Arduino Forum

Using Arduino => Introductory Tutorials => Topic started by: Geek Emeritus on Mar 12, 2019, 07:30 pm

Title: Relay pro tips - Commutating diode
Post by: Geek Emeritus on Mar 12, 2019, 07:30 pm
one of those things that is obvious when it's explained, but not before:



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:

(https://electronicsclub.info/images/diopro.gif)

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.

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/inductor-commutating-circuits/

SSRs do not need commutating diodes on the  5 volt logic side, but use them on the load side.
Title: Re: Relay pro tips - Commutating diode
Post by: lastchancename on Mar 12, 2019, 10:07 pm
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.