Hm… with such a nice switching 5V pulse, it’s sad to tear down yesterday’s circuit again.
One more experiment then. Let’s switch a little relay (the one used in the Relay Plug) at 10 Hz, and give it a serious beating. I’m going to put my dual power supply to work and put 5V @ 3A straight across the relay’s contacts. In other words, let’s see what happens when we switch the full 3A across those tiny relay contacts.
At first sight, it all looks reasonably ok:
That’s 5V across the relay while it’s open, and about 0.3V when it closes. So these contacts seem to have a resistance of about 0.1 Ω. The overshoot when the relay opens up again is probably in the power supply, as it recovers from just having been shorted out at 3A. Note that any inductance in the wiring will have this same effect. Inductance is a bit like a flywheel – it wants to keep going after such massive current (i.e. magnetic field) changes.
It’s quite impressive to see how this little relay rattles away at ≈ 10 Hz, working just fine.
But those transitions look quite different when we zoom in:
You’re seeing a mix of arcing and contact bounce, as all mechanical switches do.
Note that the switch hasn’t quite closed yet – there is still about 1.5V between the contact points, 30 µs after switching – due to contact bounce. This drops to 0.3V after some 400 µs, at which point the contacts are really firmly closed.
Here’s a much better example, this time using a 10 Ω resistor in series so the power supply just delivers 5V @ 0.5A without going into current-limiting mode:
Opening is again not quite what it seems, once you zoom in:
Two things happening here: the mechanical release of the switch (less resistance as the pressure on the contact decreases, followed by some arcing), and then a fairly linear ramp and some overshoot as the power supply recovers its 5V setting after having been shorted out. Think of a stretched rubber band, and how it “overreacts” when released.
So as you can see, a relay does some nasty things while switching on and off!