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selecting between 2 different sources

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dvandlp

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Hello all...my first time posting. I need to create a circuit that selects between 2 dc power sources (PS A and PS B) with a push button. It is ok if cause an slight interruption in power during the switching process. I can fire the relay to switch power sources well enough, but as soon as I press the button to switch, the relay dies and it defaults back to the PS A. (assuming PS A is where I am starting from.) I know it has something to do with keeping the relay coil energized during the switchover. I also need to be able to switch from PS B to PS A again after that, with the same button. I am stumped. any ideas? thanks in advance.
 
Can you post a schematic of your circuit so far?

If your supplies are fairly low current you could use a latching changeover switch for the pushbutton, if it is acceptable to have a switch that bulky. That way you wouldn't need anything more than the switch itself.
 
A change-over switch, rather than a button, seems the obvious answer. Couldn't you use that?
If you must use a relay to do the switching then perhaps you could supply power to the relay from both sources via diodes.
 
The cct looks good, Mike odom, but can the +RLY_PWR be driven from PS A and / or PS B?? I want to keep this thing kinda self-contained.
thanks
 
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