50-ohm power resistors

Hah, I'm gonna need a bigger heat sink! Running at full tilt my nanosecond 500V pulse generator will be dissipating 700 watts, mostly in 50-ohm resistors. I don't think banks of the 100W Caddock 9100 TO-247-package parts I've been using will be the best parts for the job. So I'm looking for good heat-sink-mountable low-inductance (150nH max) 50-ohm power resistors. I'm finding microwave resistors, but I'm a bit worried about their fragility. Should I be? I need two-terminal series, rather than termination types.

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 Thanks, 
    - Win
Reply to
Winfield Hill
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Am 15.09.2016 um 14:49 schrieb Winfield Hill:

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Beware, some may contain BeO.

regards, gerhard

Reply to
Gerhard Hoffmann

Is there anything to be gained by looking at dummy loads for RF amplifiers?

I suspect the frequency/power is outside the normal radio ham experience, but some of them are surprisingly exploratory and inventive.

Reply to
Tom Gardner

Have you looked at the Vishay range? At Cambridge Instruments we bought them in TO-3 and TO-220 packs - amongst others. They do seem to offer a range of relatively low-thermal resistance styles.

They are planar parts, so the inductance won't be all that high.

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None of the data-sheets I looked at said a word about inductance, so you'd probably need to contact them directly to find out anything specific.

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Bill Sloman, Sydney
Reply to
bill.sloman

Have you considered liquid cooling? Back in my more active ham radio days I built a 50ohm resistor that could pack away a kilowatt for 10min or more, cooled by cheap oil for agricultural equipment. That lasted about 30 years when the bucket rusted out and leaked. Threw it out, and in hindsight I shouldn't have.

Lately I needed load resistors again and resorted to just dumping them into a pan with filtered water. That works but strangely the gold-anodized kind of power resistors leaves some funky discoloration and weird floating matter in the water.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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Reply to
bloggs.fredbloggs.fred

Seven hundred watts!!! You are threatening my explanation point budget.

How are you going to get rid of that much heat? Concentrated into a single spot, it would be hard to heat sink. An array of the Caddocks or some AlN RF resistors would work, with water cooling or a hurricane of air on a heat sink. I have got down to about 0.05 K/W on an air-cooled sink, but that's across about a square foot of surface area.

Does your pulse generator need to be a 50 ohm source? Could you do a totem-pole driver?

These guys claim 800 watts, but I'm skeptical about how you could heat sink them.

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Maybe a copper heat spreader and then a big aluminum heat sink. Or an array of smaller parts. I assume that water cooling is too much trouble.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Win, how long does the pulse last for? I'm wondering if you need to spec your resistor in terms of energy and not power.

So something with a lot of resistive mass. Some piece of nichrome?

George H.

Reply to
George Herold

A length of nichrome wire could be located in a machined trench, with a 50 ohm transmission-line impedance. It would get red hot and radiate away the power.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Soup for robots!

Reply to
JM

A loooong time ago I read an article on making dummy loads for amateur radio use. Sort of a Heath Cantenna but for more power and higher frequency.

According to them the proper transmission line to make is one with an impedance that varies along the length of the termination -- IIRC they used a resistor as long as two paint cans; with a cylindrical sleeve it was good to 144MHz, but to make it good up to the GHz required a conical taper (narrower at the far end) -- and they made some comment about how "doing it right" would require a hyperbolic taper.

So by all means put a wire in a trench -- but do some ciphering, and figure out how the trench needs to taper.

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Tim Wescott 
Control systems, embedded software and circuit design 
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Reply to
Tim Wescott

Nichrome grows and changes resistivity a lot when it gets hot - probably not practical to come up with a means to keep it centered and at a constant resistance.

It was once possible to find lengths of carbon rod or carborundum/metal rods which could be carved to shape, metalized for connection and dunked in a bucket of dielectric oil but I'm coming up dry at the moment. The best I can find right now:

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But there's no resitivity listed.

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Grizzly H.
Reply to
mixed nuts

People still sell carborundum cylindrical resistors. They can get very hot without damage.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

No thanks; it's not my problem. I only do math if I get paid.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

I found 'em - the old source - Carborundum Corp - was purchased by Kanthal:

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Sales office in Amherst NY - no distributors I could find.

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Grizzly H.
Reply to
mixed nuts

The advise was directed at Win who, as a guy with a PhD, undoubtedly does such math in his sleep.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

At one time they were used as light sources for FTIR spectrometers, but originally iirc they were used as ballasts in series-filament radios. The original trade name was "Globar".

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

The Nerst Glower (a mix of rare earth oxides) was a common IR source early on but because conductivity was low when cold, had to be preheated

- generally long lived and stable. The Globar could be started w/o preheat.

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Grizzly H.
Reply to
mixed nuts

I have little to add to this conversation, but I'm curious about why you need to dissipate that much power. Is this your dummy load? Why does it need to be 50 ohm resistors?

700 Watts is a lot, but it would seem you could use any resistor value for a dummy load if you are going to spread the heat over multiple resistors. Would a parallel/series string of SMD devices on a large PCB work? You could use a heat sink that mounts over the entire board with a suitable conductive pad between the heat sink and the SMDs. I recently used an old skillet as a heat sink for a 100 Watt LED module. Worked ok except the LED module seems to have poor conductivity internally. The heat sink had barely started to warm up and the LED

My point is a large area heat sink with less airflow can do a very good job compared to a smaller heat sink with lots of airflow. Do you need to minimize the run connecting all the resistors?

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Rick C
Reply to
rickman

Den torsdag den 15. september 2016 kl. 14.49.59 UTC+2 skrev Winfield Hill:

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-Lasse

Reply to
Lasse Langwadt Christensen

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