need a capacitor across relay contacts to arrest arcing. wondering about type and value ?

[harry794]

wondering what capacitor type and value across relay contacts at 7amp ac load to avoid arcing

 

[Marvo]

You will get an arc when you make or especially when you break a 7 Amp load, this is normal and usually with an AC supply it will only last for a half cycle which is a few miliseconds. The relay contacts should be designed to handle the arc if the relay is sized correctly and there's no need to retrofit any components to mitigate the arc.

 

[harry794]

You will get an arc when you make or especially when you break a 7 Amp load, this is normal and usually with an AC supply it will only last for a half cycle which is a few miliseconds. The relay contacts should be designed to handle the arc if the relay is sized correctly and there's no need to retrofit any components to mitigate the arc.

thanks marvo- luv the icon btw says it all-- as for the relay its an RS 356-758 and rated at 10amps but seeings as the load i am running is resistive and not inductive the spark looked rather excessive and i would have liked to get rid of as much as possible to prolong the life of the thing.. harry

 

[westward10]

Have you thought about a solid state relay.

 

[Lucien Nunes]

For suppressing 230V AC contacts, 0.1μF is typical. It's good to add a series resistor of a few hundred ohms to prevent contact welding if the capacitor is still charged when the contacts close. The ideal values depend on the inductance of the load and the permissible leakage current that will flow through the capacitive reactance into the load while the relay contacts are open. A common solution is to use a commercially made snubber network combining R & C in one unit.

See:
RC snubber networks from RS Components

 

[harry794]

For suppressing 230V AC contacts, 0.1μF is typical. It's good to add a series resistor of a few hundred ohms to prevent contact welding if the capacitor is still charged when the contacts close. The ideal values depend on the inductance of the load and the permissible leakage current that will flow through the capacitive reactance into the load while the relay contacts are open. A common solution is to use a commercially made snubber network combining R & C in one unit.

See:
RC snubber networks from RS Components

thanks for that- i have probably got numerous parts somewhere tucked away in my workshop. i would have thought that the value may have been slightly different. btw i am supplying a print bed heater for a 3D printer (hence resistive load) so when the 7 amp heater switches out the arcing can be considerable and i didn't want to knacker the contacts too soon as it switches at quite a rate.

 

[freddo]

If such a load is being switched on an off a lot such as to closely maintain a temperature, it is common now to find solid state relays used for this purpose. Be aware though there will need to be some sort of thermal protection fitted to the heater as solid state relays can fail permanently on.

 

[harry794]

If such a load is being switched on an off a lot such as to closely maintain a temperature, it is common now to find solid state relays used for this purpose. Be aware though there will need to be some sort of thermal protection fitted to the heater as solid state relays can fail permanently on.

Hi Freddo. yes that's one reason why electronic relays don't really figure at such high currents plus having to add other protections and making things more overly complicated than they need to be- i find my self already having to build a box of tricks for this project and one can go on and on and what was a simple project can become cumbersome. and as my mother used to say > the more there is the more you've got to go wrong thanks anyway. harry

 

[freddo]

Solid state relays should be more reliable than mechanical relays and are available with contact ratings over 100A. Most are 0V switching which reduces switching interference. I spent a few weeks a while back helping a local manufacturer build control panels for milk pasteurisation equipment, the design used a small solid state relay to switch a 3KW immersion element.

 

[harry794]

If such a load is being switched on an off a lot such as to closely maintain a temperature, it is common now to find solid state relays used for this purpose. Be aware though there will need to be some sort of thermal protection fitted to the heater as solid state relays can fail permanently on.

Hi Freddo. yes that's one reason why electronic relays don't really figure at such high currents plus having to add other protections and making things more overly complicated than they need to be- i find my self already having to build a box of tricks for this project and one can go on and on and what was a simple project can become cumbersome. and as my mother used to say > the more there is the more you've got to go wrong thanks anyway

Solid state relays should be more reliable than mechanical relays and are available with contact ratings over 100A. Most are 0V switching which reduces switching interference. I spent a few weeks a while back helping a local manufacturer build control panels for milk pasteurisation equipment, the design used a small solid state relay to switch a 3KW immersion element.

hi. i was saying to someone last night about hearing the click of a mechanical relay is quite satisfying. however i take your point about being able to switch hefty loads with electronic relays.