The most common are push-button, but toggle switches too may be spring-loaded to return to one position after activating. In the catalogues, an (ON)-OFF description means it's only ON while you're holding the lever in place. With push-button, they're more often described as 'momentary-on' (push to make) or 'momentary-off' (push to break).
Here's how the two types could be used together to control ON-OFF action by latching a relay, a very common system on industrial control gear.
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| Relay Latching Circuit |
The relay needs to have two or more poles (the switches) which can be change-over (double-throw) or single-throw types.
The "On" switch S2 is a push-to-make (momentary on) type, and allows power through to the relay coil when pressed. This becomes an electro-magnet and pulls all the relay switches over to the 'no' (normally-open) position. (With relays, 'normally-open' and 'normally-closed' refer to the switch positions with no power applied to the coil.)
Relay switch Sw2 now keeps power running to the relay coil after the "On" button has been released, latching the relay into the 'on' position.
Relay switch Sw1 is used to switch power to whatever load you have attached, (Extra relay switches would be used, for example, to control a 3-phase electric motor.)
Pressing the "Off" button S1, a push-to-break (momentary off) type, breaks the power circuit, deactivating the relay electro-magnet and sending its switches back to the 'nc' (normally-closed) position.
The diode D1 gives an easy path to any voltage spike produced when the relay coil turns off (back-EMF) protecting any sensitive equipment on the same circuit.
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