Values of Capacitor and Resistor in an RC Snubber

Without knowing the inductance of your load, it is difficult to give an optimal value.

Trying different values empirically is one approach and one that I have used in the past, but I was trying to dampen EMI at a small, 400 Hz (< 1kW) medium voltage transformer primary due to continuous operation as opposed to a discrete switching transient.

You may find this link to be informative:

https://electronics.stackexchange.com/questions/162129/how-to-calculate-resistor-and-capacitor-size-for-snubber-circuits

0.01 uF and 51 ohms isn't a bad place to start if you want to just wing it. How big a spark are you trying to quench?

Anonymous,

It is not clear what you are switching the solenoid with. A relay contact or a hand operated rotary switch. The speed of opening of the contact and the gap have a big effect on how long an arc can last & hence contact wear. Solid state switches have the great virtue that there is not an arc with wear and radio frequency interference.

Using a mechanical contact, it is best if there is no arc, once it arcs it is difficult to stop.

The graph below shows the limits for arc free switching....

Sorry, CR4 is not working to insert a picture!!!

However, at 3 amps [2 amps x 1.4142 = 2.82 amps for sine wave peak and 2 amps r.m.s.] the maximum voltage is about 15 volts for silver contacts. Call it 10 volts.

There is another requirement - rate of rise of voltage across contact should not exceed 1 volt per microsecond. A capacitor of 1 microfarad per amp across the contact will give that. So use 3 microfarad for 2 amp r.m.s.

Usually, discharging capacitor when contact closes can wear the contact or capacitors. However, a 3 ohm carbon composition resistor in series with the capacitor will only drop 9 volts at 3 amps - that is within the maximum 10 volts above. So RC is 3 ohms in series with 3 microfarads.

Seems Good?? But usually there are safety reasons for not having the capacitor across the contact, if it shorts the load is not turned off. Usually, the cap is across the solenoid, leaving the breaking of an unknown supply inductance.

Also, for DC, air at 1 atmosphere does not breakdown for any gap at less than 300 volts, but generally may be 2 kV/mm - so gap must reach 0.15 mm in 300 microseconds. But an AC relay [switching solenoid] that must not release in 10 milliseconds [because voltage goes to zero every 10 milliseconds] is slow.

Consider an inductance L of 1 henry, at 3 amps it stores 0.5L I² joules = 4.5 joules. Suppose all that energy goes into the capacitor C, which stores 0.5CV² joules.

So 0.5x 3 x 10^-6 x V² = 4.5: or V² = (4.5/1.5)x 10⁶ ; so V = √3 X 10³ = 1730 volts. It requires 1mm gap to avoid breakdown. the resonant frequency of 1H & 3 μF is 1/(2pi x √(3 x 10-6)) = 92 Hz, a period of about 10.9 milliseconds - in 1/4 period, 2.72 milliseconds, reaches peak voltage.

The capacitor needs a rating of about 3 kV, 1730 + 680V = 2410 V, + tolerance of 480V & solenoid current. Peak voltage of 380V r.m.s. is 680V.

In practice, it is usual to parallel the capacitor with a voltage dependent resistor [VDR] to dissipate the solenoid energy - otherwise the L & C can resonate for an unknown time depending on solenoid resistance etc.

It is also difficult to get an inductance value - in practice, an AC solenoid has a thick shorted ring turn as well as the winding - by carrying a current out of phase with the main winding it stops the magnetic field [and holding force] falling to zero twice per cycle, avoiding excessive vibration of the mechanism. Ideally, the current in the main winding switches instantly to the shorted single-turn ring. In addition, as the solenoid moves it changes the air gap with major effect on the inductance, or to be more relevant, the energy stored in the iron of the solenoid.

In practice, it is usually a matter of brute force in the switching contact, springing open quickly to a big gap to extend the arc and burn away the energy quickly. Power relays can have a ceramic permanent magnet next to the contact to push out the arc into a loop which is longer and loses more energy and/or a double break which makes 2 arcs. Two or more contacts of the rely in series can speed up the break & increase the breaking capacity.

My experience

I once had to determine the RC suppression of a power station tripping relay contact. The tripping relay had been changed to a different make from one which had done the solenoid switching reliably many times on many sets [I think it had double-break contacts]. The contact of the new relay [rated 110V DC 3 amp resistive], turned sooty black after one break with big arc and soon failed to make properly.

The 130V solenoid drew 1.06 amps and was claimed to be 1 henry. A VDR clamping to 220volts was tried both across contact and across solenoid. Result, less severe arc but still to full contact gap. An inverse diode across the solenoid was no better [and would slow the cut-off of fuel to the gas turbine required in a few tens of milliseconds].

A 1 microfarad capacitor in series with 10 ohms was put in parallel with solenoid - no visible spark on break at all.

To prove the effectiveness, a miniature relay, contact rated 1 amp 100V 30 watts DC resistive, was used to switch the solenoid 1500 times at one operation every 2 seconds. Slight spark on make, no failure or sign of wear at end of test. A hermetically sealed relay, of type which had soon failed doing normal start/stop switching of solenoid, was tested to 9000 operations with the RC network suppression.

In service, both RC network and VDR were put across solenoid.