The question of selecting relays for a low thermal EMF scanner pops up now and then. And often turns into a competition who knows the most exotic (and expensive) relay type, contact material or contruction technique. But has anyone actually tested if those things are important in real life?
When designing my new 120 input scanner, the decision was easy because I got 150 Panasonic TQ2-5V relays for free. It is a small footprint
non-latching general purpose relay. The datasheet values may not be the best in the market, but according to my experience in other applications, it performs very well in low level switching.
A latching relay driven by short pulses would of course make things easier because it produces the minimum amount of heat. But ordinary single side stable (non-latching) relays are widely available, much cheaper and also easier to drive. May be meaningless in an eight channel scanner, but for example in my 120 input system both the component price and the drive circuit complexity become very important.
Luckily the temperature rise itself is not crucial because the circuit is balanced. The most important thing is the mechanical symmetry of the relay i.e. how evenly the heat distributes. Based on the tests that I made many years ago there are expensive but poorly behaving special relays, as well as cheap general purpose relays which perform very well. Try before you buy! The contact material had very little to do with the actual performance.
And don't forget the physical orientation of the relay because there is air convection inside and outside. Mounting the cards vertically (like Dataproof) is not a good idea.
Test circuit.
![](https://www.eevblog.com/forum/metrology/low-thermal-emf-scanner-and-ordinary-non-latching-relays/?action=dlattach;attach=268085;image)
Test jig.
![](https://www.eevblog.com/forum/metrology/low-thermal-emf-scanner-and-ordinary-non-latching-relays/?action=dlattach;attach=268080;image)
And the results.
![](https://www.eevblog.com/forum/metrology/low-thermal-emf-scanner-and-ordinary-non-latching-relays/?action=dlattach;attach=268082;image)
Some recovery time is needed to achieve the thermal balance again. If the swithing period is shorter, you will see a cumulative error component. But in a multi-channel system the ON time will be only a fraction of the OFF time anyway.
Note that the point of interest is the energized state (green). What happens when not energized doesn't matter because the relay is not in the signal path.