Hi,
I am looking for a 2uF capacitor and 2 ohms resistor to make RC network for protection of my relay outputs against spark.
The relay signal can be as high as 250V AC. So capacitors and resistors should be rated for high voltage. Besides, The capacitor maximum discharge rate di/dt can be as high as 100A/us.
Can you suggest some part numbers based on best practice for this application.
Thanks
Kaveh,
We would need more information.
Capacitor.
What type of capacitor?
Voltage rating?
Through hole or surface mount?
Resistor.
What type of resistor?
Wattage rating?
Through hole or surface mount?
2 Likes
This is an RC network for spark protection of relay outputs.
It just takes effect during open and closure of the relay contacts.
I believe voltage ratings should be at least 250V AC. Type of the resistor or capacitor I dont know. What is the general practice for the material of R and C for such application?
Through hole or surface mount is not important.
Selection of components for snubber circuits is usually dictated by peak pulse tolerance characteristics. A worst-case peak power dissipation for the resistor in your diagram for example, would occur during contact closure when the capacitor is charged to peak line voltage. For a 250VAC source and 2 ohm resistance, the peak power dissipation is (250v * 1.414)2/2Ω =62.5kW. Bulk-element resistors are the best choice for this sort of application, for reasons described in that link.
Similarly, polymer film capacitors are about the only reasonable choice for snubbing AC-fed switches. Product series that are well-suited to pulse loading are usually described as such in the “Applications” column, but this is a rather subjective characteristic, and cannot be trusted entirely either for including or excluding products for consideration.
Finally, please note that a snubber of this type will probably shorten the life of a relay considerably, because the capacitor discharges through the contacts while they are bouncing during closure. Contact arcing during opening is normal, and usually of lesser destructiveness since the arc current is limited by the load impedance. A properly-specified relay will function for many, many contact cycles without a snubber in most cases, and if the absolute maximum in durability is needed, implementing a solid-state relay in parallel with a mechanical relay permits the use of solid-state techniques for dealing with contact wear, while retaining the low on-resistance benefits of mechanical switching for steady-state operation.
3 Likes