[SI-LIST] Re: Kelvin current sense resistor
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Richard Feldman <Richard.Feldman@xxxxxxxxxxx>
- Date: Thu, 25 Feb 2010 16:10:55 -0800
That works ok IFF: In addition to the 3900ppm / C tempco, you don't
need much in the way of basic precision due to Cu weight and etching
variations ( or are prepared to calibrate and/or trim ), and you can
tolerate a very low bandwidth due to the big L/R that results from the
high bulk conductivity of the relatively thick Cu.
Steve.
Richard Feldman wrote:
> 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
>>
>>
>
>
>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
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