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Topic: Relay pro tips - Commutating diode (Read 422 times) previous topic - next topic

Geek Emeritus

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.
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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 -
Experienced responders have a nose for laziness, (they were beginners once)... Sure, there are trolls, chest-beaters, and pretenders - but the help you'll get here is about as good as it gets - if you try to help youself!.

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