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I'll try explain my problem:

I am trying to make a simple circuit that powers on when a momentary switch is pressed but then stays on until the power source (feeding the lamp) is then removed.

I want to be able to turn on the circuit with momentary power through switch1, once the power of switch1 has been stopped (press has finished,) then lamp1 should stay alight until we disconnect the wire where the red circle is marked.

When power is reapplied (the red circled area is reconnected,) lamp1 should be off until switch1 is pressed again.

The scenario: I have a pump that should start when a button is pressed (not a switch with constant power,) the pump should stay running until the power to the relay->pump is removed. Then when it is removed, the pump will obviously stop pumping. Then the relay should reset to off so that when power comes back on the pump should not start until the button is pressed again.

Can the relay can self feed itself as shown below,or is there an easy way to make this work?

enter image description here

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    \$\begingroup\$ You're basically describing an SCR/thyristor action. What is the voltage/current/power rating of the pump? Maybe an SCR circuit can be used. \$\endgroup\$ Commented Sep 10, 2021 at 4:02
  • \$\begingroup\$ @EdinFifić Interesting, I've never heard of that before. Its a fuel pump so it should be around 13 volts and 5 to 10 amps \$\endgroup\$ Commented Sep 10, 2021 at 4:07
  • \$\begingroup\$ Works as shown if lamp = pump. what is red circle?. Some simple variant of this can work if pump is different voltage. What is pump voltage and power ? \$\endgroup\$
    – Russell McMahon
    Commented Sep 10, 2021 at 5:37
  • \$\begingroup\$ FYI, I was looking at your drawing trying to figure out where to insert a deadman (momentary) switch that would cut off the power when released. The red circle would NOT work. Pressing the start button would still activate the lamp even if the deadman switch was open. You would want to insert the switch just before or after the battery (before any branches) to fully shut down the circuit. \$\endgroup\$
    – mstahl
    Commented Mar 1, 2022 at 14:31

6 Answers 6

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Your circuit should work as drawn, but with a standard, non-latching relay. Just make sure its solenoid/coil is rated for 12V, and its contacts for at least 10A. A latching relay would remain stuck in its ON position until another pulse of current is sent to its solenoid, so it wouldn't serve your stated purpose.

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This will work. The ‘off’ normally-closed switch must carry the entire load current, in the circuit shown, of course. The relay is a normal non-latching type and must have the same coil voltage as the load. You also have the option of just interrupting the coil current to the relay, which should cause it to drop out (provided the contacts have not welded).

This is an entirely standard arrangement on woodworking and machine tools- it is considered undesirable for them to restart by themselves following a power failure and subsequent restoration of power.

You can even source mains-voltage (230VAC, 120VAC) modules that integrate two momentary push buttons and the relay- sometimes called a “magnetic switch“.

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Just look at a standard stop-start circuit: -

enter image description here

Now you don't appear to need the stop contact so just imagine that shorted out then, re-arrange in your mind that your relay contact is more clearly in parallel with the start switch like this: -

enter image description here

And you have the basic start-circuit used in industry.

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Just for fun I made this circuit to latch on and off a standard (monostable) relay with a single push-button. Basically it works by coil histeresis. I used a 12 Volts DC power supply and a 5 Volts - 125ohm relay coil. I don't remember the relay model exactly. I just know it is a finder with 2 SPDT **(NO-NC)*, usually mounted on circuit boards. Obviously you can use any other model/brand (which may be better). However it is important to use correct values for R5 and R6, depending on the features of the coil: nominal, trigger and maintaining voltage, resistance. If supplied with 2,5 Volt, the relay coil I used can not trigger; but if it is supplied with 5 Volt or a bit less, then the voltage becomes 2,5 Volt, it keeps on: this is th coil histeresys. When the push-button is open, the relay coil takes about 5 Volts; when it is open the coil takes about 2,5 Volts. In the image below you will see the circuit schematic and how I found the correct values for R5 and R6. Momentary latching relay, how it works Video Momentary latching relay

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Assuming all of the components are rated for the voltages and currents involved, it should work as shown.

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  • \$\begingroup\$ I hope you understand what a latching relay is and how it works. \$\endgroup\$ Commented Sep 10, 2021 at 4:56
  • \$\begingroup\$ I do. The TS is using a standard relay in a circuit that makes it perform a latching function. \$\endgroup\$
    – AnalogKid
    Commented Sep 10, 2021 at 12:12
  • \$\begingroup\$ That may be it. I was confused by the title which starts with "Latching relay" and I assumed he was using such a relay. \$\endgroup\$ Commented Sep 10, 2021 at 12:27
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It's a standard latching circuit, using a normal electromagnetic relay, and should work.

enter image description here

An 'Off' push-button switch, with a 'normally-closed' contact, has been added. Its unlike the 'On' push-button contact, which is 'normally-open'.

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