Why does my ESD wrist strap beep?

Question

I have an ESD mat and wrist strap. The wrist strap plugs into a little box that beeps when either the mat or the wrist strap do not have a good connection to ground.

I also have a standard Phihong 5V wall wart. I have noticed that when the wall-wart is plugged into a device, and the device is sitting on the mat, the box indicates a fault with the mat. If I touch an exposed ground pad on the device, I get another fault indicator for the operator, as if my wrist strap is improperly grounded.

If I attach a serial cable to the device, this problem vanishes. I assume this is because the serial cable somehow provides a proper ground to the device.

This happens even if the device is off. However, the wall-wart is still active; if I turn the wall wart off by flipping the switch on the surge protector, the problem vanishes.

The problem also appears sometimes when I merely approach my mat with the device connected, without even necessarily touching it.

Is there an explanation for this odd behavior?

Answer 1

A fault is registered when a threshold voltage differential is measured between the banana plug of the mat or wrist strap and the provided ground. The datasheet states no more than 50mV ripple, at whatever frequency it switches (assuming it is a switcher, as a linear regulator would be hard-pressed to claim "High Efficiency"). If the connections are long enough, the magic ESD box may be registering up to 100mV or 0.1V differential.

Hook up your oscilloscope in differential probe mode between the ESD box ground and banana plugs to see if it's an issue.