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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.

Autistic
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Pedro Almeida
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    This sounds to me like a simple configuration of one or two relays. – Hearth Mar 21 '20 at 22:35
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    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 – jsotola Mar 21 '20 at 22:54
  • Why do you need this? – Bruce Abbott Mar 22 '20 at 08:15
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    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. – D Duck Mar 22 '20 at 10:00
  • 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. – Pedro Almeida Mar 24 '20 at 07:32

1 Answers1

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schematic

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.

Transistor
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  • 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. – Hearth Mar 21 '20 at 22:47
  • 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! – Pedro Almeida Mar 24 '20 at 08:56
  • 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. – Joshua May 04 '20 at 02:30