Relay Coil Supression

[Martinelli21]

When are you supposed to use a suppression device on a relay DC doil? Are there any general guidelines that anyone knows about? In my application, I am using 125 VDC relays with a coil burden of 16 W max. We have placed diodes across these relays in the past, but I am wondering if if its really necessary. There is no digital or solid state equipment affected by the relays and it is not much power. Any help would be appreciated.

 

[ScottyUK]

You will improve the life of any mechanical contacts switching the DC coil if the diode is present because there will be less arcing at those contacts. If you are switching DC using semiconductors the diode is pretty much mandatory.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[walker1]

The thing with sending DC current through a coil is that when you disconnect, the coil will try to keep the current flowing.
This can create some quite high voltages.
If you control the relay with a mechanical switch, the voltage will keep climbing until an arc is created over the just disconnected switch.
If you are using a solid-state switch (transistor, FET etc.), such an arc is usually only created once. (Burn, Hole through)
The reverse diode across the coil gives the current a way to flow when disconnected, keeping the voltage across the switch to the power supply voltage + 0.65V.

 

[itsmoked]

To answer your question; "When are you supposed to use a suppression device on a relay DC doil?"

A L W A Y S.


If "doil = coil".

 

[dbaird]

Just a thought----> in todays world most, if not all, DC relays have the back EMF diode incorporated integral to the coil. You may want to check manufacturer to see if this is not the case with yours.

David Baird
mrbaird@hotmail.com
Sr Controls Engineer
EET degree.
Journeyman Electrician.

 

[buzzp]

The coil is different than the contacts. The diode on the coil is to reduce the voltage rise when the coil is de-energized. If the diode is not there, the voltage rises well above the coil voltage, potentially causing a flashover in some other equipment with a dielectric/spacing which is inadequate.

Chances are, it will never get to the coil contacts (after all this is the reason we use relays - lower control voltage can switch a higher voltage through its contacts). So the statement concerning contacts is invalid. The suppression of arcs across contacts requires another suppression across the contacts not the coil.

If you do a search I have posted many links in another thread talking about what your inquiring about.

 

[BobM3]

Careful with just a diode across the coil. It causes the magnetic field to collapse slower and the relay will break slower which can cause burning of the NO contacts. P&B (now Tyco) has some coil suppression circuits that allow the field to collapse quickly while keeping the voltage spike fairly low.

 

[melone]

Add a resistor in series with the diode, and you will disapate the energy quicker. However, the larger the resistor, the higher the voltage will rise.