RC snubber circuit design and calculations for inductive loads

You should not switch inductive loads without some form of flyback or snubber protection.

Using simulations we identify the problem of using switches or relays to control inductive loads. We then see how to include an RC (resistor-capacitor) snubber circuit to absorb energy from the inductor and therefore reduce transient voltages. We then go through the calculations step-by-step to choose appropriate RC values for your circuit.

Note to my students: RC snubber circuits are not an examinable topic for GCE A level electronics but are a useful topic to be aware of, particularly if you plan to switch inductive loads (e.g. solenoids) in your project work. The video draws on some topics that appear may appear in GCE A level electronics and physics.

Useful references:

[erichfeit]

Thank you. A very useful and excellent video. I used to know this but have long forgotten it.
Erich a 76 year old teacher

[nrdesign]

Super useful video. I was just learning about how to design a snubber circuit for a TRIAC in an AC application. The catch there is that you also have to keep in mind the reactive power that will be flowing through the snubber when the switch is not closed.

[ministeriomundialliberando]

Excellent. Very good explained. But when using high frecuencies switching, calculating snubber circuit I think it would be more complex.

[ajibadesamuel]

This is snubber simplied. Thanks for the video. I did deduce something from your explanation, the RC network can also be calculated from the inductance voltage formula thus: VL = L I/(RC), by substituting those calculated values and the inductance value into above formula the predetermined spike voltage arrived at.
Thank again

[jorgerive]

Great video. Thank you. What does the power dissipated in the snubber resistor look like as a function of switching frequency? The 1.2A is the current charging the capacitor, but not the current through the snubber circuit when the switch turns back on and the capacitor discharges through the switch, forming the loop Rsn, Csn, and Rsw (sn= snubber, sw = switch). So, Rsn sees both charging current and the discharge current. What is the best way to compute the power dissipation of this snubber resistor?

[davitberishvili]

it's well simplified example and calculations were very easy to understand, thank you

[MihirShahLNM]

Thank you very much for an easy explanation of a complicated subject. I appreciate the flow of explanation was fantastic. I was just curious if the calculations remain the same with 230V AC signal? Could you refer some site on this subject.
Further, if Snubber is in parallel to the load, will it be better option.

[UpcycleElectronics]

Thanks. I tried playing around with a snubber circuit, but it seems my 50MHz scope is not fast enough to see the transient voltage spike. My effort was kinda pointless anyways. I don't know how to effectively calculate a universal motor's inductance. My circuit just uses the default 100nF/100R values and with my scope, I guess ignorance is bliss because it works. Thanks for the upload.
-Jake

[galvanmedellinjaret]

Sir. Excelent work.
It’s gonna be so usefull for school. Thank You very much.
My best regards from Mexico 🇲🇽

[ThomasHaberkorn]

Is there some impedance matching going on in this example? Great video

[tomcarroll]

Nice job. You made it simple and easy.

[petertyrrell]

Great video. As you doubtless know, a flyback diode across the coil will clamp transients to around .6 volts. I suppose a snubber network is used in situations where a diode can't be used, like in an ac circuit.

[gerardogonzalezpalero]

I've been looking for this for quite a while. Thank you!

[kentharris]

I'm using an electromagnet that is rated at 12 volts at 0.5 amps. Checking with the Oscilloscope the half inch ball bearing hits the aluminum foil every 0.8 seconds and arcs that are visible. I purchased both a electrolytic capacitor kit and 1 watt resister kit. Hopefully the wattage on the resisters is more then enough. I take it on your resister and capacitor values you pick the next highest value out of the kit?

[remcovantriest]

A very nice example, for AC voltage would the same formula still apply?. I have a small coil with hydraulic motor that measures 4106 mH/10VA according to the plate and 240V /60 Hz. supply. the maximum amperage we could permit is 1 amp

[davidq]

Nice video.
I have a question though, the energy stored in induction is 0.36J, then we use this same amount of energy to workout the capacity of the capacitor. Because of the existence of bottom R(the one 41.7 ohms), some of the energy is consumed by resistive loss. So I assume there is a step of the calculation proving the energy consumed on R41.7, for some reason, has to equals to the enemy provided by battery AFTER switch is turned off. Would it be possible to elaborate a bit on this?
Thanks a lot

[CoolDude-vcfl]

I like that you included calculation. One question, for example i need to install snubber in my circuit which controls motor via relay (because micro controller restarts from the motor spike). i don't know the inductance of motor. Should i use LCR meter to measure the inductance of motor? Is that a correct way? If not then can you calculate the Energy from the spike using oscilloscope? Thanks.

[BrentLeVasseur]

When you are calculating the energy stored in the coil, as you assuming that it is oscillating at the coil’s resonant frequency? Because a coil’s voltage and capacitance varies depending on the frequency of the LC circuit and it only achieves it’s maximum energy storage value when it is oscillating in resonance.

[NeoGio]

A very well explained snubber video, probs the best on YT! I love well explained videos! Everything can be simplified and well taught. I'm curious about one thing however. If our coil had quite a few turns (say primary of a transformer), it would have its own resistance. Let’s say our coil measured 10 Ohms for example. Wouldn't this greatly affect the calculations due to the current restriction of the coil itself?

[ThomasHaberkorn]

In practice, I found it difficult to get reliable values for the inductance L for solenoids. The iron losses are highly dependent on frequency, which is not well definable for this switching circuit. If there is a better way, please let me know