Electronics > Metrology

Relay choice for scanner

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fcb:
I'm building a scanner to automate the measuring of voltages references. Any thoughts on the suitability of Panasonic TQ2-12V relays?

Echo88:
For a good scanner you need latching relays, so they dont produce thermal emf voltages during their switching. So yours arent the best choice. I suggest you read the following threads about suitable relays:

https://www.eevblog.com/forum/metrology/low-thermal-emf-scanner-and-ordinary-non-latching-relays/ <- Tests using a TQ2-5V
https://www.eevblog.com/forum/metrology/measurements-on-emf-error-of-switches/ <- general discussion
https://www.eevblog.com/forum/projects/diy-low-thermal-emf-switchscanner-for-comparisons-of-voltage-and-resistor-stand/ <- scanner built from G5A-relays

Andreas:
Hello,

I would use the latching version of these relays.
I use TQ2-L2-5V see here:

https://www.eevblog.com/forum/metrology/measurements-on-emf-error-of-switches/msg1220467/#msg1220467

with best regards

Andreas

fcb:
Thanks Echo88 & Andreas,

So it looks like I'm not barking up the wrong tree with the TQ2 series at least.

For my application the relay will only be on for a few seconds every so often so the offset from heating may not be relevant - that said the cost difference (latching vs. non-latching) is not that significant and there seems to be good stock availability.

martinr33:
The high-end meters all use FETs for switching. For the kind of accuracy you are looking for, you might look the same way. Check out TiN's work on an exotic Keithley scanner card.

https://www.eevblog.com/forum/metrology/volt-nut-test-for-eevblog-readers/

FET switches. Fully isolated - has batteries on the other side of the optos so that nothing can cross over from the main power supply.
Tin's notes indicates that there could be some improvements. Might also be possible to use the packaged switches, although the discrete design might be easier from a battery perspective.

www.mouser.com/ds/2/256/MAX4661-MAX4663-58511.pdf

This isn't too hard to implement. I know that relays are temptingly easy, but they are just trouble if you are looking for high precision. The little optos are very easy to use. If you want to be tricky, you could add series LEDs to the opto drive side. That's very helpful with debugging. Plus, you won't be worrying about coil fields and inductive spikes.

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