Damn, burned out my 100-watt 50-ohm resistor!

Winfield Hill · 2016-08-18T18:04:56+00:00

A major meltdown last Friday for my 8ns 1kV SiC MOSFET pulser. After set of tests at 400V and 4MHz repetition rate, with everything working fine, I had to increase the frequency. A loose hand slip and I was at 10MHz. That's when the smoke appeared. Blew out a 1-ohm gate resistor, both TO-247 MOSFETs, both 24V gate drivers, the flying regulator and the 150Mbps 50kV/us isolator, and a 0.1-ohm current-sense resistor! Plus a scorch damage region on the PCB. In the painful process of getting it going again took out another gate driver. (No more stock now for those.) So yesterday after Tuesday's rebuild, I took it slow and easy, checking temps with the Flir IR camera as I went, looking for the operating limits. Can do 10MHz in long bursts, check, 8MHz continuous with HV off, check, 5MHz at 500V, nice. 500V, 5MHz, 5A square waves anyone? OK, saved scope screen shots, turned everything off, went home. Now I find the burned out 100-watt 50-ohm non-inductive resistor, mounted on 100W heatsink + fan. (Need 50-ohm back termination to drive output coax.) Sheesh, but thankfully my 150-watt attenuator is OK. Those are expensive, even on eBay! -- Thanks, - Win

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Tauno Voipio 9 years ago

I had a similar thing with two parallel groups of 1 kohm 2 Watt carbon composition resistors. There was 40 resistors in a group, and two groups in series. I used brass sheet instead of FR4.

-Tauno Voipio (OH2UG)

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Jim Thompson 9 years ago

Yep. It was an 8 x 5 array spaced far enough apart to allow good air flow. ...Jim Thompson

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Cursitor Doom 9 years ago

I housed mine in a bean can (Heinz). Carefully removed the label, topped up with oil, soldered the lid back on and replaced the label. Dead flat VSWR up to about 7Mhz. This was immediately before I learned the importance of minimal lead length. :-/

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Jim Thompson 9 years ago

You used the wrong bean can, should have been Bush's ;-) ...Jim Thompson

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Joerg 9 years ago

The only 50ohm resistor I had that could really withstand power was the one I built myself. Specially shaped copper structures to keep it a real

50ohms even at hundreds of MHz, lots of 2W resistors, then the whole thing submerged in tractor oil. It lasted 30+ years and had to take the occasional kilowatt. A few years ago I had to scrap it because the bucket it was house in (a huge metal honey bucket from a bee keeper) began rusting out and leaking.

Regards, Joerg http://www.analogconsultants.com/

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Damn, burned out my 100-watt 50-ohm resistor!

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