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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?

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    \$\begingroup\$ Power the microscope from a better USB charger. \$\endgroup\$ Dec 16, 2020 at 18:00

2 Answers 2

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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.

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  • \$\begingroup\$ Yes, that was it! With an higher quality USB power supply I don't feel anything when touching the metal parts of the microscopes. Also the "humming" vibration coming from the microscope chassis, near the USB plug, that I felt with my finger (with or without wrist band) when the cheaper USB charge was used, has greatly reduced. Thanks! \$\endgroup\$
    – haku15
    Dec 16, 2020 at 21:38
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Is this normal or should I be worried for my safety?

Using touch to detect electricity is not the best way as it's very subjective. Use a DMM and find out what the actual potential is between the strap and the camera. While this will not detect if there is an electrostatic potential, but it will detect other dangerous potentials that might result from a non standard USB charger.

Also Ohm out the chassis of the camera to ground, you should find that it is low impedance. Another thing is the microscope is resting on the mat, which should also help keep ESD low if the microsope has a metal frame. One thing you could do is also ground the microscope frame to ground with a Megaohm range resistor.

USB devices with a metal chassis will not develop an electrostatic potential on their own. If the strap is a metal expansion type it is common to feel pinching from the metal. Another experiment is turn everything off, and ground the microscope to the mat. If you still feel pinching with everything unplugged then it's probably the strap itself.

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  • \$\begingroup\$ Thank you, I appreciate your suggestions. Measurements with DMM between microscope metal focus ring and earth bounding point (I also tested AC Voltage, I don't know if it makes sense here): - Cheaper USB Charger: DC Voltage about 0.07V, AC Voltage: up to 80V - Better USB Charger: DC Voltage about 0.02V, AC Voltage: up to 33V I could not measure the resistance, as the DMM kept changing range. The strap is textile, not metal expansion. \$\endgroup\$
    – haku15
    Dec 16, 2020 at 21:44
  • \$\begingroup\$ The AC measurement is concerning 33VAC may be enough to suggest electric shock. Now the question is how much of a source you are seeing. Put the meter in series with something like 10 Ohms of resistance (the resistor is to protect the meter in the case of a high current VAC source) and put the meter into AC current mode and measure the current to ground. The current should be lower than 1 amp. If the current is lower than 1 amp then don't use a resistor at all and measure the current to ground, it should be in the mA range or lower. \$\endgroup\$
    – Voltage Spike
    Dec 16, 2020 at 21:50
  • \$\begingroup\$ Sorry, I don't feel confident enough to do those measurements now. I did however more tests and noticed that the 33VAC (when using the quality USB PSU - Aukey PA-T14) are only in some areas of the microscope (focus ring and screw above it). Other areas show 15 or 1VAC. I also tried a cheap battery power bank and there was 1VAC. I don't understand how to interpret this AC values and what should I do to improve the situation. I am guessing it's due to the common mode voltage on the output? Or maybe also some problem with the microscope? \$\endgroup\$
    – haku15
    Dec 17, 2020 at 0:24
  • \$\begingroup\$ A voltage is only dangerous if there is current, but in reality there should not be any VAC from a USB PSU, its probably a cheap PSU from a country with little or no safety regulations and could potentially be dangerous, discard it and don't use it. \$\endgroup\$
    – Voltage Spike
    Dec 17, 2020 at 1:09
  • \$\begingroup\$ I've tested with yet an other PSU, a DJI (drones) one, which has to comply with regulations and has plenty of certs on it (TÜV, GS, UL,....): I've measured 65VAC and no "wrist pinches" felt. So I am unsure about those VAC values...maybe I'm measuring it wrong: DMM is in AC mode, one tip on the microscope metal ring and the other on the Earth bonding box snap button (so there is still 1MOhm before ground) \$\endgroup\$
    – haku15
    Dec 17, 2020 at 18:14

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