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In my system I want to measure multiple DC voltages with a single multimeter. I'm thinking of using relays to switch between the different signals.

But relays specifications tend to indicate, that I actually can't use them for this purpose.

One of my signals is from a high precision current shunt (Vishay HZ series) with an expected signal range of 550 mV to around 4.3 V. The others are in the range of 0 to 10 V.

I'm not too concerned with the switching capacity of the relays. It's used in a production equipment and can be replaced on a regular basis; I expect around 20000 switch overs per year.

My research:

Solid State Relay

Solid state relays usually have a minimal output voltage specification, which is higher than 550 mV. So it can't be used for the current shunt and for the 0 to 3 V range of the signals.

Furthermore, they typically have leakage current specification which would defeat the whole purpose of the high precision current shunt (leakage > 1 µA).

Standard electromechanical relay

The problem with standard electromechanical relays is the requirement of minimal load current. To get anywhere near the load of a voltmeter (1 µA at 10 V, or even less if switched to >10 M\$\Omega\$) I need gold-plated contacts.

But even gold-plated contacts are rated for a minimal load current in the order of 1 mA at 24 V.

Reed relay

In most of the datasheets I haven't found a mention of a minimal required load current or voltage specification.

I'm not sure if that means that it doesn't matter (because of the sealed enclosure there is no oxide build up?), or that the manufacturers don't care.

Currently I'm leaning towards reed relays, but I'm not sure if I'm missing something.

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    \$\begingroup\$ There are plenty of MOSFET based Solid State Relays that can work with both AC and DC signals. Their output is just two back-to-back MOSFETs and they have no minimum output voltage requirement. With respect to leakage, most do have >1uA leakage, but there are some made by Toshiba and Omron that are specified less than 1nA leakage @ 100V. \$\endgroup\$
    – user4574
    Oct 20, 2022 at 14:36
  • \$\begingroup\$ Depending on how reliable this needs to be, I would look into a either reed relay with gold-plated contacts, or a vacuum relay (which, as the contacts are in an evacuated chamber, doesn't have to deal with oxide buildup). The best option would be a relay with self-wiping contacts (like used in many switches and connectors), but I'm not sure if those even exist. \$\endgroup\$
    – Hearth
    Oct 20, 2022 at 14:43
  • \$\begingroup\$ @user4574 Something to keep in mind when dealing with solid-state relays, though, is that leakage in semiconductors is exponential in temperature. If you have 1 nA leakage at room temperature, you might have tens of μA at upper limit of the temperature range. That might not be a problem; it depends on the conditions in which it's used. \$\endgroup\$
    – Hearth
    Oct 20, 2022 at 14:45
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    \$\begingroup\$ For some applications, I used "wetted" (?) mercury Reed relays. \$\endgroup\$
    – Antonio51
    Oct 20, 2022 at 15:10
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    \$\begingroup\$ Omron suggests that contacts will normally recover from the problems of minimal load when "ordinary" loads return. Is there any way to run your tests and have some kind of downtime where you switch larger loads? \$\endgroup\$
    – jonathanjo
    Oct 20, 2022 at 18:52

4 Answers 4

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At least some conventional relays seem to have very low contact currents. For example, I see that Kemet EA2/EB2 series has a contact current of 10 uA. Any good?

Datasheet

enter image description here

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Put a capacitor and a small series resistor in parallel with the meter's inputs. Then when you switch, the capacitor will draw a small surge current and keep the contact clean.

0.1 uF and 10-100 Ω should work depending on your operating range.

Be careful that when switching from a high input V back to a sensitive circuit that the now-charged capcitor doesn't overdrive the sensitive circuit.

Also ensure that you have a delay between opening one pair of switches and closing the next - some relays may take longer to open than close, so you might briefly short two signals together

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  • \$\begingroup\$ That's a good suggestion and costs way less than those railway relays. \$\endgroup\$
    – Arsenal
    Nov 3, 2022 at 14:41
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After the helpful recommendation from @jonathanjo I did some further research concerning electromechanical relays.

From my findings I'd conclude the following for switching low current signals:

  1. Use a relay with gold plating, sometimes also called gold clad. Be sure that it is a thick gold layer. Gold layers of 0.2 µm are only to prevent the contacts from corroding during storage. A thin layer will wear off or easily evaporate if an arc occurs. I found relays with up to 10 µm gold, which shouldn't wear off as fast.

  2. Look for bifurcated contacts. Basically, this means that for each contactor there are two contact points, which make for a more reliable connection.

  3. Use more than one contact in parallel. According to Finder this will decrease the minimum switching load and increase the reliability. Sadly, I haven't found a second source for this claim and I'm not sure if this scales downward into µA range.

I have found one relay for rail applications, which is specified down to 1 µV and 1 µA (Mors Smitt D-E). For my application it makes life much easier as it is mountable on a DIN rail.

I have asked one of the biggest companies for reed relays, but there are none which could be mounted on a DIN rail directly or with a socket. Otherwise, they would be a very good fit for this application.

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For measuring voltage, something like the Toshiba TLP3317 solid state relay is a good choice for voltage measurement.

  • It has an on resistance of 7~12 ohms. If the meter has high input resistance (like > 10M ohms) then it shouldn't be adding more than 1 ppm error.
  • It's recommended for use up to 64V.
  • It has a leakage current rating of only 20pA at 80V. Proportionally lower leakage at lower voltages.
  • Solid state, so no minimum switching current or cycle limit.
  • No minimum switching voltage.
  • Both AC and DC voltages can be measured.

Omron also makes similar relays.

Also, if you are only measuring up to 10V, manufacturers like Analog Devices make analog MUX chips that could be used.

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