1
\$\begingroup\$

I'm planning one of my first electronics projects beyond basic repair soldering (it's a small pre-amplifier for a contact microphone). So I've got some JFETs and other pieces and would like to put them together without ruining them with static electricity.

Over here in Canada I can hardly walk through my apartment without getting a static shock in the winter. There are plenty of wrist straps with alligator clips, but my question is: What do I attach this to? I've got no metal chassis, I'm on a plastic table. Is there a way that I can ground to the middle screw on a receptacle? Maybe rip the alligator clip off and wrap the wire around the screw?

Any advice for this newbie is appreciated

\$\endgroup\$
3
  • 1
    \$\begingroup\$ you could also get yourself some matts \$\endgroup\$
    – Abel
    Jan 26, 2022 at 2:21
  • \$\begingroup\$ This is not meant as an endorsement of any particular product, but there are plugs such as this: [amazon.com/StaticTek-Adapter-Universal-Connection-Unbreakable/… \$\endgroup\$ Jan 26, 2022 at 4:02
  • \$\begingroup\$ do you have an old pc power supply? ... or do you have a desktop pc? ... plug power supply into power outlet with a standard pc power cord ... use the case of the power supply as a ground point \$\endgroup\$
    – jsotola
    Jan 26, 2022 at 4:17

3 Answers 3

5
\$\begingroup\$

Unfortunately, the ground strap alone is not going to do much good if you are using a plastic table which is just as likely to have static charges sitting on it as you are. A common solution to this is to use a ground strap with an ESD mat like this picture shows here (source is esdmat.com).

enter image description here

The mat is static dissipative, and can be plugged into the ground receptacle of an outlet in your home or as this picture shows tied down with a grounded screw on your outlet or a piece of grounded equipment. The wrist strap can be plugged into the mat where the mat attaches to the grounding cord. If your wrist strap doesn’t have a plug that will fit into the mat receptacle, you can use the banana clips to clip onto the mat which keeps a static dissipative path between you and earth ground.

Safety Note: you will want to make sure that there is a proper dissipative path between you and ground when using your wrist strap (\$10^6 \Omega\$ to \$10^{12} \Omega\$). Otherwise if you touch a live connection, you will be fried along with your electronics. Most wrist straps have a built in resistor for this purpose, but it’s a good idea to double check before working with live electronics. Good luck!

\$\endgroup\$
0
0
\$\begingroup\$

For professional electronics assembly:

  • Use an ESD mat which should come with a connection to the mains ground through a >10Mohm resistor.
  • The wrist strap is either connected to the mat or separately to the mains outlets through a similar >10Mohm resistor.
  • Get some ESD slippers.
  • All tools need to be ESD qualified.

For hobbyist purposes (quick & dirty hacks):

  • Use a wood surface for your workstation. Not a surface made of plastic or rubber materials.
  • Don't wear fuzzy sweaters, fleece materials, shoes (with rubber soles).
  • Grab a big metal surface like a radiator before handling electronic parts.
  • Avoid touching the parts with your bare hands, always use tweezers/flat nose pliers.
  • You could put an humidifier in the room you are working in - moist air is good when working with electronics to prevent ESD. But note: moist air is bad for storing electronic parts because of oxidation. Ideally keep parts in the ESD plastic package they came with and keep any "silica gel do not eat" bags inside that package even though you've opened it.
\$\endgroup\$
3
  • \$\begingroup\$ And don't pet your cat while soldering :) \$\endgroup\$
    – Lundin
    Jan 26, 2022 at 8:29
  • \$\begingroup\$ "Avoid touching the parts with your bare hands, always use tweezers/flat nose pliers." What difference do you suggest that makes? You'll discharge any static electricity through a pair of tweezers just as well as touching the part directly. \$\endgroup\$
    – Klas-Kenny
    Jan 26, 2022 at 8:33
  • \$\begingroup\$ @Klas-Kenny Because you leave the tools on the work surface when not using them. Then you pick them up and hopefully get the discharge there, in case your body's potential is different. Or alternatively they have plastic handles. If you stick the tweezers behind one ear or something when not using them, then that's probably not a great idea... \$\endgroup\$
    – Lundin
    Jan 26, 2022 at 8:36
0
\$\begingroup\$

Triboelectric suppression, static dissipative workspace and deionized air are 3 factors used for Electro-Over-Stress (EOS) protected workspaces.

The 4th (reminded by @Stevesh) is humidity control above 40% minimum or around 50%. In your case, boiling water might do the trick before potential EOS operations to achieve that in very dry winter conditions.(20%)

You can always buy the ESD mat, wrist-strap etc, but if not , there are handy alternatives.

If you have a Kraft Paper, large pc. of wood or glass to put on the plastic table, those do not generate as much static as plastic and can be cleaned with a damp cloth to moisten. Dust+moisture= static dissipative. A typical residence may have from 100k to 10M particles per cubic foot > 0.1 micron in the air. The output of a good HEPA filter might be 10k or <1% .

  • But you do need to have 2 conductors with charged dielectric in between to make an electrostatic discharge (ESD) after tribolectric (motion) friction.
  • In the '70's when we had zero ESD protection, we would exchange boards between engineers by touching fingers first (zap) then hand over the board and always with a finger on the ground or power contacts then release fingertips.
  • in the '80's when I had a very sensitive 0 pF 10 Gohm charge meter in the office with terrible nylon carpets. I performed this test. Touch grounded cable cover with shoe , touch meter test point, measured 0V. Raised same shoe with foot/leg. touched meter again , measured 200V! Repeatable within 20%. Q=CV I was changing my body C and Q remained constant.

There are also static dissipating sprays that work well on nylon carpets famous for generating 2~3 cm arcs of 20 kV with neoprene shoes. It's enough to reduce the resistance to 10 M or less but might only last a week or few.

Of course, you could always put a wood panel on the stove which is grounded and moisten it with a cloth and attach a wrist strap to any metal on the stove /oven.

The iron should also be checked and kept clean with a moist sponge which as you touch the sponge equalizes the charge if there is no 1 M resistor to earth.

Then avoid the plastic soled shoes.

Anecdotal

  • I was responsible as test Eng Mgr for plant-wide EOS prevention at Burroughs in the 80's and a wide range of interventions was used. Warehouse concrete floors were humid and needed no treatment but the 100ksqft factory concrete needed shot-blasting then dissipative epoxy. I had a metal wire-caged secure labratory full of electronic test equipment and computers on a concrete floor and the MC6800 emulator and an Apple computer would reset everytime someone came and touched the cage until EOS protection was added.

  • The Tektronix FET buffered Diff Probes were rated for 25V and would can move your arm and generate that voltage. So many rookie engineers blew the FET buffers and pissed off the female repair tech. The probe has a shorting tip to be removed prior to use, so using my fingertip to neutralize finger charge to its ground ring then tip was safe for me.

  • My former LED client in NZ received about 50k LEDs /yr from me at one time and was starting to get 1% field failures that caused profit losses from installations in Swiss tunnels that had to work for 1 mos error-free before full payment. But there were 20 LEDs per Stud luminaire so the early failure rate was significant. I was confident his process was damaging the fully tested LED's so I flew down at my expense (20h). I found 12 opportunities for ESD damage and changed the chip design to include ESD (reverse diode) protection on the next shipment. Problem solved. All LEDs are only rated for -5V leakage < x uA (1uA for R or Y an 10 uA for W, B or G). ESD can weld or fuse a part of the junction and still work but degrade faster until field failures. A curve tracer or scope with 2 probes with hum can see this effect from the added capacitance and a Lissajeau circle figure.

\$\endgroup\$
2
  • \$\begingroup\$ I would add relative humiduty control to the three you mentioned. I think the typical range is 40% TO 60% RH in factories and labs where sensitive assemblies are built. \$\endgroup\$
    – SteveSh
    Jan 26, 2022 at 2:40
  • \$\begingroup\$ @SteveSh Yes We always used 50% in production, and Appt's may be OK. some house windows in winter will sweat and ice up around the edges. if RH is 50% perhaps 40% is OK but depends if triple pane or old 2x \$\endgroup\$ Jan 26, 2022 at 12:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.