My design needs a very low contact resistance electro-mechanical device, such as a reed relay. The solution needs to be small (PCB mount). It needs to have low leakage, with a contact resistance smaller than 10 mΩ, as that is the largest resistance value in my circuit (shunt resistor).

My "goal" would be:

  • 1 mΩ contact resistance.
  • Voltage less than 40 Vdc,
  • Current from 1A to ~1 μA.

Some have suggested using mercury wetted reed relays, which may be a possible solution, although they are not easy to find (currently manufactured ones).

Could I use multiple low contact resistance relays in parallel, to both increase current capability and decrease total contact resistance? I have seen that at the small currents that my device sometimes experiences there can be an issue with relay contacts.

The design cannot use MOSFETs, the entire point of the device is that it has very low resistance.

Basically the same question was asked here, but I have other requirements than the ones in this other topic.

In basic terms, it is a current measurement device. It goes shunt -> buffer -> x10 -> PGA -> ADC. The circuit is (almost) outputting the expected values (3-digit precision down to μA level; I want 5-digit precision).

There are two relays in my design. I'm using two 0.01 Ω shunts for mA/A/inaccurate μA, and 10 Ω for accurate μA/low-end mA. Relay #1 shorts out 10 Ω shunt, not used in most measurements. Relay #2 shorts out fuse. Relay #2 could be eliminated along with the fuse.

EDIT: I edited the question to best of my ability so that it does not ask for specific manufacturer parts.

  • 1
    \$\begingroup\$ I think you might have an issue with the contact resistance increasing over time as the relay contacts degrade. You might need a mercury wetted relay. I just looked and you can still get them. \$\endgroup\$
    – Drew
    Feb 17, 2022 at 2:01
  • 1
    \$\begingroup\$ I would suggest you rearrange your circuit so the contact resistance of the relay is not critical. It is usually possible to do that in switching shunt resistors. \$\endgroup\$ Feb 17, 2022 at 2:24
  • \$\begingroup\$ Relays have not only a maximum current, but also a minimum current ("wetting current") to prevent corrosion at the contact points. So mercury wetted reed relay may be your best bet. If I understand correctly, you're using a 10mOhm shunt to measure supply current of a device-under-test (DUT), and want to use the relay to bypass the burden voltage of the shunt when in normal operation, correct? Does your DUT power supply have remote sense (Kelvin sense)? Maybe you could leave the 10mOhm shunt always in circuit, and just regulate the DUT voltage after the shunt? \$\endgroup\$
    – MarkU
    Feb 17, 2022 at 2:56
  • 2
    \$\begingroup\$ @super7800 Here's what I use, for example. COTO relays. These do work well over a wide dynamic range. They are NOT cheap. But I could not afford leakage and they provide sub-picoamp leakage for me. These are used over about eight orders of magnitude current range. But not up to an amp, granted. Perhaps up to some milliamps. (Sorry about the dusty board... but I pulled it from storage to get the photo. It hasn't been used in about a decade, now.) \$\endgroup\$
    – jonk
    Feb 17, 2022 at 5:48
  • 2
    \$\begingroup\$ You will very greatly improve the presentability of your text by following basic English rules: Capitalise the first word in sentences, capitalise "i" , Use proper English construction so sentences are 'complete thoughts'. \$\endgroup\$
    – Russell McMahon
    Feb 19, 2022 at 11:40

1 Answer 1


There are MOSFETs that have on-resistances of less than 10m\$\Omega\$. They will have high capacitance in the off state. Relays are finicky since the contact resistance will vary and low currents won't break oxide layers on the contacts. Mercury wetted, as you found out, are hard to come by due to RoHS restrictions.

Perhaps use two amplifiers as shown below. You can sum the outputs of U1 & U2 (high level symbols for instrumentation amplifiers) or multiplex using an inexpensive analog switch. This gives you 4-wire (Kelvin) connections on your sense resistors.


simulate this circuit – Schematic created using CircuitLab


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.