# Question on how stimulus current affects resistance measurements

#### BDMN

Joined Sep 6, 2017
3
This seems like it should be simple, but I'm having trouble working through the logic.

I use two different testers to verify continuity on circuit boards. Both work by sending a constant current through the trace to be measured, measure the resulting voltage drop, and calculate the trace resistance. (Also, both testers have some cabling involved so the resistance is actually across the trace plus the cabling).

Anyway, one tester can use up to 200mA for the stimulus current, while the other is limited to about 5mA. Some boards will require a resistance of less than 5 Ohms on these continuity tests. I'll have much better luck measuring less than 5 Ohms using the tester than can push 200mA for the stimulus current.

Is there some simple explanation for why this may be?

Thanks!

#### AlbertHall

Joined Jun 4, 2014
12,028
5mA flowing through a 5Ω resistor will result in a voltage of just 25mV whereas 200mA will result in a voltage of 1V. It is easier to measure 1V than 25mV.

Are these bare PCBs (no components)? In that case there is no possibility of damaging anything with 200mA.

#### BDMN

Joined Sep 6, 2017
3
5mA flowing through a 5Ω resistor will result in a voltage of just 25mV whereas 200mA will result in a voltage of 1V. It is easier to measure 1V than 25mV.

Are these bare PCBs (no components)? In that case there is no possibility of damaging anything with 200mA.

Yeah they are bare boards. The tester that can do 200mA is just a bit more of a pain to use, but looks like it will have to be used!

So more or less, since the 25mV isn't able to be measured as accurately, there will just be a bit more fluctuation in the calculated resistance value in the 5mA tester, and that will cause some of them to jump above that 5 Ohm requirement?

#### AlbertHall

Joined Jun 4, 2014
12,028
So more or less, since the 25mV isn't able to be measured as accurately, there will just be a bit more fluctuation in the calculated resistance value in the 5mA tester, and that will cause some of them to jump above that 5 Ohm requirement?
If they were both designed to do the job then they probably will. I think I would choose the 200mA version.