Antistatic wrist straps advice

BobLowry

Hi everyone,

I've been working with exposed Basic Stamps, MOSFETS and similar devices which can potentially be damaged by static discharges, and so I think it's probably best if I start adhering to some sort of anti-static routine. I believe I need at least an antistatic wrist strap, and maybe some ability to remove any static from my soldering iron too(too hot to touch whilst weating my wristrap). I need some advice on how I should use an antistatic wrist strap and what I should connect it to.

I'm really ignorant when it comes to what I need, but maybe some possible grounding points I have access to are: a regulated linear lab power supply(3-15V) which I use to power my circuits, a computer case and my garage tin shed which is bolted into a slab of concrete. Up to this point I have been regularly tapping the computer case and my shed wall every so often, in the hopes this would dissipate any static, but I have no idea if this is actually working.

Some knowledgeable advice on theses issue would be very much appreciated.

Post Edited (BobLowry) : 10/17/2007 10:07:37 AM GMT

Bean

Bob,
What is really more important that every is at the same potential. Even if that is not ground.

In other words, if the bench is charged to 1000 volts, and you are charged to 1000 volts then that is not a problem.

Since most object disapate charge they will eventually be at ground potential.

A conductive work mat that is connected to the wrist strap is a good idea. Connecting the mat to a ground connection (you supply might have one) is an even better idea.

Make SURE you wrist strap has a 1 meg resistor in it. Otherwise you could get a nasty shock.

Bean.

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Post Edited (Bean (Hitt Consulting)) : 10/17/2007 3:21:59 PM GMT

LoopyByteloose

Bean points out the key reason that many of us ignore wearing such a device.

If you are grounded and don't include a 1meg resistor, your hardware may be safe - but you are in danger of a serious shock if you touch the wrong hot wire [noparse][[/noparse]like AC].

High humidity and floors without rugs go a long way toward removing static. Winters in the north create problem conditions.

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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········

···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Sparks-R-Fun

Most likely the metal of your computer case will be grounded. Also the round hole in a three-prong outlet (at least in the US) and possibly the electrical cover plate screw are other likely places for finding a ground connection.

I have a grounded soldering iron. Radio Shack sells these and anti-static wrist straps as well for under $10 each. Also a Ground Fault Circuit Interrupter (GFCI) is a nice device to have added to your workbench outlets. It is designed to cut the power if too much current is leaking (possibly even through your body) to a ground connection rather than the neutral wire. You can get these for under $10 as well.

CMOS devices can be very sensitive to static charges… so much so that touching an exposed wire or metal connection can be seen as a change in logic state! Other devices are not so sensitive but almost all can be permanently damaged by a high enough voltage difference. If you tend to receive a small shock when you touch metal objects you will definitely want to consider some form of static protection!

- Sparks

T&E Engineer

I have never heard of the 1 Meg resistor being in the wrist strap. Does it come this way or is it something you are supposed to attach. If so, how is it done. Most wrist straps I have seen are pretty identical. We have anti static lab bench with wrist strap attachements for it at work. I don't remember seeing any resistors.

Interesting...

Mike Green

Usually, there's a blackish strip that's part of the wristband. This is usually a carbon impregnated strip of cloth or paper that acts as a high value resistor (megohms). The specific value doesn't matter much.

RDL2004

http://en.wikipedia.org/wiki/Antistatic_wrist_strap

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- Rick

Whit

Here is a good way to start for about $25. This should be avaliable locally - They had it at our local store. It has good instructions too. It also has the In-line 1M ohm resistor. It can be carried easily with you in your tool box.

http://www.radioshack.com/sm-electronics-anti-static-service-kit--pi-2102871.html


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Whit+


"We keep moving forward, opening new doors, and doing new things, because we're curious and curiosity keeps leading us down new paths." - Walt Disney

Post Edited (Whit) : 10/19/2007 3:04:30 AM GMT

StarMan

I'm curious about when to stop worrying about static.· Once a component is installed in a circuit board, is it less susceptible?· Or when the circuit board is installed in a product?· If not, what precautions are typically taken in product design in order for the electronics inside to meet the harsh consumer environment?· I realize this is a pretty open ended question because I know the number of variables·is quite diverse,·but I'd like to hear what kinds of rules of thumb people follow.

I have a small company that makes some LED lighting products.· Our circuits boards are built and burned-in by a contract manufacturer and we receive them in nice little pink plastic bags.· But once we receive them, we don't pay much attention to static issues and we install them in the product and ship.· Being a mechanical engineer, I didn't pay a lot of attention to static related failures in the design stage.· But it hasn't seemed to matter.· We've had units in the field for a few years now with no failures.· Did I just get lucky?

Chris I.

LoopyByteloose

Generally, once the FET device is in circuit, it is safe. But in hobby work, you may be plugging and unpluging modules that have direct access to the FETs inputs. These can easily get zapped. Also, there is a tendence to 'hot plug' [noparse][[/noparse]not tern off power] when using a breadboard. That is another source of trouble.

In some cases, MOSfet transistors - like the 2N7000 are provided with a protective resistor of a few hundred ohms. The resistor is not absolutely needed in the circuit, but it is there for protection from the outside. The Art of Electronics has some discussion about this.

In the realm of marketing, it is quite attractive to have everything wrapped in static protection. The customer feels more confidence in the delivered goods, unlikely to try to return as defective, be more loyal to the supplier, and may even accept a greater cost. But on the downside, all those warnings really enhance doubts in ones personal abilities. Ideallly, all components should be tested upon receipt to verify they haven't been damaged by static, but I am sure that I have only tested about 50% of what I have on hand as the low cost itmes, I accept the risk. If you have been receiving tousands of one componet, a statistical sampling of a small portion is standard proceedure for quality assurance. in other words, you try to catch probles at the delivery rather than months later.

I must say that in Taiwan, the over-the-counter service handles product very roughly - no static protection, bent pins, tossed together in to cellophane bags and stapled shut. Still, I haven't gotten a bad device in all the time I have been here. The high humidity and lack of carpets are preventative.

I also suspect that the early stories of failures were part of an intial phase in product development and that devices may actually be less prone to mishandling due to improved design and better handling in production.

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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········

···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

BobLowry

Thanks for the all responses guys, definitely some more things to consider before I buy the wriststrap.