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current sensing with 2 parallel resistors

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Simon:
Right, I'm doing this design (now becoming well known as i keep asking questions about it  ;D) that is passing 20-25 amps and I'm thinking of using two parallel resistors for the current sensing to reduce strain, however I can only sense the voltage on one resistor, is there a risk due to me using 4.5 milli ohm resistors that track resistances and solder resistance will alter the 2 nearly symetrical paths and give me false readings due to one passing slightly more current than the other.

Attached is the section concerned of my PCB R00 and R01 are the sensing resistors, each coming from a relay IC3 is the current sensing chip.

Have I just opened a can of worms ? what if I took the line from the current sense chips to both resistors maybe through a say 1K resistor each ? so as to sum/average the two readings and not put 10A down a 0.64mm trace ?

scrat:

--- Quote from: Simon on August 19, 2010, 11:52:31 am ---Have I just opened a can of worms ? what if I took the line from the current sense chips to both resistors maybe through a say 1K resistor each ? so as to sum/average the two readings and not put 10A down a 0.64mm trace ?

--- End quote ---

I can't understand the sentence "and not put 10A down a 0.64mm trace". Where do 10A pass on a 0.64mm trace, in your layout?

However, summing the two differential signals could work.

Simon:
if I were silly enough to run the 0.64mm trace past R00 and onto R01 I would be be linking the two together, but then a significant amount of current would try and go down it too messing up the measurement as the trace would become part of the main current path, if you look I have made sure the tracks to the sensing chip are not in the main current path.

you think "summing" the two voltages with resistors in series would work ?

jahonen:
I would just put the sense resistors as close as possible each other and take the sense lines in between of resistors. Sense lines need not to be so thick, you can easily make them much thinner, if that makes it easier.

Regards,
Janne

Simon:
that's a good idea, I'll have to do some pushing and shoving and make room to bring them together, I think I placed each one between relay pins, if they are in the middle they need more free room as they will clash with the relay pins

using the resistor adder does not work anyhow at least not in simulation