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electrical switch question

 
 
tomahok
 
Reply Thu 14 Jan, 2016 01:13 pm
in our lab we have a pneumatic valve that is controlled by a 24V DC power supply. That means if the 24V is applied, the valve will be open; if the 24V is off, the value will close. However, the function that I want to achieve by a relay-like device is that if the valve is closed due to a electricity outage of the 24V power supply and electricity comes back after a certain amount of time, the valve doesn't automatically open, but instead requires a manual operation to open it. I feel such function can be achieved by a relay-like device but I don't know what exactly it would be called. Do you have any idea how to achieve this function with a simple device?
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Tes yeux noirs
 
  1  
Reply Thu 14 Jan, 2016 01:34 pm
Sound like a "no-volt relay". They are widely used in motor circuits to prevent the motor restarting after a mains failure.
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engineer
 
  1  
Reply Thu 14 Jan, 2016 02:13 pm
@tomahok,
You use the power downstream of the relay to keep the relay on so that once the relay is tripped, it stays tripped until a manual button is pushed.

Wire the positive output of the relay to the input to the relay coil with one line and run a line from the positive 24vdc line to the same input using a line with a push button on it. Run the negative of the coil to the 24vdc ground. Now, when you press the button, the relay is closed and it is then kept closed by the 24vdc running through the relay to the valve. If the power fails, the relay opens and can't close again until the pushbutton is used.
engineer
 
  1  
Reply Thu 14 Jan, 2016 02:18 pm
@engineer,
I found a diagram. This shows a switch, but should be a normally open pushbutton. If you add a normally closed pushbutton in the light blue line, that would be a manual off button for the circuit.

http://www.diychatroom.com/attachments/f18/50983d1337205673-power-outlet-stays-off-when-power-fails-309px-relay_symbols_svg-copy.jpg
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tomahok
 
  1  
Reply Thu 14 Jan, 2016 05:06 pm
@tomahok,
thank you for the help. do you know if there is something that is commercially available ?
engineer
 
  1  
Reply Thu 14 Jan, 2016 08:20 pm
@tomahok,
Don't know, but you could put this together for a few dollars.
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