Assuming you're working with a standard SPDT relay:

I'd like to place a +12v source on the N/O (87a) terminal. I'd also like to place another +12v source on the N/C (87) terminal. When the coil activates/deactivates, will the device attached to the common (30) terminal see a voltage change? Will there be a brief moment of no voltage? Will there be a voltage spike or dip? Or will it be a seemless switch from being powered by one +12v source to another?

Thanks in advance.

 

I am not sure because I've never measured it, but I think the device will see a drop in voltage because most relays are break-before-make type switches. This means that the connection is broken between terminals (30 and 87) before connecting to the the other terminal (30 and 87a).

With that being said, you'll want to put a diode between the coil (terminals 85 and 86) to provide a path for current when you are de-energizing the coil (switch from N/O to N/C). This will allow the coil's field to collapse without generating a voltage spike. You'll connect the diode's cathode to the positive terminal of the coil and the anode to the negative terminal. And no, it does not matter if 85 is positive and 86 is negative or vice versa, just make sure you connect the diode to the correct polarity.

 

Right. Ok, now I figure placing a capacitor on the relay "output" (common/30) lead will fill in that voltage drop. I'm trying to switch between an ignition 12v to a battery 12v. How can I determine the capacitor size I need?

 

I just wired my Demon Eyes in a similar way (wanted them to be on with ignition and also with running lights for when my headlights stay on after the key is removed). I used a SPST automotive relay. Just connect both the ignition source and the switch source through a diode each to the coil and ground the other side of the coil. Then, just connect the battery source to the 87 (or is that 87a) terminal and your output comes off the 30. Done, and no spikes or momentary drops.

Would this work for you?

Curt

 

heh, yeah, that would work. I guess what was confusing me was that the coil would be getting two 12v sources simultaneously for ~ 48v. For some reason my brain translated that into meaning the output on 30 would be ~ 48 volts also. Forgot the coil is just the signal and the output on 30 is passed through from 87 which would be 12v from the battery. Good call, this is a lot easier. Thanks.

 

Cool. Glad it will work for you.

Curt