Electronic – How to protect bistable latching 2 coil relay with diodes

All this hanging contacts around the relay coil can and does work .The red and green start and stop buttons seen in machine shops are older than me .There is another way that uses the fact that the pull in voltage is always much greater than the dropout voltage .Get your relay and a DC power source thats the same as the coil volts .The relay coil will off course pull in reliably.Now place a series resister thats the same as the coil resistance in your relay coil power supply circuit.The coil now only gets 6V and wont pull in .If you momentarily short the resister the coil will pull in .If you momentarily remove the power the relay will drop out .So you can make a normal relay have a bistable latch function .The time to do this rather than the orthodox approach is when you need the contacts for other things and you are not worried about power.I have tried this with small DC relays but have not done this on AC relays.

It depends, if your worried about an overvoltage condition on the line, then yes you do need a diode. The relay will open and the magnetic field will cause an overvoltage on the lines connected to the coil. If you have other IC's connected to the power side I would strongly consider a flyback diode. If it's an automotive application a flyback diode would also be warranted because you don't know what else is on the 12V line. If it's a motor or resistive load I wouldn't worry about it too much.

As far as the switch debouncing goes, it depends on your application again. The relay could follow the switch depending on the timing of switch 'bounce'. If the application is a 'dumb' load and it can handle being turned on and off for a 10's of microseconds (potentially) then you don't need to worry about debouncing. If it is a microprocessor or something with digital logic, then debounce the switch. If you have a high current load its probably a good idea to debounce the switch to be nice to the relay's contacts.