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Old August 9th, 2019, 12:50 PM   #1
NetNathan
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24vdc coil surge suppressor

For a 24VDC relay coil...
For surge suppression....would you use Resistor/Capacitor OR Varistor?
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Old August 9th, 2019, 01:03 PM   #2
Steve Bailey
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Here is a discussion of various suppression techniques. It is written from the standpoint of relay coils but applies to all inductive loads.

https://www.te.com/commerce/Document...=CS&DocLang=EN
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Old August 9th, 2019, 01:40 PM   #3
NetNathan
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Quote:
Originally Posted by Steve Bailey View Post
Here is a discussion of various suppression techniques. It is written from the standpoint of relay coils but applies to all inductive loads.

https://www.te.com/commerce/Document...=CS&DocLang=EN
Thanks...nice read.

They are Automation Direct - Fuji relays and only "Varistor" or "Resistor/Capacitor" is offered.
I am probably going to use resistor/capacitor, due to fact that a varistor is "one time only device".
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Old August 9th, 2019, 02:13 PM   #4
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I think both Varistor and RC network are for use with AC coils. DC coils are more usually used with diodes. I was going to Google to confirm but we have had major electrical supply problems and the internet hasn't fully recovered yet.

I was using Moeller contactors and they don't offer suppression for their DC contators because they have it built in. Maybe that is why Fugi don't offer anything for their DC coils. Varistors aren't 1 time devices unless they are too small for the job. They should discharge the energy and then recover back to normal as their temperature drops.
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Old August 9th, 2019, 03:32 PM   #5
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DC surge suppressors are simply diodes wired reverse across the coil.

When the DC voltage is dropped to a coil it may generate a pulse of reverse DC that the diode would short out and protect the control circuit / PLC output.

EDIT: Some DC relays have this built in, that is why if you wire it backwards you get a direct short, while other brand relays still work wired reverse - and some don't show a polarity preference.
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Old August 9th, 2019, 04:18 PM   #6
BITS N BYTES
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I've used IN4001 diodes for all 24VDC relays and coils for over 25 years. Never had a problem. Cheap and simple to install.
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Old August 9th, 2019, 10:06 PM   #7
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Good discussion. Some may not be aware that many PLC transistor output cards also have a diode on each output. Not sure if that's intended for surge supression but I would have to think it works for that. Would be curious if anyone knows how effective it is (diodes too small?) or if supplemental surge suppression is still recommended.
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Old August 10th, 2019, 07:03 AM   #8
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Originally Posted by rupej View Post
(diodes too small?)
The commonly used 1N4001 is small itself. The reverse pulse that would go through it is so small of a current that the 1N4001 is overrated for the job.

Just to CYA I would still put a diode on the relay itself, maybe the PLC or output card will be changed in the future that doesn't have them, or the output diode does die because 24V got hooked up to the wrong terminal, or ...........
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