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

Back in my amateur radio days (K7ZAE, 2M, ~1964), I made a dummy load with two sheets of FR-4 and 40 2K/2W/Composition resistors in parallel between the two sheets (and a fan of course). Gave an outstanding good VSWR at 144MHz:

================= _|_ _|_ _|_ | || || | | || || | |_ _||_ _||_ _| | | | ================= ...Jim Thompson

Look at " RF Termination Microwave Resistor Dummy Load RFP 150W 50ohm 150wa tt G150N50W4B " on Ebay.

eBay item number:

Dan

Hey Dan, that's a simple dummy load, not a GHz bandwidth precision 20dB attenuator, with configurable connectors.

Yep. Mmodern RF power resistors are nice, my 100-watt TO-247 part is only $11 on DigiKey,

But as I said to Dan, I worried for my precision attenuator, those are invaluable in RF power work. E.g., $500 at Fairview Microwave for 150W units.

That will give you indigestion ;-) ...Jim Thompson

I am just confused. I thought you said your 100 watt 50 ohm load was burned out. And that your 150 watt attenuator was okay.

Dan

The two are in series and dissipate the same power level. Right, I'm thankful the $11 100W TO-247 guy burned out, and saved the $250 150W attenuator guy.

Next I'm going to tear up the PCB layout, double the power capability of the heatsink + fan, and use two of the 100-watt resistors in parallel.

Still confused. Assuming the resistors are 50 ohm, would you not need to put two in series and parallel those with two more 50 ohm resistors ?

Do not worry about replying. I am sure you found a better way like using 100 ohm resistors.

Dan

Right, although 100 ohms is the highest value available and there's low inventory at Digi-Key, two 25-ohm in series is better. I'm beginning to like these guys.

Do those have a ceramic back? We had issues with production over torquing the mounting screw and cracking the ceramic... split lock washers were also removed and replaced with expensive belleville. (besides the torque wrench/driver.)

George H.

Oh, boy, yes they do. I've just been using a screw w/o anything, and making it tight. Sounds like trouble brewing. What if I'm tightening down on a flexible Sil-Pad?

You may want to use something other than just a screw through the resistor, so that the pressure on the top of the device is spread out. Possibly a large washer? Some people prefer to avoid using the screw entirely, and like to clamp the device to the heatsink... run a flat, rigid metal bar across the top of it and screw down the ends of the bar, or use a clamping spring.

If you do use a screw, respect the device manufacturer's torque specification for the part.

If I recall correctly, Sil-Pads have rather poor thermal conductivity, compared to some other solutions. They might reduce pressure stress, but make it harder to keep to the "25C case temperature" part of the rating spec.

I wouldn't use a sil pad, unless there was some voltage isolation involved. Belleville washer and proper torque was caddock's recommendation. If you are doing them all, you could do the torque by hand. Production people like to make things "tight", and some of them are strong as an ox.

George H.

Thanks, George!

Now you know why they have such things as interlocks and overstress protect ion circuits built into the high power stuff.

People are making surface-mount resistors up to 200 watts

but not on my circuit boards!

There are flange-mount versions too

Of course, 1KW into 50 ohms is only 220 volts RMS. They may not be good for kilovolt pulses.

The author of AoE can surely get samples to try.

I have a potential application for a 6KV or so high-current pulser. I looked at SiC but they seemed to have high gate resistances, which would make them slow. GaN is too low voltage, so I'm left with stacked-and-racked mosfets, with insane gate drivers.

Doncha just hate it when some mistake blows everything up?

Thanks for providing the little diagram, there, Jim. I was finding it kinda hard to picture parallel resistors. ;->

The SiC transistor I'm using doesn't show the gate resistance in the datasheet (other manuf mention 10 ohms, etc), but I had to lower one gate resistor from 4.7 ohms to 2.2 ohms, to get my 7ns rise/fall time, so apparently it's not high enough to badly damage the risetime.

I also got better results with +16V, -3V gate drive rather than the recommended +20V, -4V. The gate plateau was +10V. So I could have used a 20V driver IC instead of a 36V part.

I wouldn't worry about it, Win. I blow things up on a daily basis. Something went **bang** only a couple of hours ago, actually (stray crock lead hit a 240V live line). Fortunately the neighbours have got accustomed to all the explosions and fires over the years and the police don't even bother to call any more.