>-----Original Message-----
>From: John Barnes [mailto:[email protected]]
>Sent: Tuesday, February 19, 2002 4:45 AM
>To: [email protected]
>Subject: Re: Resistors pulse loading capabilities
>
>
>
>Massimo,
>All the tests that I've seen for evaluating the peak-pulse-power
>handling capabilities of resistors are based on charging a bank of
>capacitors to a high voltage, then applying this energy (E =
>1/2 * c * V
>* V) to the part/equipment-under-test through a low-resistance
>low-inductance switch or relay. You need to be very careful running
>these tests, because there is enough energy involved to kill you!
>
>For the Keytek surge generator that I used to run IBM Lightning Surge
>Susceptibility tests on network adapter cards, this came to something
>like 4 joules at 800 volts, with the tester rated to deliver up to 200
>amps maximum. That tester required you to toggle two widely-spaced
>switches to trigger a discharge, to make sure that you had
>both hands on
>the tester. I always triple-checked my test setup, then had another
>experienced engineer double-check me, before I even plugged in
>the surge
>tester. I blew up quite a few components, and fried a few circuit
>boards, while developing/helping develop some 35 IBM and Lexmark
>networking/digital-office products from 1990 through January 2002.
>
>NOTE: You also want to put a direct short across the
>capacitors when the
>tester is not in use. Dielectric absorbtion stores some energy in the
>dielectric of a capacitors as a physical displacement of the atoms.
>This can take seconds to hours to relax, and as it does it induces a
>corresponding charge on the capacitors' plates. Without a bleeder
>resistor of some type across the capacitors, up to 10% or so of the
>initial charging voltage can appear on the capacitors after they have
>supposedly been discharged. This could give you a nasty, if non-fatal
>shock, next time you want to use the tester.
>
>As for help choosing the resistors for your circuit, check out
>The Resistor Handbook by Cletus J. Kaiser (98 pages, 1994, CJ
>Publishing, Olathe, Kansas, ISBN 0-9628525-1-1). This is the best
>single source I've found for information on resistors. On
>page 3, under
>the subheading One Short Pulse it says:
> "The theory of pulse handling depends on the pulse width.
> One
> short pulse of 100 milliseconds or less is assumed to never
>have time enough to do more than heat the element. Therefore
>the
> calculation is based on the total mass of the element (wire)
>being heated to the maximum internal hot-spot temperature."
>
>As Bob Wilson said, this will depend on the overall type of resistor,
>and on the specific details of its construction (i.e. manufacturer/
>series). You need to consider the bulk sections of the resistor, which
>have relatively-uniform construction, and the interfaces between them.
>
>So I would expect bulk-metal resistors, such as those made by
>Vishay, to
>have the highest peak-power/rated-power capability because they are
>essentially one piece of metal. From there I would look at:
>1. Carbon-composition resistors.
>2. Wirewound resistors.
>3. Untrimmed metal-film resistors.
>4. Untrimmed carbon-film resistors.
>5. Untrimmed cermet resistors.
>6. Trimmed metal-film resistors.
>7. Trimmed carbon-film resistors.
>8. Trimmed cermet resistors.
>
>The untrimmed film resistors have rather sloppy tolerances, on
>the order
>of +/-20% or so, but have much higher pulse-power handling capability
>than their trimmed brethren. (We got bitten by this, when purchasing
>had trouble getting the part we had specified for a Token-Ring
>card, and
>substituted a part that the salesman said "was even better
>than" the one
>we wanted.) If you must use a trimmed resistor, because of tolerances,
>abrasive trimming would probably be better than laser trimming, because
>it makes a smoother cut and leaves less debris.
>
>I found it very educational to look at resistors under a microscope
>(about 10-30x magnification sufficed), both before and after
>running the
>Lightning Surge Susceptibility test. The failed resistors looked like
>they had been machine-gunned, with a vertical line of pits across the
>surface of the resistor from the corner of the "L" (left by
>trimming) to
>the edge of the resistor.
>
> John Barnes
> dBi Corporation
>
>-------------------------------------------
Let me add one more type of resistor, right at the top of your list. I have
used silicon carbide resistors (made by Carborundum, then Ceseiwid, now the
name has changed again) as pulse generator loads and source impedance
adjustors.
These resistors have the inductance of a similar sized piece of rod stock,
and can handle tremendous short duration currents. They can function
properly right up to the point of glowing. They withstand high voltages
pretty well (best to choose a rod shape rather than a thick washer or hockey
puck shape).
These resistors withstand power only slightly worse than a bulk section of
metal, but they offer resistance values well beyond practical metal
resistors.
My experience with metal film resistors is that they work only once (or
less) in this role. And the trimming causes poor distribution of the current
density and an increase in inductance.
Wire-wound resistors have too much inductance to serve in a pulse generator
or load. It's tempting to use some of those Dale 200 watt finned ceramic
"brick" resistors, because they look so robust, but some models have
significant inductance. Don't assume a resistor is always pure resistance.
Regards,
Ed
Ed Price
[email protected]
Electromagnetic Compatibility Lab
Cubic Defense Systems
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (Fax)
Military & Avionics EMC Services Is Our Specialty
Shake-Bake-Shock - Metrology - Reliability Analysis
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