Tuesday, 9 October 2012

Relay Bounce

I was using an oscilloscope to do a little troubleshooting on a project that involved having a microcontroller flip a relay and I stumbled across something interesting.  I was graphing the voltage across the relay contacts (there was a good reason for this !) and I spotted this:-

When the relay snaps shut the voltage is supposed to suddenly drop to 0 as the contacts short.  As you can see that's not exactly what happened.  Instead it bounces between open and shut 4 times before finally settling on closed.  This all happens over 16ms so you might never know about it if you weren't looking at it on an oscilloscope.  I figure what is happening here is that the moving relay contact is quite literally bouncing off the fixed one.  Note how the closed period (0v) of each successive bounce becomes longer until it finally settles.