From: John Larkin
Subject: Re: Open air current sensing PCB mount shunts...
Date: Sat, 14 Dec 2002 09:51:59 -0800
Organization: Posted via Supernews, http://www.supernews.com
X-Newsreader: Forte Agent 1.91/32.564
On Sat, 14 Dec 2002 17:33:52 -0500, "Klaus Vestergaard Kragelund"
>"John Larkin" wrote in
>> On 13 Dec 2002 15:59:17 -0600, boB gudg wrote:
>> >I'm looking for sources for current sensing shunts in the 1 to 2
>> >milliOhm and 2 to 5 Watt range. I have seen OAR series from IRC
>> >and RCD and Token but it looks like they aren't very popular... ie,
>> >long lead times etc...
>> >I have a couple of IRC samples and they work pretty good actually.
>> >Made of Manganin I think with good TempCo.
>> >Any preferences ??
>> Isotek makes pcb-mount shunts, too.
>> We punch our own from sheet manganin, but that's a hassle unless you
>> need a whole lot. Manganin does have pretty good tempcos...
>I've used Manganin in a previous job. Great tempco as you say - but some
>people in our department was worried about the thermal expansion coefficient
>when the shunt is mounted on FR4. They worried about the thermal cycles
>destroying the PCB. Have you experienced anything like this?
we normally punch a shape into the manganin that includes both the
current and sense contacts. The contacts are bent at 90 degrees to fit
into PCB pads, so there's some flex designed in. IRC's 'oar' shunts
are omega shaped, which also gives some compliance. Some of the Isotek
parts are really planar, like a huge surface-mount resistor, but I
wouldn't suspect this to be a problem; the solder joints will be huge,
and although the pcb material has a high tce compared to almost
anything else, it's soft so the forces won't get very high. But if the
shunt self-heats a lot, that might be nasty... in that case I wouldn't
solder a flat shunt to a pcb without strain relief somehow; that could
well rip the pads off the board.
We often heat-sink the shunt, which helps.
This is similar to the problem of attaching a big ceramic package to a
pcb. I've read of cases like that that failed after a few dozen wide