Unknown short in an MC-E mod...

[prayzrofGod]

Hey CPFers.:wave:

I'm currently making a 4 MC-E mod of a WP-800 from DX. I'm mostly just upgrading the XR-Es to MC-Es and changing out the driver board. It's currently planned to be a 4s4p rig with either 2x 18650 or 4x rc123s depending on the driver I end up using.

At this point I've removed the old leds, mounted the new ones, and wired them up. When I tried to test the mounts, I found that DDing the 4 MC-Es with an adequate voltage leaves one of the MC-Es not lighting up. I figured it was a short or a bad led so I dropped the voltage and tested each led individually. All MC-Es light up the night.

With these encouraging results (I now know that all MC-Es work), I tried all 4 again. Yet again the 4th MC-E in series refuses to light up and the other 3 aren't at full power. That tells me it should be a short so I tested each of the combinations of connectors starting with the legs of the malfunctioning MC-E and moving out at each connection point.

All legs, solder joints, and wires work without problem. The only time that the MC-E shorts is when I send the voltage through another led. I've checked for wires touching the heatsink, the mcpcbs, or other joints.

I've gone so far as to remove all the leds and remount them. It seems to be the same board, not a specific led.

PLEASE HELP ME!! I am sure this is a SIMPLE issue, but I'm at a complete loss for a fix. Any advice would be greatly appreciated.

 

[Morelite]

How are your MC-E's wired? Edit: I just seen that you using a 4S4P setup.

With your setup powered up measure the voltage across each LED connection, I guessing you will find that you don't have enough voltage at the dead LED.

 

[prayzrofGod]

That's the thing though. I'm using a ~14 supply for the moment to find the problem. That's 3.5v to each MC-E. On top of that, I forgot to mention that I don't get any voltage reading from the malfunctioning MC-E. Edit: when all 4 are being run together.

 

[Morelite]

I take it that the four dies in a single MC-E are your parallel circuits and the four MC-E's make up the series circuit and it is the last one (closest to the positive lead) that doesn't power up.

 

[Morelite]

That's the thing though. I'm using a ~14 supply for the moment to find the problem. That's 3.5v to each MC-E. On top of that, I forgot to mention that I don't get any voltage reading from the malfunctioning MC-E. Edit: when all 4 are being run together.

Now measure the voltage across the whole setup. What current are you providing?

 

[prayzrofGod]

The supply is ~14v as I said in the last post. I measured ~9.54v across the system. In an attempt to see if I was really dropping that much voltage (which is conceivable since I'm using batteries not a dedicated psu) I also tried the same supply across two of the leds, one of which being the shorting led. I got 5.something volts across the system and 2.75v across the working led. Yet again, no reading on the dead led. (edit)The odd thing is that I am direct driving these things. I don't have any driver/resistor to limit the pull from the batteries based on what is connected to it.(edit)

It seems to me like the dead led is acting like a resistor almost. Taking the voltage but not sending it to the dies. I don't profess to be an expert so I could be mistaking the effect. I thought that a short would send the voltage throught the system, not eat it...thereby lowering the overall output. (edit) I mean, shouldn't a direct drive 14v power supply send 14v (minus inefficiency) through the system?(edit)

I am thinking the problem must be in the mcpcb, but what's really bugging me is that it only shorts when in series with other leds. It works great when powered on its own.

(edit) Please ignore these statements, I realized that my batteries are being limited by their ability to supply current. Hence the system dropped voltage when I didn't change the battery voltage. Speaking of which...I'll be right back...(edit)

 

[prayzrofGod]

Thanks Morelite. I figured it out. Turns out that I forgot that my batteries couldn't supply the 2.8a needed to direct drive an MC-E. I had the voltage figured but not the amperage. (Though I wish I had...I can't type to well after just blinding myself by looking into two fully driven MC-Es.:candle::naughty

This is why I like forums. Different perspectives.:twothumbs