PCB Current Shunt Design

Question

I want to use a shunt resistor to measure the current flowing though a section of my circuit. I am deciding between using an off the shelf shunt resistor or simply drawing a wide track on my PCB and calculating the resistance (and hence voltage drop) of that section.

What are the pros and cons or each method, and from your experience which method is best?. For my application the accuracy is not highly important (avg of 10A flowing and I need to detect current to an accuracy of 1A).

From what i can see the advantage of using an off the shelf shunt resistor is the precision and accuracy of the resistance, however it is more costly and the requirement of soldering will add uncertainty to the resistance.

Drawing a section of copper track on my PCB seems cheaper and eliminates the uncertainly brought by the solder joint.

Answer 1

The three largest errors for using an etched copper resistor are the copper thickness, etching tolerance on the copper width, and the temperature coefficient of the resistivity.

The fab house ought to be able to give you tolerances for the first two.

If the board contains plated thru holes, there will be a build up of copper on your shunt track, that will not be as well controlled in thickness as the original copper. It will help both track thinness and thickness tolerance if you can get the shunt area masked before hole plating.

At 10A, you will need a wide track, so the effect of lateral tolerances will be small.

The resistivity tempco of copper is about 0.4% per degree. That's a 10% change in 25 degrees. That means a copper current shunt will give you an 'indication' rather than a 'measurement'. You could measure the temperature of the board and correct for it, but that sounds a lot more trouble then specifying a low tempco resistor in the first place.

If you can tolerate the uncertainty in resistance from all these terms, then an on-board copper track is a cheap and cheerful way to get what you want.

If you want higher accuracy and go for a soldered component, consider using a '4 terminal' resistor. That will reduce uncertainties around track and solder conductivity, and geometry details.