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Looking to add a "high oil temp" warning on a machine tool by adding a temperature switch into the hydraulic circuit, right after the radiator before returning to tank. I know monitoring from the tank would be better, but tapping into the tank is not possible.

The switch I would like to use....

https://www.mcmaster.com/7079k95

....is rated for 120/240 AC.

I would like to use the switch to trigger an input on my PLC. The switch is normally open and would close at over temperature, allowing a +24vDC to connect to the PLC input(PNP).

Can this switch, as only rated for AC, be used to carry the low-amperage +24vDC signal?

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  • $\begingroup$ Commenting as only 95% sure, and I think someone who knows 100% should answer, but yes. I'm sure this would be OK. $\endgroup$ – Jonathan R Swift Oct 30 '18 at 19:30
  • $\begingroup$ I used a hit water tank stat to control electric fans on my v8 once - worked great, the adjuster knob allowed me to get it to cut in fine... $\endgroup$ – Solar Mike Oct 30 '18 at 22:46
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There are important differences between switches designed for AC and those designed for DC, the main distinction being "breaking speed", i.e. how fast the contacts are separated when the switch is turned off.

However, that is unlikely to matter at the current levels you describe. The problem is greatest when there is an inductive load like a motor involved, which leads to arcing across the contacts when they are separated. Briefly, energy is stored in an inductor in the form of magnetic energy and it needs to go somewhere when the circuit is broken causing the magnetic field to collapse. The voltage rises until the energy can be dissipated and this causes arcing across the contacts that can degrade them.

I expect your PLC load is low inductance and low current, so it shouldn't be any problem in your specific application. Other readers might want to look further and pick a suitably rated switch for motor, transformer, or other inductive loads.

https://www.mouser.com/blog/which-switch-who-cares-if-its-ac-or-dc

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yes, it is OK. the rating of the switch is the MAXIMUM SAFE limit and you are nowhere near that if you are switching 24VDC. The AC and DC ratings of switches like that are sometimes different but this has to do with how the AC voltage measurement method is specified (peak, RMS, ACV, etc.). It is not a specification that the switch may not be used with DC.

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