Is it possible to overburden a driver and still have it work?

[fry_school101]

I'm playing with some Chinese LEDs and drivers and I need someone to straighten me out. Supposedly I have a 30w driver which delivers 2A at 16-18v. When I run one led it measures 15.9-16.1v of drop and 2A of current. If I understand correctly 15v * 2A gives me my 30W right? I can hook up four LEDs in parallel and my voltage drops to around 15.0V but the current still says 2A. Am I measuring something wrong or is this possible? Does it just mean that the driver is capable of more than the 30w since everything is still running? How can my voltage drop a little with each additional LED and still have everything running at 2A? Is it possible to test the actual output capability of a driver?

[Origami]

I'm playing with some Chinese LEDs and drivers and I need someone to straighten me out. Supposedly I have a 30w driver which delivers 2A at 16-18v. When I run one led it measures 15.9-16.1v of drop and 2A of current. If I understand correctly 15v * 2A gives me my 30W right? I can hook up four LEDs in parallel and my voltage drops to around 15.0V but the current still says 2A. Am I measuring something wrong or is this possible? Does it just mean that the driver is capable of more than the 30w since everything is still running? How can my voltage drop a little with each additional LED and still have everything running at 2A? Is it possible to test the actual output capability of a driver?

Yes, Voltage * Current = Power. Thus, 15v * 2A = 30W.

 

Where are you measuring the current when you hook the LEDs in parallel? If you're measuring it before the split or after recombining the strings, then you're getting the total power dissipated in all four strings of LEDs.

 

The driver is unlikely to be giving you more than the rated output. Do you have a datasheet on the driver?

 

Are you using some sort of (LED) diode array? The reason I ask is that 15 volts across a single LED is excessive. The drop is more in line with the drop that I'd expect across four serial LED's.

[icecool2]

Current is through the whole circuit. You want to control the voltage to match the forward voltage needed by your circuit. That can be done through either voltage or current control.

[Origami]

How can my voltage drop a little with each additional LED and still have everything running at 2A? Is it possible to test the actual output capability of a driver?

The driver is a constant current source. Using feedback, it adjusts its output voltage (within its control range) to the load to try to provide a certain amount of current.

 

When you say "output capability," what do you mean? It's a 30W driver. The data sheet, if one is available, should be able to tell you the max and min voltages are within its regulated range. Given a particular voltage drop across an LED, this should give you an idea of how many LED's can be driven. Roughly speaking, you should be able to drive somewhere around 10 3W LED's.

[fry_school101]

It is one of those big led chips the have 100 or so diodes on them. The spec for the driver says 30w 16-18v. The chips are running 16v so I should only be able to run one, right? How is it that 4 will light then? Could I be taking measurements at the wrong points?

[Origami]

It sounds like you're running them in parallel and splitting the current between four devices. Is that what you're describing? What is the rated wattage on one of the 100-chip arrays?

[fry_school101]

Yes, that is correct. It is one of these guys: http://dx.com/p/30w-2400lm-led-emitter-metal-plate-warm-white-16-18v-47262?item=8

 

It is rated for 30w

[Origami]

I suspect, then, that you're splitting the current between arrays and driving each array with less current. That results in less light output per device. The total current supplied by the driver remains constant, though. If you were to measure the current going through each string (array), you'll find that it's being split and is less than the total.

 

I don't have the datasheet for the device that you've shown (in fact, it's unclear whose product it is, but I suspect that, given a search that I just performed, it's from China. But, let's use a Cree XP-E LED (single device) as an example. The datasheet for this LED is here:

 

http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/Data%20and%20Binning/XLampXPE.pdf

 

The plot at the top of page 10 will show you what I mean. It's a plot of luminous flux (light output) versus current. From the plot, you'll see that with increasing current you get increasing light, up to the rated current for the device. You can see that you still get light output from half the current, even a quarter current, but that the luminous flux decreases accordingly.

 

So, if your driver is supplying a total of 2 Amps to 4 strings (arrays) wired in parallel, then (assuming each array is identical), the driver output is being split into 4 paths of 0.5 Amps each.