Coil Suppression on MC4x Outputs

Seneca 4 years ago in Master modules / MC4x updated by Gustav Widén (System support) 4 years ago 1

Hardware Question:

In the Documentation for the MC4x family of controllers - it says in many places to use diodes for external coil suppression, for example when driving a relay with the Digital Out HS driver from the MC4x.

A relay that I have specific to my application requires (highly recommends) the use of varistor coil suppression, which is less "harsh" on the relay device during transients. The specification requests a clamping voltage of 3x the coil voltage, i.e. 36Vdc for a 12Vdc relay coil.

Are there any adverse affects of using a varistor instead of a diode for coil suppression? 

Perhaps this could be also compatible with the hardware design of the MC4x series of controllers?

(Hint: I already have tested this configuration on my system to many thousands of cycles under load with no discernible affect on system operation or relay operation)

In my experience, the tricky part when considering other types of transient suppression (than the regular clamping diode) is to know that the clamping voltage will be sufficiently low. 

In 24V systems I have seen some designs with zener diodes that had no effect in protecting the output, as the clamping voltage was too high.

If you have no external transient suppression, then the high-side driver of the module will take the hit and cut the transient. 

If you have installed transient suppression (varistor or zener diodes) with a too high clamping voltage, the high-side driver of the module will also take the hit. 

In a 12V system, you can select a lower clamping voltage, so maybe your solution with a varistor (or other solutions with zeners diodes) could work in the 12V case. 

Worth noting is that in an application with a relay coil, the load might be so small it falls within the acceptable range for maximum allowable load inductance without clamping diode.