0
\$\begingroup\$

I'm building my first CNC machine, it's a 40W Laser Cutter. I'm planning on adding a security feature that cuts the laser power supply if the enclosure door is open during while operating.

I want to use this Solid State Relay I want to use a simple mechanical switch like this and just wanted to see if you think it would be a terrible idea for any reason. I would send 5v to the switch and then to the relay. When the door is closed, the relay would get the 5v and allow electricity to go to the laser. If the door is open, it would kill it.

Thanks in advance.

\$\endgroup\$
  • \$\begingroup\$ Beware of Fotek SSRs! Counterfeits are all over the place and can be a reliability and safety hazard because they use underrated TRIACs. See protosupplies.com/… \$\endgroup\$ – Enric Blanco Aug 1 '19 at 21:43
  • \$\begingroup\$ You might need a heatsink for the SSR as well. \$\endgroup\$ – Enric Blanco Aug 1 '19 at 21:44
  • \$\begingroup\$ This is not a good idea. SSRs usually fail as shorts. \$\endgroup\$ – EinarA Aug 1 '19 at 21:51
1
\$\begingroup\$

You are right to be thinking of safety. I offer the following observations:

A safety switch shouldn't rely on a spring to driven into the off position when the guarding is open. That way if the switch spring fails then the system fails in a safe mode. This often relies on the use of a cam or similar mechanism to operate the switch. Safety-rated magnetic switches are also available but these are probably out of budget for your application.

The problem with the solid state relay is that you can't predict its failure mode. If the switching element fails it is as likely to fail short-circuit as open-circuit. Mechanical relays are much more reliable in this regard and their major risk is contact welding but this can be mitigated by using a high safety factor in the contact rating.

Industrial safety circuits use redundant circuits: the safety switch will have two contacts and drive two relays in a supervisory unit. Both relays must drop out before the system can be reset. (That way you catch the first fault if a switch or relay fails.)

In summary:

  • Use forced disconnect when the guard is open.
  • Use an over-specified relay or two.
  • See if you can add a supervisory circuit to alarm if the relay doesn't match the switch position.
  • Add an indicator LED (bright) to indicate when the laser is powered. After a while you'll get used to the LED turning off when the guard is opened to the extent that you should recognize a fault if it occurs.

I have described the operation of a safety relay in my answer to Symbol or marking on safety relay. You might find it of interest.

\$\endgroup\$
  • \$\begingroup\$ A snubber may help prevent some causes of contact welding too. \$\endgroup\$ – Enric Blanco Aug 1 '19 at 21:49
  • \$\begingroup\$ A relay with cadmium used in the contacts also helps. The cadmium being there means the switch was designed with arcing in mind. \$\endgroup\$ – DKNguyen Aug 1 '19 at 21:58
  • \$\begingroup\$ "Forcibly guided" is the term for industrial stuff if you want to explore that route, though it will require a controller. The less stringent approach that is easy to do at home is to get a little redundancy by using a double-pole relay and looping your signal through both switches on the theory that they will probably not both weld closed at the same time. \$\endgroup\$ – IceGlasses Aug 2 '19 at 1:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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