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:



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.
Title: Re: Relay pro tips - Commutating diode
Post by: EclectricCraftsman on Nov 07, 2019, 11:59 pm
So... I shouldn't just wire a nano digital output directly to trigger a relay? It does trigger the relay but I have a project that I am working on and am new and not sure where to seek advice.
Title: Re: Relay pro tips - Commutating diode
Post by: TheMemberFormerlyKnownAsAWOL on Nov 08, 2019, 12:06 am
So... I shouldn't just wire a nano digital output directly to trigger a relay?
Unless it is a reed relay, drawing much less than 40mA, that's right, you must not connect the coil to an output pin.

But you don't want to be asking questions here, in six month old threads.