Electric power off safety circuit

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zawminoo

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I would like to build a circuit that prevent automatic power on while power on from power off condition.
For example...
Firstly I switch on motor and it run.
And then power off, motor stop but I forget to switch off.
After power on again, I don't want to run motor automatically.
I would like to a switch on to run motor.
clear....?
Please help me if you can...thank..
 

Shops that use power tools need a similar system. This is a working schematic:



You press the ON button. It activates an independent circuit which sends power to the relay.

The ON switch is momentary-On. It opens as soon as you release it.

Later you press a separate OFF button. It interrupts the circuit which powers the relay.

The OFF button is momentary. It closes as soon as you release it.

An SCR is the device which maintains power to the relay. If a blackout occurs, the SCR resets. Thus when power is restored, the tool will stay off.

The SCR stays on as long as current flows through it. The diodes and capacitor provide a DC supply. All these components must be selected to be compatible with the relay coil specs.
 
This is a typical "motor starter". they have a relay inside with two sets of contacts. One set (normally open) are wired across the start switch and in series with the coil and the mains , the other side of the coil goes to the other side of the mains. So you switch on, the relay pulls in and the contact that was open , now closes and will hold the relay in for ever (or until the mains fails ). The off switch just breaks the mains circuit to the relay. The spare set of contacts is used to switch the actual mains to the equipment.
Frank
 
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