Why my did my led blow?


I made a fet driver from a nanjg 105c for in my SRK. When i attached it with 4 ncr18650a batteries, but only 1 xml LED for testing, the LED blew instantly (ie, about 1/10th of a second?). When I then connected another LED to the driver, but with 1 battery only, it was fine. Did the LED blow due to 4 batteries? I wasn’t expecting this would be a problem.


FET = electronically controlled switch, more or less a direct wire that is opened and closed like a relay

With 1 battery it pushes many amps, with internal resistance it can push alot of amps but starts heating up and sagging voltage/current wise

If you have 4 batteries in parallel now they can share the load, thus sag slower and keep pushing more amps to the emitter…eventually it just can’t take it

Measure the current being pulled by 1 battery, then just imagine what it would look like if it was almost 4x as much (electronically speaking it wasn’t but the capability of MUCH higher sustained current is there)

Was the emitter on an aluminum star or copper direct thermal path star (sinkpad/noctigon)

That is a very simple explanation, the actual readings and whatnot and calculations for current flow, resistance, and the technical aspects fill many binders

Were the batteries connected in series or parallel? You didn’t say. Warhawk assumes parallel and says that the extra current draw will overwhelm the emitter. While that is true (or at least can be) I don’t know that that is what happened. To me, it sounds like you had it in series, because you said the LED blew instantly (almost). While LED’s are not really sensitive to high voltages directly, they will over-current instantly when high voltages are present without current limiting of some sort. Of course, we’ll know better when you tell us whether you had series or parallel batteries. Bottom line is that without current limiting, both can cause failure.

If the LEDs were stock mounted on a single aluminum MCPCB, the traces probably burned out. They can’t handle much more current than the stock driver provided. With a FET driver, they should be mounted on Noctigons.

Thanks for the suggestions. Yes the batteries were in parallel. The led was on an aluminium star in air. I intend to mount the leds on sinkpads for the final setup, but I assumed an led could handle a second or two for testing the driver without any good mounting. But I guess not. I stated the led on one battery was ok, but realise this was because I was testing it in that instance externally from the torch with alligator clips. I think this limited current. When I tried one battery mounted correctly with the driver in the srk the next led was smoking almost instantly. Thanks again.

An SRK utilizes parallel cells.

I’ve burned up the traces in at least three of those boards. They can’t handle any current over stock driver.

These are high power LEDs and direct drive drivers, never power this stuff up unless the LED is mounted to something. Bench testing with a single cell is fine, you don't need to run it on all 4 unless you're taking measurements, and if you're doing that you need to be using the LEDs mounted on the MCPCBs that will be used in the final build. LEDs on copper are better cooled than when on Al and at different temperatures the LED will have different voltage/current curves (in other words, you won't learn anything of value by testing with different parts that what you plan to use in the light).

Actually I believe Djozz found thru his testing the direct thermal aluminum and copper both excelled over stock until the current levels in the LED went well above 5A, then from there copper pulled ahead slightly up to 7A I believe

Stock stars have the PCB substrate UNDER the emitter, the heat has to force it’s way thru that substrate, direct thermal the heat immediately flows to the larger metal mass and spreads out to then flow to the pill (P.S. this was for the OP not for comfy…we KNOW you know what you are talking about :wink: )

Yeah, I knew this answer was coming. But, he didn’t at first say that he was testing it inside the SRK, so I was just checking to be sure. All moot now, it was parallel. But the second LED smoked too, after removing some resistance from the circuit, using only one 18650. So series or parallel doesn’t matter. A single cell was enough to blow it.

if the led blew instantly it did not matter if it was on copper or not, the current was just too much. Was it a XM-L2? I guess 4 NCR18650A cells parallel could keep 4.0V or so under 1.5A load, for 6A, a XM-L2 should handle that, I'm not sure about an XM-L however, never tested that at 5+ amps, neither did match.

It was an xm-l on aluminium board as pulled from a uf-t20.