Sprague KoolOhm

[Winfield]

I had samples once of porcelain-on-steel thick film resistors
that you might look at.
http://www.rcdcomponents.com/rcd/rcdpdf/TP-TPS-FA048.pdf

You might arrange to get some kind of special terms, if you're
ordering for an educational facility - good targeted advertising.

http://www.welbornelabs.com/mills.htm
http://www.heiresistors.com/ovalstan.htm
http://www.upe-inc.com/resistors/
http://www.globar.com/ec/resistor.php.html
http://www.micro-ohm.com/powerceramic/ce.html
http://www.peccomponents.com/powerresister.htm?gclid=CMH7s83cyY8CFQS4...

RL

I will check these out, thanks!!!

 

[Winfield]

That's about all that's in an Agilent 54006A probe, $2500.
http://www.home.agilent.com/agilent/product.jspx?nid=-536900317.53688...

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

 

[John Larkin]

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

 

[none]

Joerg a écrit :

Only the caps are left of Sprague. To me it all looked like a major
corporate shriveling-up process. Maybe one of those 3rd generation
downfalls.

I used to buy Dale for low-L resistors, they come in a heatsink
mountable enclosures:

http://www.vishay.com/docs/50013/rh.pdf

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

 

[redbelly]

Joerg a écrit :







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

 

[The Phantom]

Yep!

And what did you find?

 

[Fred Bloggs]

Now, now, you know exactly what I mean. Low-value power
resistors are well characterized as a resistance in series
with an inductance. They "become" primarily inductive
above f = R / 2pi L, where the inductive reactance exceeds
the resistance.




No. A good LCR meter will show the same value for series
inductance over a wide range of measuring frequencies (to
be sure of the measurements, I do a quick check to verify
that I'm measuring with a high enough frequency to get a
sizable phase shift from the inductance, e.g. 5MHz or more
if needed). --- Sorry, here's a missing datum, the 48nH
measurement was on a 3-ohm 5-watt KoolOhm resistor, so
f = 3 / 2pi*48nH = 13MHz. Oops, where'd 10MH come from?
That was from an earlier 66nH measurement, except after
re-zeroing the meter I got 48nH. There are just too many
numbers floating around.



L = R / 2pi f = 23nH, or 2x better than 48nH (another
measurement yielded 66nH, hence the 2x - 3x estimate).

The carbon is close to a piece of similarly-shaped wire.




The breakpoint frequency. It's convenient to say the part
acts primarily as a resistor until a 3dB point, afterwhich
it acts primarily an an inductor. Plot Z vs. frequency: a
straight line from DC to a breakpoint, then rising with f.
You know the drill, I assume.

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.

 

[Joerg]

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

Mark

Yep.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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

 

[Fred Bartoli]

Joerg a écrit :

Yep.

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

 

[Joerg]

Joerg a écrit :

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.

 

[Joerg]

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

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

 

[JosephKK]

john jardine [email protected] posted to
sci.electronics.design:

Farnell stock as standard, the planar, non inductive, thick film
types, BPC(Bi tech) and (Tyco) MCP. These are quite cheap and have
proved surprisingly rugged.

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.

 

[Fred Abse]

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

Only *really* expensive probes. IME.

 

[Joerg]

Only *really* expensive probes. IME.

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.

 

[Rich Grise]

Only *really* expensive probes. IME.

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

Cheers!
Rich

 

[Joerg]

john jardine [email protected] posted to
sci.electronics.design:


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

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.

 

[[email protected]]

Does anybody know what became of the Sprague
KoolOhm non-inductive power-resistor line -- who
owns it now, what did it evolve into?

I don't think Vishay got the line, since apparently
Ohmite owns the tradmark. Did they discontinue
them? What happened?

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

 

[JosephKK]

Joerg [email protected] posted to
sci.electronics.design:

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.

Now that was a serious act of determination.

 

[Joerg]

Joerg [email protected] 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.