View Full Version : Negative (-) is it ok to do this

computer guy
04-10-2007, 09:44 AM
Can someone tell me if it is alright to share a negative (-) terminal on 2 devices.
More specifically the power source to the relay (5v) and the propeller (i am using a Proto Board now).
Only the negative (-) terminal.

5v (+) -------------------------------------------(+)
(-) ------------------prop (-) -------------Relay (-)

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Paul Baker
04-10-2007, 10:31 AM
In general it is ok and in fact preferable, however with inductive loads such as a relay you need to be careful since inductive kickback can damage electronics. To be on the safe side you may want to use an optoisolator so that you can control the relay without having an electrical path between the two.

Paul Baker (mailto:pbaker@parallax.com)
Propeller Applications Engineer
[/url][url=http://www.parallax.com] (http://www.parallax.com)
Parallax, Inc. (http://www.parallax.com)

computer guy
04-10-2007, 10:42 AM
So replace the transistor with an optoisolator.
Is this what you are suggesting?

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Mike Green
04-10-2007, 10:53 AM
You could use an optoisolator, particularly since your power supplies are already separate, then you don't need a common ground. On the other hand, it is unlikely you would have any significant inductive kickback with a reed relay or other low power relay with the diode across the coil as you have it. If your supply already has a common ground, just leave it and use the transistor rather than an optoisolator. Be sure to put the diode physically close to the relay coil terminals and keep the switching transistor and relay power source relatively close to the relay.