Cool Components V: ESD Safe Buttons
This was supposed to be a quick post on one piece metal buttons.Â But it’s spiraled totally out of control, zooming past one post before finally settling down, I hope, on three posts.
I first researched metal buttons because I needed an ESD-safe button, and I couldn’t find one.Â Plenty of buttons have specs for ESD immunity, but I needed one that wouldn’t cause ESD (Electro-Static Discharge).
ESD can create a high voltage spark which can kill nearby sensitive electronic circuits.Â Everything close to the ESD-sensitive part needs to be either conductive and grounded or dissipative (material with resistance of 10^6 to 10^9 ohms/square so current will flow, but not too rapidly) and grounded.
Normal plastic is especially bad, because it is an insulator, and can be tribocharged: friction caused by rubbing the plastic part can create a large static charge.Â You can get dissipative plastics, but I don’t know of any buttons that use them.
Anodized aluminum is also an insulator; for an ESD-safe aluminum part you have to use electroless nickel plated aluminum.Â For example, Banner’s ESD safety light curtains use electroless nickel plated aluminum for the bodies and static dissipative plastic for the optical covers (BTW, as far as I know, they are the only readily available ESD-safe light curtains).
You can’t reliably ground through a moving part.Â So if a button has a moving button, then the moving part has to be grounded with a ground wire as well as the stationery part.
While there aren’t any buttons that are advertised as ESD safe, there are some that might work.Â What characteristics would help?
- Be able to ground the both the body and the actuator.Â A single piece, non-moving body is ideal if it can be grounded and is conductive or dissipative.
- Everything that an operator could touch must be made of conductive or dissipative materials such as stainless steel.Â If the button is illuminated, the plastic lens would have to be made of dissipative material.
So what are some possible solutions?
- One piece conductive metal buttons, such as a Schurter 1241.2611 PSE16 16-mm stainless steel piezoelectric button (~$45) or a Texzec T01-012203 22-mm stainless steel ActiveMetal ultrasonic button.
- Two piece conductive metal buttons such as a stainless steel vandal-resistant pushbutton (available from Schurter, ITW, and many others).Â As noted above, you’d have to figure out how to ground both pieces.
- Use an ESD-safe cover: cover a regular pushbutton with a fixed body that captures a moving part (to depress the button’s actuator); the cover parts have to be conductive or dissipative.Â One advantage: if you use a clear, dissipative plastic for the moving part, you can use an illuminated pushbutton underneath.Â This ESD-safe cover will probably cost substantially more than the pushbutton.
- Spray on anti-static spray.Â Although anti-static spray should help short term, I’m skeptical that it will continue to work well for a substantial period of time.
All of these possible solutions would have to be verified: you will need to verify that all external parts of the button are grounded and that the button will conduct or dissipate any static charges.