one of those things that is obvious when it's explained, but not before:
- if you run a current through a wire, you generate a magnetic field
- if you pass a magnetic field across a wire, you generate a current
- if you pass a current through a coil of wire you get a larger magnetic field, because more wire
- if you cut off that current, the larger magnetic field collapses
- back into the coil
- generating a reverse polarity voltage spike back into the current source
- which explains the vast blue spark when you unplug a coil
and your Arduino eats that reverse spike. On an old Triumph motorcycle, 6 volt coils, I measured 321 V spikes when the points opened.
which is why NASA was insistent that the products they bought have a relay across the diode, with a high enough PIV Peak Inverse Voltage rating to shunt the voltage spike to ground or the regulated power supply:
you have the same situation on the load side. in the drawing above, if COM is a power source, and NO & NC both go to inductive loads, both NO & NC need a diode that is reverse biased to the load voltage, shunting the reverse spike from the load to ground. A diode on COM can't work, because it will be disconnected at the precise time i8t is needed.
it is called a commutating diode. this is not a particular kind of diode, like a zener or Schottky diode. It's a generic diode that is used to mitigate the effect of polarity reversal.
SSRs do not need commutating diodes on the 5 volt logic side, but use them on the load side.
A decent post, but the title is trying to make it sound important.
Adding a relay isn't a *pro* tip - it's fundamental.
Perhaps thread should be renamed -
WHY DO I NEED A DIODE ACROSS MY RELAY.