I am looking for a switch that only keeps contact while current is running. Once the current drops to zero the switch opens contact. Something like a circuit breaker that trips with no current. It should require the user to press the switch again after tripping. This is for a low voltage (24V) and low current circuit (<1A). Is there a name for this component?

If there is no off-the-shelf component that can do this I wouldn't mind assemble it from other components, but at the moment I'm not sure how to do it.

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    \$\begingroup\$ This sounds to me like a simple configuration of one or two relays. \$\endgroup\$ – Hearth Mar 21 '20 at 22:35
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    \$\begingroup\$ one double pole relay ... the relay solenoid in series with a push button switch ... pressing the switch energizes the relay .... one set of relay contacts connected in parallel with the switch to keep relay energized after initial button press \$\endgroup\$ – jsotola Mar 21 '20 at 22:54
  • \$\begingroup\$ Why do you need this? \$\endgroup\$ – Bruce Abbott Mar 22 '20 at 8:15
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    \$\begingroup\$ The standard reason for something like this is a machine tool - say a drill press. You press the start button and the drill turns. Press stop and it stops. But if you press the emergency stop, (or the mains power fails) when the power returns - you don't want to the drill to turn until you press start again. \$\endgroup\$ – D Duck Mar 22 '20 at 10:00
  • \$\begingroup\$ D Duck is spot on. In this case I'm working with a laser that I want to turn off under certain circumstances for safety reasons, and I would like to keep it off until the user presses a button. \$\endgroup\$ – Pedro Almeida Mar 24 '20 at 7:32


simulate this circuit – Schematic created using CircuitLab

Figure 1. A thyristor latching circuit.

Thyristors have the characteristic that once triggered by injecting enough current into the gate will latch on until the current through them falls below the hold-on value - typically a few tens of mA.

Pretty much any regular thyristor should do the job and you should be able to find the rated current and the hold-on current in the datasheet fairly easily.

  • \$\begingroup\$ To the asker: Note that this won't work for AC. You'll want a triac instead if the source voltage can be negative. But also the current will go to zero and turn off the triac or thyristor every cycle--this may not be what you want. \$\endgroup\$ – Hearth Mar 21 '20 at 22:47
  • \$\begingroup\$ This is intended for 24VDC, so your solution should work. Once I manage to test it, and if it works, I will accept your answer. Thanks a lot for your help! \$\endgroup\$ – Pedro Almeida Mar 24 '20 at 8:56
  • \$\begingroup\$ Old-school version: self-keeping relay. My relay can be used with either current direction, but I have no clue if AC would work or not. \$\endgroup\$ – Joshua May 4 '20 at 2:30

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