Electronics > Circuit Studio

Example Project: 20 Channel solid state scan card for K2000 DMM

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aronake:
I am thinking about building a scanning card for DMM6500.

What are experiences from those of you who have built this model? Recommended? Tried any other DIY scan card? How was that in comparison to this?

voltsandjolts:
Well, I designed it, so I'll leave others to provide critique.
I don't think many have been built, maybe Anders Petersson has some feedback (ping'd him).

I think the 10 channel card (which doesn't need an mcu) is more popular, as long as you don't mind the relay clicking noise.

Anders Petersson:
I actually modified and built another PCB, since that one was drawn in KiCAD so I could cad in an edge connector to plug in a whole set of wires. Here's the one I started from: https://github.com/cozdas/CozScan2020

Mine is working well with DMM6500. It doesn't work with Keithley 2010 however -- possibly solvable with some firmware modification. Maybe voltsandjolts version works with older DMMs.

With lots of channels, even the touchscreen of DMM6500 is inconvenient when setting up channels and navigating the data. I'll make PC scripts instead.

I also have an official 2000-SCAN card with 10 channels and clicking relays. I don't need the electrical advantages of the electromechanic relays and hearing the ticking wearing on the relays gets on my nerves. 2000-SCAN works with Keithley 2010 though.

aronake:

--- Quote from: voltsandjolts on April 24, 2022, 04:26:58 pm ---Well, I designed it, so I'll leave others to provide critique.
I don't think many have been built, maybe Anders Petersson has some feedback (ping'd him).

I think the 10 channel card (which doesn't need an mcu) is more popular, as long as you don't mind the relay clicking noise.

--- End quote ---

Could you elaborate on:
"Switching between 2W and 4W resistance on the same channel causes the channel relay to open.
It works fine changing between channels in 2W and 4W."

In the first post? So possible to do 4W meassurement but need to set up in a certain way?

otherwise all works well for you?

voltsandjolts:

--- Quote ---"Switching between 2W and 4W resistance on the same channel causes the channel relay to open. It works fine changing between channels in 2W and 4W."

--- End quote ---

When my PIC firmware receives a command (from the DMM) to open a relay, it opens all relays. I do this because it makes it simple to behave in a safe way and ensure there are no unintended conduction paths between channels.

This method works fine on the K2000 DMM, because it always sends commands to opens all relays before closing any.

But the DMM6500 doesn't do that - it remembers which relays are open and which are closed. So, when changing between 2W and 4W it just sends a command to open or close the sense channel relay. My firmware sees the open command and opens all relays. So its a safe failure mode. Workaround is to select 4W mode, then change to the channel you want to measure.

Firmware can be changed to fix this, but you would be best to do some checks on which relays are closed, to be safe.
It doesn't affect me because I just do 2W voltage scanning, so I did firmware in simplest/safest way which suits me.

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