Designing an RC snubber for AC solenoid lock (inductive load)

Question

I am designing an AC solenoid lock. Here is my design:

• Resistance 8 Ω
• Main outlet power source 220 V - 50 Hz
• Inductance 150 mH
• Impedance 47 Ω
• I (RMS) ~ 5 A

I read that the inductive load needs a snubber circuit to protect the switch and electric components from inductive turnoff transients (but some say there's no need for a snubber).

If I want to add a dimmer voltage regulator circuit to this design, do I need to add a snubber to the circuit?

Dimmer specifics

• 50 - 220 V
• Less than 1000 W

If so I will add an RC snubber as follows:

Resistor = 44 Ω (I chose 44 Ω because I want 220 V to be the max. voltage across the switch and the dimmer)
V = I·R = 5·44 = 220 V

Energy in inductor:
E = ½·L·I² = 0.5·0.15·5² = 1.875 J

E = ½·C·V²: 1.875 = 0.5·C·220²
C = 0.00009375 = 93 μF

So, are my calculations correct and do I need a non-polarized ceramic capacitor 93 μF, 220 V?

Should I add the RC snubber parallel to the switch or parallel to the inductor?

Edit:

Thanks guys for your suggestions.

But please i decided to make RC snubber (only resistor and capacitor) to just protect my switch and dimmer

So i will only need answers about how to make RC snubber to this circuit and how to calculate the capacitor value i need

Answer 1

The most efficient solution would be to switch the coil with a triac. This handles turn-off gracefully, as the triac will keep conducting until current in the coil reaches zero.

The same triac can handle dimming, so there is no need for a separate dimmer.

If you want a dimmer and a switch, you could just use a dimmer, and put the switch in series with the dimmer's potentiometer, so that the dimmer's output goes to zero when the switch is off.

However, if you use dimming, you should be aware that, under certain circumstances, if there is enough inductance, the triac can conduct for enough time that it misses the next triggering pulse. If this goes on, it will only conduct during one of the half-waves on each cycle, always on the same polarity, and it will miss the opposite polarity half-wave. If that happens, the coil will be supplied DC instead of AC, its inductance will no longer limit the current, and it will burn. This is explained in this ST appnote.

Answer 2

I provided an answer for a similar question: Sizing a RC Snubber - microcontroller restarting when switching off an inductive load. You don't really need to size the capacitor that large. Generally, you can use about 100 ohms in series with 220-470 nF, and the snubber is usually placed across the inductor. A 93 uF capacitor will have a reactance of 34 ohms at 50 Hz and with the 44 ohm resistor will be impedance of 56 ohms and will draw about 4 amps.

Some other similar answers:

How to design an RC snubber for a solenoid relay driving an inductive load?

Designing AC kick solenoid lock

How to make a 220 V AC solenoid lock

Here is a simulation showing the action of a snubber:

I did another simulation using your values, and found that I had to increase the capacitor to 4.7 uF to reduce the transient peaks to 800V. The inductance and capacitance form an LC tuned circuit that will "ring" until the resistance dissipates its energy. And the snubber resistor dissipates 25 watts:

There are probably ways to calculate the values, but I find it's easier to use LTSpice. This snubber draws about 400 mA so resistor R1 will dissipate about 7.5 watts.

[edit] As can be seen, a snubber sufficient to limit inductive spike to a peak value near that of the applied voltage will waste a lot of power. A resistor of 44 ohms and capacitor of 93 uF will have an impedance of 53 ohms at 60 Hz and the resistor will dissipate 750 watts. So that is clearly impractical.

Another method is to absorb the inductive energy in a TVS diode. A 5000 watt TVS can handle 2 Joules (Watt-seconds) for 400 uSec.

Perhaps the most efficient method, short of using a TRIAC or SCRs that stop conducting at a current near zero, is to add a bridge rectifier and large capacitor across the solenoid. It might be possible to use the DC voltage for some purpose rather than waste it in a resistor.

Here is a simulation showing a 100 uF capacitor and a 25k bleeder resistor that dissipates 5W:

Answer 3

Note that snubbers are used "only" when always "same working" conditions.

Something as this could also be used (VDR or MOV).
Just dimensioned for Energy to be "released".
See applications notes and this. Case of Energy absorption. Switching off inductive loads.
R2-C1 (and R4 for C1 discharge) could also be added and adjusted as one wants.

The theoritical best solution is this, but the capacitor to be used is very "prohibitive".

With the use of VDR - MOV

Here, another positioning of VDR.

Answer 4

RC snubbers are good but for your setup they waste too much power when you want the peak volts to be quite low. Why not look at a RCD clamp but because you are using AC the diode is a standard mains bridge rectifier. Try big C like 220 microfarad electro (remember that the cap only sees DC from the bridge) and a say 100k discharge resistor. If this goes across the AC solenoid the little power that is wasted is only burning when the solenoid is active. Also remember that a snubber across the switch terminals could bleed enough power to make the solenoid stick.