40A Relay Reliability enhancement

Thread Starter

Jonathan Foong

Joined Mar 13, 2021
27
I am using a 40A relay of a 12V coil of 140mA with a diode for a 24V 25A load. I suppose during the opening and closing of the contact, which is likely under strong impact of 25A current flow of sparks, in the long run, the formation of deposition of oxide and worn out will lead to poor current conduction.
Is there any circuit enhancement to improve or to reduce the high impact of spark? If SCR is a better choice?
 

dendad

Joined Feb 20, 2016
3,909
Including an R/C snubber circuit may help.
I have seen articles that use a FET in parallel with the relay contacts.
Drive the relay and the FET together for switching on. The FET will close the circuit first, then the relay will carry most of the current.
For switch off, release the relay first, than after it drops put, turn off the FET.
It could be better to just use a big FET alone.
An SCR will not work on DC without a turn off circuit.

Actually, I think the article in question was for an AC application, a relay across a TRIAC as the TRIAC will switch off itself at zero crossing.
 

MaxHeadRoom

Joined Jul 18, 2013
23,618
If switching large DC currents with a relay, it is recommended to use arc suppression style relays, I have used one by a popular manuf that is fitted with magnetic arc blow-out.
 

crutschow

Joined Mar 14, 2008
27,718
If the load is DC, than a diode across the load (cathode to positive) will greatly reduce any arcing.

An SCR will only switch AC loads.

You could use a solid-state relay (SSR) for longer life.
 

MaxHeadRoom

Joined Jul 18, 2013
23,618
One thing to consider when using a rectifier across the load and where high current exists, it delays the drop out of the device, in some cases can be detrimental.
 

Thread Starter

Jonathan Foong

Joined Mar 13, 2021
27
Including an R/C snubber circuit may help.
I have seen articles that use a FET in parallel with the relay contacts.
Drive the relay and the FET together for switching on. The FET will close the circuit first, then the relay will carry most of the current.
For switch off, release the relay first, than after it drops put, turn off the FET.
It could be better to just use a big FET alone.
An SCR will not work on DC without a turn off circuit.

Actually, I think the article in question was for an AC application, a relay across a TRIAC as the TRIAC will switch off itself at zero crossing.
Thank you for your advice, sorry I had missed out that it is a 24VDC 25A for a solar heater element controlled by a temperature controller.
 
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