# Replacing the mechanical switch with digital that has low on-resistance

Please take a look at the picture below.

What I am trying to do is designing a "switch" that can close the circuit so that the resistance meter can measure the resistance of the resistor. I have to do it digitally, it will has a MCU to control the switch.

I try to use the relay, but most relay has pretty high on-resistance like 30mΩ, 50mΩ which seems will affect the measurement.

What solution can close the circuit and has low on-resistance that won't affet the measurement? Thanks in advance!

• What maximum current can your R1 allow without damage?
– jonk
Commented Sep 6, 2022 at 2:41
• @jonk Actually, this is the coil on the motor, so, at least it allow 2A current go through.
– John
Commented Sep 6, 2022 at 2:51
• If you're going through motor brushes, you have much bigger problems to worry about than the resistance of the of the switch. Motor brushes have a resistance that varies with contact and with applied current. Commented Sep 6, 2022 at 3:42
• A low Rds(on) MOSFET would be the most obvious choice. However, I don't see why you need to close the circuit for the purpose of measurement? If there's a current sense amplifier measuring across R1 then the MCU can decide to measure the current through its ADT or not to measure, because the motor is running etc. Commented Sep 6, 2022 at 8:19

but most relay has pretty high on-resistance like 30mΩ, 50mΩ which seems will affect the measurement.

This is not true. The only reason it seems this way is because you are limiting yourself in your relay selection to lower current relays. Higher current relays will have lower resistances.

Besides, many solid states switches have significantly higher resistances than what you listed. Sure, they could be lower but so could a larger relay.

But using a bigger relay is not the answer either. For measuring low resistances use Kelvin connections: use different wires to send the test current and different wires to measure the test voltage right at the terminals. Since the measurement wires carry virtually no current they do not experience a voltage drop.

• The advantage of this method is that you could use a solid-state relay to interrupt the current, but when the current is flowing, it's the resistance across the test resistor that matters, not the resistance used to set the current or the resistance presented by an in-series solid-state switch. The kelvin connection permits the voltage across the test resistor to be measured accurately through, say, an instrumentation amplifier. The output of that amplifier could be input to an analog-to-digital converter, which makes it easy for a microprocessor to infer the unknown resistance. Commented Sep 6, 2022 at 2:45
• Thank you very much! I took some time to learn about the Kelvin connection. I find that the resistance meter I use is actually using Kelvin connection. The connection is 4-wires, do you think it will works if I use 4 relays (or 2 relays) and close them when measurement starts. Maybe better solution? Thank you in advance.
– John
Commented Sep 6, 2022 at 3:54
• @John For what reason do you need to disconnect it in the first place? Commented Sep 6, 2022 at 3:56
• @DKNguyen As you know previously, this is a coil on the motor. We have to start measuring (close the loop) after the power to the motor is off, otherwise it will break the resistance meter. We are now doing it manually by using some mechanical switch.
– John
Commented Sep 6, 2022 at 4:08
• @John Then yeah a four-pole relay would work. Commented Sep 6, 2022 at 4:35