How to remove the bite from a cube?

[G Senier]

This is not really a plc question but I am sure for many of you this will be a simple question to answer. I have an old machine that has several safety or stop motion sw. that are 24vdc to grounded frame. The switches are exposed and when they go to ground it turns on a 24vdc cube relay (470 ohm across the coil if that means anything )and stops the machine. If you happen to be touching the switch and the frame when the switch makes or breaks it will give you a small bite. Not bad but just catches you off guard if you are not expecting it. How can I prevent this. High ohm resister across the coil? how would I calculate the ohms? diode? snubber? Any thoughts?

 

[OkiePC]

For something similar in a food plant, we used these:

https://www.alliedelec.com/crouzet-automation-pnr110a/70159413/

The "probe current" is limited to 2mA. You won't feel it. You can get them with 24vdc or 110vac coil power and they have some extra features designed for tank fill or tank drain types of applications. We used them for tank fill and drain applications as well as product jam sensors where they were directly wired to a metal flap which was isolated from the machine frame, but when product piled up, it would push the flap against the frame and short the "probe" contacts. Operators could be touching those flapper bars and even with wet bare hands, there was no not enough juice flowing to feel it.

My concern for you making any changes to a system is that your description contains that magic word "SAFETY"...

 

[Lare]

Sound that you have inductance spikes.

You need flyback diode or schottky diode. Polarity is important, if you connect it wrongly for this, you have short circuit over A1 and A2 terminals.

Long time since last time have calulated values, basically current and reverse voltage is needed to know
(if you overrate diode, it only costs more. For one diode it won't make big difference if any)

Coil current is now 24V/470R = 51mA, so any 4N4x diode should work for voltage and current spike (1N4007)

https://electronics.stackexchange.com/questions/110574/how-to-choose-a-flyback-diode-for-a-relay

https://en.wikipedia.org/wiki/Flyback_diode

You can change cube relay to relay with inbuild diode and see if it makes any difference.


And Welcome to forum

 

[G Senier]

Thanks Guys , after reading the wiki it looks like the spike off the back emf is only 300v. I thought it would be in the 1,000's. It looks like the 1N4007 will cover that ( there suggestion).

 

[keithkyll]

...If you happen to be touching the switch and the frame when the switch makes or breaks ...

If it's back EMF, the "bite" should only happen when it breaks.
1N4007 is fine for this application.
When trying to protect semiconductor devices, you want the diode to be the fastest switch.
1N4937 is fast, UF4007 is faster.

 

[Lyndon A]

If it's back EMF, the "bite" should only happen when it breaks.
1N4007 is fine for this application.
When trying to protect semiconductor devices, you want the diode to be the fastest switch.
1N4937 is fast, UF4007 is faster.

The "bite" on make is probably due to the switch contacts bouncing.