I have an ESD mat, ESD wrist strap and a Chinese USB (power only) digital microscope(Andonstar ADSM201.)
I use the microscope for soldering and inspection at home for hobby projects.
The microscope is powered by a USB charger from the wall plug, so it is not connected to the same grounding point of the mat and wrist strap (because the charger has only 2 pins on the plug, not 3 like mine Earth bonding point.)
I know the microscope is not ESD-safe, but I don't know other solutions than maybe buying a professional microscope for 1000-2000USD.
So I thought: I can attach the body of the microscope to a flexible arm outside of the mat, so that it stays 15cm above the mat (and ICs.)
However, when I touch the metal part (aluminum?) of the microscope to adjust focus, I feel slightly unpleasant "pinches" on my wrist, where the ESD wrist strap is. I guess we are not at the same potential and current is flowing through my wrist. Wrist strap has 2MOhm resistance to ground.
I've tried also to let the metal part touch the ESD mat, but no changes.
If I remove the wrist strap and touch the metal parts of the microscope, I don't feel anything.
Also if I put some insulating tape on the focus ring, I feel no pinches, but that's not good for ESD.
Is this normal or should I be worried about my safety?
I see online lots of people using such USB microscopes, but nobody mentions ESD safety.
Is there no other solution than buying a professional ESD safe microscope?
Best Answer
It's normal from a cheap USB power supply. Cheap power supplies may have a lot of common mode voltage on the output, or can be otherwise dangerous. Don't power it from a cheap USB power supply.