I want to replace the water pump 12v switch with an auto-off one. I've looked for a 12v auto-off switch, but to no avail. My question is would a 240v pneumatic switch (like you see in caravan park toilet blocks etc) work on the 12v positive?
Simple answer - yes. However, there are more complications to consider. The main one is the current rating of the switch. If you are switching the same load - say 1 kW, the 12 V switch will have to handle 20 times the current (forgetting the inrush current to the motor). This is a significant increase. Then you have the difference between AC and DC. When you make contact, no problem, but when you break the contact, the DC will try to maintain an arc at the contacts a lot longer. The DC switch would not last as long. You can overcome this problem by using the 12V switch to operate a relay, which would then switch the mains power. The current required to operate the relay would be a lot less than the motor current
Simple answer - yes. However, there are more complications to consider. The main one is the current rating of the switch. If you are switching the same load - say 1 kW, the 12 V switch will have to handle 20 times the current (forgetting the inrush current to the motor). This is a significant increase. Then you have the difference between AC and DC. When you make contact, no problem, but when you break the contact, the DC will try to maintain an arc at the contacts a lot longer. The DC switch would not last as long. You can overcome this problem by using the 12V switch to operate a relay, which would then switch the mains power. The current required to operate the relay would be a lot less than the motor current
DITTO to all above
Risky , unless the switch also has a DC rating on it
Especially if a micro switch [with a very small "contact open "gap.
Switching a DC motor [ inductive arcing] is a lot different to a light [ non inductive,very little arcing]
