>-----Original Message-----
>From: Richard Hughes [mailto:[email protected]]
>Sent: Tuesday, February 17, 2004 4:10 PM
>To: Gary McInturff
>Cc: Price, Ed; 'EMC PSTC'
>Subject: RE: DC Current Probes
>
>
>Gary,
>
>I agree that the brass blocks you mention are OK for low frequencies,
>but they often use relatively long conductors so inductance could be a
>problem. Also a possible problem at higher frequencies is the added
>inductance introduced by the time you have run wires to these blocks.
>
>
>My suggestion would be to use four terminal precision current sensing
>resistors wired as close to the pcb as possible. Such devices are
>available from Rhopoint in the UK and probably other companies
>in the US
>too. The resistance starts at 1 mohm, which is probably less
>than you'd
>get from hooking up a great big bass block. This solution is
>likely to
>be higher cost however. As always, you pays your money and takes your
>choice.
>
>Surface mount type: http://www.rhopoint.co.uk/components/RHPM/rhpm.htm
>
>Leaded type: http://www.rhopoint.co.uk/components/LORL/lorl.htm
>
>[I have no connection with the above company, other than I have used
>their products in the past.]
>
>Regards,
>
>Richard Hughes
Those are interesting gadgets, and likely very useful. But the leaded 0.001
Ohm AN models would have a short life while passing 100 Amps, as they are
rated at 1 Watt, and would be dissipating 10 Watts (uhh, for a short time
<g>). I think these parts are just a little bit short of what the OP needs.
The older "meter shunt" metal bar resistors is a bit long for minimum
inductance, but you can get shorter ones. I have a couple of Solar 0.001 Ohm
shunts that are shorter, have four terminals pre-wired, and have a spiffy
nickel-plated cover. Of course, with an open design, you can vastly exceed
the power ratings, depending on how big a fan you have.
Most of the in-rush testing that I do is on military 28 VDC power busses,
and the initial surge is created by charging powerline filter capacitors
and/or starting up switching power converters. If the equipment draws 15
Amps or so steady state, the inrush in a poor design can be really awesome.
Although the metal bar resistors are nice, if I have to really draw some
power, I like to use the old Silicon Carbide Globar style resistors. These
are often available in surplus stores or on eBay. These resistors are
non-inductive, stable and very tolerant. http://www.cesiwid.com/ I haven't
found the hookup lead inductance to be a problem (although I usually use
heavy knitted wire braid straps).
Ed
Ed Price
[email protected] WB6WSN
NARTE Certified EMC Engineer & Technician
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (Fax)
Military & Avionics EMC Is Our Specialty
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