[SI-LIST] Re: Kelvin current sense resistor
- From: Richard Feldman <Richard.Feldman@xxxxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>, Joel Brown <joel@xxxxxxxxxx>
- Date: Thu, 25 Feb 2010 16:05:38 -0800
By excluding connection resistance from the sense path, 4-terminal resistors
give a well-specified resistance and temperature coefficient.
That's important because at 3.6 watts, your sense R and its soldered connections
are going to get hot.
If you don't care about tempco and are prepared to calibrate the
effective sensing resistance, why not just connect sense points
1 milliohm apart in your 60-amp copper pattern?
Rich
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of steve weir
Sent: Thursday, February 25, 2010 3:16 PM
To: Joel Brown
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Kelvin current sense resistor
Joel, you are likely underestimating the error because you assume
uniform current distribution. A 4 terminal device can bring the
connections directly from the two ends of the reference resistor out to
sense pads. With a two terminal configuration you are going to read the
voltage from some point on the pad which is going to be higher. If you
are going to go this route then you want to arrange your sense lines so
that they do not include any of the current carrying conductors, and so
that they are as close to the potential of the test element as
possible. To do that leverage symmetry and use a lead-in to the sense
resistor from either side, and draw your sense from inside the pattern
something like this:
view next part in courier 10
______ ______
=====|pppppp| |pppppp|=====
=====|aaaaaa|--- ---|aaaaaa|=====
=====|dddddd| | | |dddddd|=====
------ | | ------
| |
sense + sense -
If you just tap one of the sides, then you are going to get additional
resistance effects. You can use a tool like Ansys to figure out just
how much error you will really end up getting.
Best Regards,
Steve.
Joel Brown wrote:
> Sorry if this is not considered signal integrity related.
> I am using a 0.001 (1 milli-ohm) resistor to sense up to 60A of current.
>
> They make 4 terminal Kelvin resistors so that the voltage drop created by
> the current flowing through the PCB to resistor connection do not cause an
> error in the measurement.
>
> I can also do a similar thing with a 2 terminal resistor by making two pads
> on each side, one large one for the current and a small one for the sense.
>
> However when I calculate the resistance of the solder connection, using a
> resistivity of 1.21 E-7 ohm.m, a pad size of 2.54 x 5.46 mm and a joint
> thickness of 10 mils,
>
> I get a resistance of 2 micro-ohms, resulting in an error of 0.4% This would
> say I don't really need a 4 terminal connection and I could even calibrate
> out the error. Is there something I am missing?
>
>
>
> Joel Brown
>
> Chief Electrical Design Engineer
>
> Z Microsystems, Inc.
>
> 9820 Summers Ridge Rd.
>
> San Diego, CA 92121
>
> Tel: 858-831-7011
>
> Fax: 858-831-7001
>
> <mailto:joel@xxxxxxxxxx> joel@xxxxxxxxxx
>
> http://www.zmicro.com
>
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