Sprague KoolOhm

I got mine on eBay for less than 10% of that, IIRC. Comes with spare resistors - what, do they burn out?

Maybe the wires break off the ends?

I got one on ebay, too. Once we figured it out, we bought a bunch of additional Caddocks for fun. The longer, higher-value parts are handy for monitoring high voltage pulses.

John

RH series are definitly not low inductance. I just measured one RH25-82R I have at 1.4uH

Look on p. 2 of Joerg's link, under Ordering Information. There is an option for non-inductive winding.

Mark

And what did you find?

Winfield wrote:

I don't know what your fixation on low inductance is about, but it does not pertain to wirewound resistor design for broadband applications as practiced in the industry from 1940 onwards. I guess that was a time when engineering did not shy away from things like transmission line equations and expansions of their solutions into usable approximations which today would be described as "math-uh-magical bullshit" by the zeroids. The wirewounds were modeled as distributed transmission lines with "shorted return" and uniform R, L, G, C per unit length, configured in such a way as to make the characteristic impedance the same as the DC total R. They went with resistance wire to reduce high frequency skin and proximity effects, and they balanced R,L,C to cancel any reactance effect out to 100MHz. And even then, they could design in the high frequency "residual reactance" to be either inductive or capacitive. Any number of winding techniques were available to them, including the Chaperon dual winding, loop or hairpin winding, or concentric winding. Your inductance measurements are meaningless without the R and C, since to a first approximation, in the general case, L=1/3 x R^2 x C is used to neutralize quite a bit of the residual reactance. I scoped out an IRE Proceedings advert from Sprague Specialties Co. KoolOhm from 1941 and they don't mention anything about special winding techniques, just their ceramic wire insulation rated to 1000oC. Anyhow, originally, the L was not minimized but deliberately introduced.

Yep.

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Don't know what's readily available these days though but I've successfully used some around 10MHz.

Sorry, I mistook that one with some other vishay series where the first letter of prefix changes when you want the low inductance winding.

They must have bought Dale, like so many other companies, and then kept their part numbering convention.

Probe kits are notoriously exposed to "Oh s..t, I shouldn't have touched that!" situations.

john jardine snipped-for-privacy@idnet.co.uk posted to sci.electronics.design:

So are you saying buy stock thin film parts and build up what you need?

DPA of a few KoolOhms as a kid and sound engineering and physics allows me to make equivalents onesie-twosie near cost (i eat the labor) for myself. If you want a very few i can explain the tricks i remember or can figure out to make your own. For any kind of production quantities it is a different matter.

Only *really* expensive probes. IME.

-- "Electricity is of two kinds, positive and negative. The difference is, I presume, that one comes a little more expensive, but is more durable; the other is a cheaper thing, but the moths get into it." (Stephen Leacock)

And they can be ramarkably tough. I had a Philips probe and accidentally used it at a place with 230VAC. Didn't want to calibrate, hmm, why? Then I realized that its wall wart was freaking hot. It was at least 10 minutes. Oops! Switched it to 230VAC, everything worked fine.

That's what that little plastic sleeve in the accessories bag is for. ;-)

Cheers! Rich

That's what I did as a kid when I needed a 50ohm resistor that could take a kilowatt for 10 minutes or more. Waited until there was a sale of some carbon overstock, took an old one-gallon metal bucket with a lid (honey container, almost like a paint can), cut copper to make a cone shaped structure, soldered all day long, made a vent hole and filled that with oil. Worked nicely all the way up to around 100MHz.

Don't know what became of them but Surplus sales of Nebraska has some in stock. al

Joerg snipped-for-privacy@removethispacbell.net posted to sci.electronics.design:

Now that was a serious act of determination.

It was just one method to stretch the allowance :-)

The metal can was free. The oil wasn't and I resisted the temptation to get some they drained from a tractor but sprung for a can of fresh oil. The cheapest I could find, of course, some brand I'd never heard of before.