Sizing RC Snubber for 120 volt A/C coil

[BeringCSparky]

Hello,
First time posting to this forum and not sure I am posting to the correct place so forgive me if I am not.
I am an electrician and just starting to dip my feet into the world of electronics, I learned from the school of hard knocks and have no formal education so I apologize if my question seems elementary.

I was hoping someone could help me choose or narrow down my choice of some RC snubbers for 120 volt A/C coils.

I have a plc digital relay output controlling a relay, holding amperage of the relay coil is 100ma @ 120 volt A/C I was thinking to add a snubber to the coil to help protect (make last longer ) my reed contacts in the plc output.
Also the relay N/O contact is controlling a 120 volt A/C solenoid valve and I was thinking to install a RC snubber on the solenoid valve coil as well to protect ( make last longer ) the contacts of my relay the holding amperage of the solenoid valve coil is 500ma.

I have done much reading on the subject and have spoken to several manufacturers of RC snubbers but am now more confused than ever.
RK Electronics recommended using 220 ohm resistor with 0.47 uf capacitor for both coils.
Red Lion recommended using 47 ohm resistor with 0.1 uf capacitor for both coils.
Canfield Connectors said they only make one type 10 ohm resistor with .005 uf capacitor that they install in there DIN connectors and they use it for all coils no matter what the resistance.

So you see where I am scratching my head, I have looked at many formulas for calculating the values needed but at the moment they seem a bit over my head. Have also read that you can connect an oscilloscope and try different value components until you see which combination dampens the transients best. (Don’t have scope yet)

I need to order something quick for this project and hope to get some useful advice from any electronics guru's on this site.

Thank you in advance for any information and I look forward to browsing this forum often.
BCS

 

[BeringCSparky]

Maybe if i phrase this a different way it would be easier.
As I understand it we want the resistor in the snubber circuit to be as close to but not more than the resistance of the device coil. (is this correct)
Assuming this, out of the three manufactures RK Electronics which recomended using 220 ohm would be more on the money.
(is this correct)
120 volt / .5 amps = 240 ohms ( so 220 ohm resistor would be right in the ball park for this solenoid coil )
However using this same assumption for the coil of the relay 120 volts / .1 amps =1200 ohms ( a bit further off (20%)but should still provide a path for the capacitor to charge and give the contact time to get air between them without arching. ( is this correct)
So using this same theory if i were to use the DIN connector with inbedded snubber circuit with 10 ohm resitor ( from Canfield Connectors ) it would only be 5% of the resistance of the solinoid coil and less than 1% of the resistance of the relay coil yet the manufactures say it will provide me with the protection i need. ( does this sound correct )
If so can you breifly explain why the resistor value varies from manufacture to manufacturer yet they all say there value is just fine to use and will protect the contacts no mater what the resistance or amperage of the coil being used.
(RK does offer other sizes but said this on was the most common used) (the other two mftr only offered me the one size and just by doing the math seem to be on the small side)

Also if possible can anyone give me a brief explanation of how to come about the proper size capacitor if you were engineering this example.

Again I am new to electronics so I might be wrong in what I am saying, and would like to hear from others with more knowlage on the subject.

Thanks again in advance for any feed back.
BCS

 

[chuckey]

I have had a problem with relay coil snubbers, the fault was that when the relay was energised it would not de-energise. In the end it was due to the change of inductance of the relay coil when the armature moved and completed the magnetic path.
The circuit was a triac that had a slugging circuit on its gate, so after 4 seconds after the supply came on , the triac was switched off and dropped out a AC plug-in relay (motor start circuit). As a precaution, I put a .047 MF cap across the triac. When the supply came on, the relay pulled in, after 4 seconds - nothing. The triac must be faulty so I changed it, same again. it was only when I replaced the relay with a resistive load and measured the voltage I figured out what was happening! When the triac was off I had 160V across the relay (240 V mains).
Frank