Test a diode?

Gata · May 9, 2009

Hey guys. I bought a GGW sled in scopeguy's group buy an planned on putting it in a Pocket Mini by jayrob. But after all was soldered and I got it into the host I turned it on and for a few seconds. Everything was working good for about 5 seconds when the diode started to rapidly get dimmer and dimmer. Eventually it died completely and I got pissed and started breaking things and yelling and...well you understand. I unscrewed everything and everything looked fine but I resoldered the connections on the driver anyway. So I powered it back up again and I got nothing. Anyway I want to know if there is anyway to see if a diode is even working without having to connect it to a driver and do all that jazz. I am assuming there wouldn't be but before I go and, out of uncontrollable rage, destroy it I want to make sure that was what went bad. Any help is appreciated guys. Thanks ahead of time.

scopeguy20 · May 9, 2009

Sorry not much help here, but I would want to examine it with a microscope just to see if I could spot anything. -Glenn

dr-ebert · May 9, 2009

Still not much help, but it does sound like far too much current (resp. heat)... I got similar behaviour from a red/IR diode off a PHR sled when running it with 65mA, without heat sink to boot (the diode is ~15mW AFAIK so max current should probably be around 30mA).

BuildAbluraylaser · May 9, 2009

Have you checked the batteries?
Were you using litiums or regulare alkaline ones?

rkcstr · May 9, 2009

Most diode failures I've seen involved a sudden drop in output, not a "dimming" drop.

To me, it sounds like batteries, but I can't say for sure. More info on your build would be helpful, ie batteries, driver, etc...

Gata · May 9, 2009

I have been using CR123 primary's that I know are good. I had a flexdrive set to 185ma and that shouldn't have killed a 6x diode would it? I was going to test the driver with a dummy load last night but I pulled my leads off by accident. So I will check that later this weekend. Thanks guys

rkcstr · May 9, 2009

Hmm.. well, not sure then. The reason it sounded like a battery is the "dimming" which could occur as the battery voltage drops below the driver's operating threshold. But, if you say the batteries are fine (and one 3.7V lithium should be adequate), then I'm not sure. I'm not entirely familiar with troubleshooting those drivers.

Did you remember to discharge the output caps on the driver before connecting the diode?

Testing with a dummy load would be a good idea before you try to power up the diode again. There are a number of things that could be wrong, so ruling out the driver would be a good start.

Gata · May 9, 2009

Discharge output caps? I had no idea I had to do that. Tomorrow I will solder leads back onto the driver and test it again.

When it did dim it went down to a very very dim dot and stayed there until I turned it off then it wouldn't turn back on. Not that that helps much but I am starting to think it wasn't the diode that died.

rkcstr · May 9, 2009

Yes, if you just short both output pads, that will dissipate any charge left in them. It's one of the things you should always do before connecting a diode (or possibly before disconnecting as well). Otherwise, any residual charge may spike into the diode and damage it.

BuildAbluraylaser · May 9, 2009

And it might be too late...

Gata · May 10, 2009

BuildAbluraylaser said:
And it might be too late...

Lets hope not.

dr-ebert · May 10, 2009

Also... a capacitor cannot "spike" (in voltage). During normal operation, the capacitor cannot charge to a voltage that would damage your diode. The only (minimal) danger is if you operate a high-V (e.g. blu-ray) diode with your driver and then directly connect another diode with a lower voltage drop (e.g. red); then you would get a short current pulse that (due to the low resistances involved) could damage your diode.

Since blu-rays are the highest voltage consumers you're likely to connect, the danger for them is effectively non-existent.

However, disconnecting the load from the flexdrive while it's on can damage it, since under open-circuit conditions the output voltage of the driver will rise as it attempts to get the current flowing; this might reach level where it damages itself. Even if the driver is ok, under these conditions, the cap might be charged to a voltage that will then kill the diode... the primary error would be disconnecting the load without switching power off first.

Bionic-Badger · May 10, 2009

He's not talking about spiking voltage, he's talking about a risidual charge on the capacitor rapidly discharging through the diode (current spike) when reconnected.

Gata, how many volts were hooked up to the Flexdrive in the first place? Also was the diode properly heatsinked? I've had diodes die a death like that when they burned up in operation.

dr-ebert · May 10, 2009

I know - still, it's not possible, except under the conditions I mentioned.

rkcstr · May 10, 2009

I'm sure you understand the physics better than I do, but that was just the general guidelines most agree on. Sure, it may not happen, but why take a chance? It takes a second just to make sure. And, who knows when someone here who isn't very experienced with working with these electronics does something unusual and creates a condition where it is more likely to happen?

So, it's just a "safety precaution" mostly. Not saying that it killed the diode because it wasn't done.

Certainly an intermittent connection between the diode and driver are more dangerous. The version previous to this most recent lavadrive had a zener on the output as protection for open circuit to prevent over voltage on the output, but I'm not sure what it employs now. Also, when you get an open circuit, I imagine you'll have a fully charged cap that will have no problems sending a rush of current through the diode when it is reconnected.

Gata · May 10, 2009

Bionic-Badger said:
He's not talking about spiking voltage, he's talking about a risidual charge on the capacitor rapidly discharging through the diode (current spike) when reconnected.

Gata, how many volts were hooked up to the Flexdrive in the first place? Also was the diode properly heatsinked? I've had diodes die a death like that when they burned up in operation.

I was using a CR123 primary giving out 3 volts. It was cooled with the heat sink the is included with a Pocket Mini kit.

Test a diode?

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