Screw Terminals (App Note)

This application note is really about improperly measuring the voltage at a screw terminal with a DMM or voltmeter.

Don't count on touching a DMM probe to the screw head of a loose screw-terminal.

The screw terminals are only guaranteed to conduct properly to a conductor securely clamped inside the screw terminal.

• Touching a DMM probe to the screw head of a loose screw terminal is hit or miss.
• Touching a DMM probe to the screw head of a tightened screw terminal is always valid that we have seen.
• Inserting a wire inside the opening of a loose screw terminal and touching the back is always valid that we have seen, but difficult to make a consistent connection.  Useful for testing a counter but expect lots of counts.

"Some (but not all) VS terminals are not working"

This is almost never a real problem.  If the device has LED activity or you can talk to the device, the VS terminals almost certainly do have a reasonable voltage on them.  In addition, they all solder directly into the same VS plane on the PCB, so for some to be different than others would require a fundamental mechanical problem.  The problem is sometimes using a bad DMM (check if it has a low battery), but the most common problem is touching DMM leads to the screw heads of loose terminals.

"I'm setting my output to blank but it is not working"

Sometimes this is a real problem with software or the output being affected by a connected load, but if using LJControlPanel/Kipling to test and nothing is connected to the output (except DMM probe), this is rarely a real problem.  The most common problem is touching DMM leads to the screw heads of loose terminals.