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Example Project: Relay scan card for K2000 DMM

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--- Quote from: Serpent10i on May 01, 2022, 06:48:10 pm ---I built one too! Thanks for the great plans, this was a fun and rewarding project.

--- End quote ---
Glad you had fun building it  :)

--- Quote ---I only ran into one issue, the MIC5841 was out of stock everywhere EXCEPT in a weird 20PLCC package, so I made a small adapter board to compensate.

--- End quote ---
Nice solution. This component shortage is a PITA in so many ways.

I built one of these. Works very well and fund project!

Thanks to voltsandjolts for sharing!!

I have now been using this card for some time. Generally it works very well.

Once issue though: when measuring some very precis voltages that are not too well buffered, when the card scan over these voltages it loads down with around 50 to 300 micro volt. I have another 6.5 digit multimeter where i can see the voltage jump up and down with the scanning. When using the front input of the DMM6500 and tap the measurement point with test leads this do not happen. So its not the DMM6500 that load down, but switching from the card.

Is this a general thing that can happen with scan cards, or could there be some problem with mine?

I also build voltsandjolts's SSR 20 chanel card where i dont have this problem. Sometimes the last digit on the 6.5 meter i use to check voltages can move a bit more than if no scanning, but thats all ok, and just some noise.

Some load transients when switching between channels is a general problem. The inputs have a certain capacitance and the protection part may have a limit in the slew rate for some internal signals (e.g guards, bootstrapping for protection). The scan card should not add much to this (a little capacitance and maybe a little capacitive coupling to a control signal).  The VOR1142 SSR   has some 39 pf and this adds up with 20 channels. So a version with fewer channels or additional seprating groups (e.g. like the mode for 4 wire ohms) can help.
Scanning with high impedance signals will always be tricky.

Just taping with the leads by hand may be misleading as the DMM inputs tends to stay charged to the previous signal and this may be the same signal as before when doing it by hand.

Perhaps the fact that you see the problem on the armature relay card but not on the SSR card is a clue.
Could it be the impulse applied to the relay coils has this slight affect on the high-impedance signal, passing through the contacts close to the coil?
Do you see the effect permanently (e.g. when scanning just one channel) or only temporarily when changing channels?


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