.
Jim

[Nexogen]

Originally Posted by ALToronto

What happens if I run too many LEDs on a constant current driver? Or too few?

I have lots of 5 watt LEDs, and drivers that are rated to run 7 pairs (7 parallel branches of two diodes wired in series). Voltage output of the driver is 27-36 vDC, current is 2100 mA. Each diode is rated for 15-17.5 v, 300 mA.

I need to set up two light fixtures at opposite ends of a room, and 7 pairs doesn't split nicely between them. What would happen if I put 8 pairs on this driver? Or if I set up two drivers, each with less than 7 pairs?

8 pair on one driver?
8 pair goes well, about 260mA per LED.
by 2(pair) in series and in parallel the 8 pair





I do not recommend.
If a LED breaks, there is a risk of flowering...

[naoki]

Most of the time, people who is interested in efficiency runs them at much lower current than the nominal current. For example, the nominal current of Vero 29 in the data sheet is 2.1A, but a lot of us uses 1.4A or 1.05A CC driver. Some crazy people use it even at 700mA or 500mA to get extremely high efficiency for a bragging right. With these DIY underdrived LEDs, you can almost always get better PAR efficiency than commercial fixtures.

If you underdrive, you get higher efficiency (more lumen per given watt). But the total output is smaller. So we need more diodes. It is the balance between initial cost versus efficiency again. This is why efficient fixtures are expensive.

So 8 parallel circuits with 2 serial LED / circuit is better way to go (I know that you don't care too much about efficiency). Each is receiving 262mA in theory. So if you use the same driver and compare the total output of 8 parallel vs 7 parallel, you'll get slightly more with 8 parallel due to higher efficiency.

But most of us won't run it in this way (or take the risk). I'm not sure the spec of your LEDs, but the maximum current is usually 2x the nominal (600mA). But if it is the Chinese diodes, their maximum current is lower than this (and they can't handle overdriving too much). So if 4 diodes die for some reason, each is going to receive 525mA, which is pretty close to the max, and there is a chance for the LED to burn.

Now in practice, you have to measure the current of the driver. Sometime 2.1A driver may be actually giving only 1.8A if it is a cheap driver. Also, with cheap LEDs, there are variation in forward voltage. So it is likely that current hogging happens (one of the strings use up lots of current, and become much brighter than the rest).

[ALToronto]

Thanks Naoki, that's very helpful.

Of course I care about efficiency, but I'm not willing to sacrifice aesthetics for its sake. So if 2 more diodes (which cost me $2 each) will make the circuit more efficient, I'm all for it. But really, it's being able to split the fixture in a more convenient manner that is making this alternative attractive.

I'm not worried about overdriving - I look at these lights every day, so even if some diodes fail (and they usually fail one pair at a time, so far due to wiring rather than the diode itself), it will be at most a few hours that they will see the higher current.

These ones aren't so bad, they've served me well since last April. As I've said in other posts, the living wall with these lights is the only one that has orchids in bloom. An older wall, with special, expensive LED grow lights has lots of great roots and leaves, and no flowers. That's why I'm a little jaded about the so-called 'special' grow lights. They're not any better than my DIY fixture.

[Nexogen]

Best would be to use 300mA drivers but you can use 2100mA driver.
If you have a good dissipation temperature, for approximately 260mA (+/- QmA) risks are minor.
In fact, almost all companies put on chip LEDs in series and/or in parallel.

[naoki]

Alla, you already have the LEDs, so you probably don't care about this: but Vero 10 would be almost the same price ($4 for at least 2x the performance), and it would be good for your type of use (many smaller LEDs to get even lighting, and optics required to narrower beam). It is highly efficient. And the new 2015 version increased the efficiency further. So if you have another project, you might want to give it a try. Very high performance/cost ratio.

It would be interesting to see the variation in Vf to see how serious the current hogging is the problem. It's not recommended, so I've never done in parallel.

Nexogen, yes some products use parallel circuits. Almost all array type diodes (e.g. COB, XF-3535L) use parallel and serial inside. But my understanding is that they try to minimize the variation (e.g. use emitters from the same wafer or same lot). Most LED strips are designed to be for parallel. But they contain the balancing resistor for each module, I believe (and I think that this is a part of the reason they can't be as efficient as regular LEDs driven by CC drivers). With the modern manufacturing, current hogging may not be serious. Indeed, from my measurement of several Cree CXA3070, the variation in Vf doesn't seem to be huge. If you read Bridgelux Vero application note, they still don't recommend parallel circuit.