A device to prevent automatic restart of a motor (or pump) after power failure.


It is well known that aquarium fish are curious and will explore items in the tank. In the hobby, pumps called circulation pumps are often used to simulate natural water currents. Some of these pumps have finer plastic screening on the flow exit side to prevent fish from entering them when they are stopped. There are more than a few stories about fish being killed by the impeller of a circulation pump because the owner did not know a fish was in there... I also have thought that this same situation could arise from the following scenario: 1) power goes out during the night or any other time the owner is not around; 2) fish explore inside of impeller area of pump; 3) power comes back on; 4) 1 or more fish are dead..

In speaking with someone from a local industrial controls distributor, he discussed simple solutions such as screening over the flow exit of the pump. While I can do that, aesthetically it won't look good and it may decrease the flow from the pump.

My own research has revealed that I need a device known as a "automatic restart preventer" or "undervoltage release with manual reset". Basically, think of what I need as an "inverse circuit breaker". These are commonly sold for industrial motors, machines, and tooling where the auto restart after a power failure/power restore could pose a huge safety hazard. However, checking online, those devices are well beyond the budget I am willing to spend for this project.

The guy from the controls company said that I could easily accomplish my goal with an AC DPDT relay and he sent me a schematic to use. Although he sent me a BOM for parts I could buy at his company, I decided to go to local RadioShack and was able to build the circuit since they have all the parts.


Relay is a DPDT, 120Vac coil; 10A contact rating Neon lamp is a 120vac pushbutton is 120Vac momentary Motor is the pump motor (2.8W power consumption)

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He explained that the relay would function such that it would only be continually energized if the pushbutton was first actuated. And that if the power went out, there would be no way for the motor to come back on without the button first being pushed.

I built the circuit above and tested it, but it doesn't seem to be working.... When I push the button, I can hear the relay click, so I know the coil is being energized, but nothing else happens. The relay seems to be working in that there is continuity between the NC pins on each respective side of the relay when the coil is off and there is none when the coil is energized. However, I am not detecting any AC voltage on the output pins from the relay for the NO contacts....


closed as off-topic by Dave Tweed Mar 18 '17 at 11:41

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    \$\begingroup\$ There's nothing wrong with the schematic. Maybe you've miswired the relay or misunderstood the relay pinout. \$\endgroup\$ – Spehro Pefhany Sep 6 '14 at 17:11
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    \$\begingroup\$ The second relay contact is not required - the motor can be connected in parallel with the relay coil. An SPDT (or even SPNO) relay is sufficient. I agree with the others that you must have mis-wired the relay contacts for the motor. \$\endgroup\$ – Peter Bennett Sep 6 '14 at 17:58
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    \$\begingroup\$ Success!!! Thank you all for the comments. In my haste (which I know is wrong when dealing with mains voltages) to get this project finished, I used the wrong pins on the relay! I am glad that I didn't short line to neutral!! Ok, well going to coat some things with liquid tape just to make sure, then screw the top onto the plastic enclosure. On to the next one!! \$\endgroup\$ – PhilosophStein Sep 6 '14 at 20:28
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    \$\begingroup\$ By the way, what is the common name(s) for using an AC relay in such manner as above? I think he mentioned the term "AC impulse latch". \$\endgroup\$ – PhilosophStein Sep 6 '14 at 20:46
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    \$\begingroup\$ I'd just call it a latching relay. You can buy the button and relay in one package - it would normally have two buttons: "Stop" and "Start" - useful on power tools to prevent the tool from restarting after a power failure. Often the stop button is a large, easily-hit paddle, while the start button is smaller and recessed to prevent starting the tool accidently. These may be called "Safety Switch". \$\endgroup\$ – Peter Bennett Sep 6 '14 at 21:44

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