RC snubber circuit design and calculations for inductive loads
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- เผยแพร่เมื่อ 21 ก.ย. 2024
- 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:
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Thank you. A very useful and excellent video. I used to know this but have long forgotten it.
Erich a 76 year old teacher
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.
Glad it was helpful!
Name of software you used ?
Circuit Wizard
Excellent. Very good explained. But when using high frecuencies switching, calculating snubber circuit I think it would be more complex.
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
excellent analysis!
Thanks, now i am ready to over protect my circuits :)
I've been looking for this for quite a while. Thank you!
it's well simplified example and calculations were very easy to understand, thank you
great video , thanks for sharing the valuable knowledge.
Sir. Excelent work.
It’s gonna be so usefull for school. Thank You very much.
My best regards from Mexico 🇲🇽
You're welcome. I'm glad it's useful to you.
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.
correct!
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
50MHz should be fast enough unless the spike is in nanoseconds
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.
Maybe I should post a new video for 230 VAC supplies as you're not the first person to ask a similar question. Okay, let me think about it first. As for whether the snubber should be parallel with the switch or inductive load, I have always (rightly or wrongly) positioned the RC snubber across the switching element but assumed it could also be placed across the load. Thinking about this further, having the RC snubber across the load is a more direct attempt at protecting the vulnerable component (the switch) rather than across the load (which nonetheless should protect against a large transient). On the other hand, across the load would give the whole circuit protection against transients. Wikipedia and several other references show the RC snubber across the switch. Of course, a diode snubber would have to be placed in reverse parallel across the inductive load.
I found this reference that may be of interest to you - www.bpesolutions.com/bpemanuals/Snubber.pdf
Interestingly, they recommend the RC snubber to be placed across the inductive load "for best results" but mention that across the control contacts (the switch) "will also work". It's a shame they don't explain this any further
@@PizzeyTechnology Thanks
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?
Apologies, I failed to have mentioned that we are pulsing the switch (so transistor say, like a chopper circuit). Then we have rate of change!
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?
thanks so much from Valencia, Spain!
Thank you too!
great explanation sir
THANKYOU SIR
The resistor calculation is incorrect. Makes no sense to set R = Vm/Im. R should be calculated by setting the desired time constant RC, which will be dependent on the switching frequency. Al alternative is to set R by it's desired maximum power rating, which is also dependent of the time constant RC.
very useful information...Thanks lot...few doubts.... 1.if we are not using current limit resistance means what will be I?Can we take motors Coil reistance and Rds on as Resistance.....2.What will be voltage rating of snubber capacitor (C volt=V*2) 3.Power rating of snubber resistor
I may have to revise what I am going to say but I do not agree with his mathematical analysis presented in this video as the capacitor will never see the 50 volts voltage suggested across it. Here is the engineering reason for my conclusion.
1. When the switch is on, the energy calculation in the inductor was correct. The heat energy stored in the 10-ohm resistor cannot be returned, so that is correct in using only the inductive/magnetic stored energy. When the switch is on, the load to the battery consists of the resistor 10 ohms, the transient impedance of the inductor, and the zero output impedance of the battery acting as a VOLTAGE SOURCE which is basically zero ohms and the small resistor in the MOSFET.
2. When the switch is opened all the above changes, as with the inductor being present the battery and 10-ohm resistor and inductor will become a CURRENT SOURCE, with the current stated flowing, and this needs to keep instantaneously flowing at any voltage really!! So when the switch is open the current must be retained into the RC snubber which behaves as follows. Since the action is very fast, the capacitor or any value will be a dead short circuit and so the 50-volt voltage at the snubber acting as a load to the current generator, note "current generator", will be the current (1.2 amps) flowing through the resistor (41.7) ohms. This flow of current in the resistor will use some of the electromagnetic energy in the inductor with the additional energy being used to heat the 10-ohm resistor in the circuit, and hence one cannot use the full energy stored n the inductor to finish up charging the capacitor, as it does not, most of the energy goes in heating the resistor 41.7 ohm in the snubber, and the resistor 10 ohm in series with the inductor.
3. As was said before, any value of capacitor for a very fast voltage change across it will act as a DEAD SHORT CIRCUIT so the initial zero voltage of the capacitor will rise from 0 volts to 12 volts of the battery and the final value of the energy stored in the capacitor is due to the battery voltage only,. which is 12 volts. It is an interesting integral function that goes on in that capacitor as the current is slowly decaying in value.
4. 288 micro-Farads are just too high for an RC snubber where the MOSFET is periodically switched as the high energy in the capacitor will have to be dissipated by the Mosfet when it comes on, and it would not like it at all with all that energy in that large value capacitor. The approximations of theory and simulations are one thing, while the practical circuit is another.
A suggestion as to the amount of energy the snubber and other existing resistance is required to dissipate is the amount of energy stored in the snubber
capacitor. It is recommended that you choose a capacitance value that causes the resistor to dissipate one half the wattage rating of the resistor. Note that though the load /battery feeding the snubber is a current source with its own imperfect high output impedance also containing the 10-ohm resistor. Due to reasons not mentioned in this video, the snubber capacitor would be more likely to be around 1000 PICOFARADS and not MICROFARADS. It scares me to even think about that high value.
Thanks a lot, very nice and simple explanation.
Nice job. You made it simple and easy.
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.
I assume the voltage rating of the snubber cap is at least 50v ??
great VIDEO!!
Thanks very much for this awesome video, could you please explain snubber circuit in switching power supply.
Very good explanation. Thank you teacher. But, how I calculate r c when connecting a battery directly in a motor with a big current like 40A?
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
Excellent explained ! Thank you !
Glad it was helpful!
Thank you
Very well explained! Thanks
very very useful .thanks my master
Very useful explanation, thank you very much.
Thank you for a great explanation.
Glad it was helpful!
Hello my friend. This is a very usefull explanation. I wonder... how this calculus change when there is frecuency? Greetings from colombia!
Thank you very much for the video. Few Questions though
1. The power rating of the 10 Ohm resistor should be 1.2*1.2*10 = 14.4 Watt ?
2. If I am using the Inductive load of a Transformer, How do I calculate the saturation current of the transformer.?
3. I want to maximize the output voltage of a pulse transformer, How do i achieve that through circuit design?
Thanks in Advance
I very much wish to understand what the voltage will do if we take out the resistor and just use the capacitor to dampen the switch?
Thisis great, but I don't get why you use 50V for both the capacitor and resistor calcs. That would make 100V across the switch. They should both see a fraction of the voltage across the switch (and i can see that changes over time as the cap is first charged up then discharged by the resistor).
Thank you. This was really informative.
Nice approach and very simplified but it has some physics error which is the energy was being stored in the inductor will not be fully delivered to capacitor and will be less because both resistors will consume it while transfering to steady state after resonance period , am I wrong ?! ...
great vidio, thank you Please tell me about the simulator software etc.. It looks useful.
When you close the switch at about timestamp 11:51 why doesn't the simulator show the exponentional delay (due to the L/R delay) during the fall of the voltage??
THANKS MUCH!!
brilliant. i am making a scr control circuit for my welder. this helps a lot
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?
Congratulations from Brazil.
It's nice to know my videos are being watched all around the world. Greetings from the UK.
It's an estimation method to find out the values and make understanding about snubbing circuit. Plz add a real time experimental experiences
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
Very Nice, I would like to take this to next step. Can we snub the inductor instead of the switch or both at the same time. If we added a diode in parallel with the inductor; How can we explain this type of snubber across the inductor with RCD? This is problem I am having on a project now.
Very helpful. Thank you!
You're welcome. Glad to hear that people find the videos useful.
Thankyou for uploading this. Really appreciate the work. I was wondering, which software is being used for demonstration?
Thanks sir , peace upon to you and your family ,
Excellent explanation Sr!!
Glad you liked it
Thank you very much for this very informative video. I have a question. Right after opening the switch all voltage will drop across a resistor since a capacitor will be shorted and then all the voltage will be transferred across a capacitor during transient and then no current will flow through a snubber circuit. But then how does a capacitor discharge ? As per the simulation, we can see a voltage spike of 50 V acorss the switch and so across the capacitor, but after that moment voltage dropped down to 12 volt, so how did capacitor get discharged from 50 V to 12 V ? Through a resistor? If yes, then what should be the direction of current flow ?
Excellent, thanks alot for the video !
Nice explanation with calculation
Great video but what should be the max voltage for snubber cap ? 50v or 5kv ? and also the power of snubber resistor 1/3w or a 1watt resistor ?
If the snubber limits the voltage to 50V this may give a good starting point when specifying components. If the snubber works as intended, there would be no high-voltage transients in the circuit. As for the resistor wattage I'll leave that one for you to calculate. It's been a while since I watched this video but hopefully I've given sufficient information.
hi...pls clear me if i'm wrong.
why you considered 50v in the calculation of Capacitance finding. as the circuit is operating with 12v supply.
Also, you have calculated the circuit current by taking 12v voltage.
I think if took 50v instead of 12v calculation will be wrong. as we taking calculating value wtih 12v in all formulas.
Hello Shivm, those are good questions but you may have missed an essential point -- the circuit is capable of raising the voltage above the supply voltage. In the example I chose 50V but I could have chosen some other arbitrary (but acceptable) voltage. The snubber then suppresses the potentially high voltage caused by the inductor. Without the snubber, transient voltages can easily exceed the supply voltage by many times. Does that answer your question?
@@PizzeyTechnology Agreed thanks for the nice explanations.
beautiful explanation, thank you sir
Thanks and welcome
Hi, what in case it's a switching circuit where there is a time limitation where the capacitor needs to be charged and discharged in some predefined time set as per the frequency of switching.....
then when am I to use the formular C= 1/freq x Vsquared, coz you didn't mention the switching frequency?
Very informative .. I was in search of this and I do have a doubt that is ,
Why we need to keep the snubber circuit current equal to the current through he main solenoid circuit?
Hello, i need a snubber for my circuit but do not have the specific ressistors and cappacitors on hand, what if i were to use a much higher value? would it still funcion?
Excelente trabalho
Dear, i need 1000vdc x 20a its possible use snubber in contactors AC ?
I don't get it... if switch is closed or open the circuit is still complete ? am I missing something ?
very well explained. can you also explain the working of brake chopper circuit. looking forward.
Have you seen this article? It gives quite good coverage of how chopper circuits are used. www.kebamerica.com/blog/brake-chopper-circuits-vfds/
Why do you have choosen the whole inductance for the calculation ? As far as I know you choose the leakage inductance because this is the part that you can´t use (magnetic loss into air) and it comes back as the spike to the circuit. Of course I don´t know If we are talking about a transformer or air coil here ? If it is the second you are right just wanted to clearify things. :)
In this example I assumed a single coil. There should be no leakage inductance that would be associated with coupling a primary and secondary coil. Is that right?
Of course you are right ! With air coil I meant single coil too. I should have tell it more clearly. Leakage can only occur with two or more coupled coils. It doesn´t make a difference by the way If the medium is air or iron or ferrite as anybodycould think that because of my answer above. Single coil never can or will have a leakage inductance only coupled coils. The medium just changes the leakage value itself.
Regarding the RC snubber network, is it mainly to discourage charge from building up on the negative plate of the capacitor? As well as bleed any charge that might build up during normal operation?
Thank you sir.
Outstanding. Thank you.
Thanks good work.
Sir. Pl confirm RC value for 24VDC,2AMP solenoid load
Inductance is 46.70mH for 1Khz
Very nice explanation. I am designing a contact point triggered ignition, i replaced the hv contact with an nchannel mosfet 3205, triggered by p channel mosfet and the regular kettering contact point, i am having a problem i kept on burning the 3205 mosfet i already placed a diode between drain and source on 3205 but when installed it burns. Can you create a video of this using mosfet? Thanks a lot.
Usefull ty so much!
thanks for the great video. I still see RC packages on EMRs and mercury relays. Sometimes I see a flyback diode instead on the EMR coil. Any advantage to RC snubber vs flyback diode? Any reason to use both? Thanks!
Thank you very much.
No problem, Omid. I'm glad to help.
Thanks for a most useful and excellent Video, No idea what the inductance of the motor I want to use is ? However I guess I will figure it out, one (constructive) criticism, ditch the potato mic, hard to pick up some words. Maybe its the encoding settings?
Thanks for the feedback. Are you planning to control an AC or DC motor? An RC snubber can be used with AC or DC, but if your motor is DC then you may find a simple flyback diode a simpler solution. There are other snubber types too, including resistor - capacitor - diode. Regarding the audio quality (or lack of) it's a valid point and a long-standing issue that I have not yet addressed. It's due to a combination of factors not least of which is the cheap microphone and its placement. I'm working in a very cramped location (this would make a good video in itself) and I need to make myself an adjustable microphone stand. It looks like my videos will become monitized in the near future, and if that happens I will look into improving my setup.
@@PizzeyTechnology Thanks for your reply. I will try a snubber as I want to go through your steps just to get an idea of them.
I found an article
electronics.stackexchange.com/questions/182116/whats-the-easy-way-to-measure-a-dc-hobby-motors-inductance
that should do the trick for finding the small motors inductance. Its so obvious. I am being really lazy , relying on the google machine instead of thinking.
How should we calculate resistir power? Considering that peek power will be very high and impractical to use that value for selecting a resistor.
Excellent explanation.
A question: Is this method also valid for AC?
Thank you so much.
Yes.
For AC usually use varistors (MOV) to clamp the spike. Although it is good idea to install fuse or braker in line, as varistor will drain access voltage to ground/neutral side.
@@commenter5469 Thanks for the information. It is very useful.
Hello, can you please explain to me why there's a need to place a resistor in series with the capacitor? Can't I already get by with only a capacitor? Thank you.
Hello, thanks for the video. Some questions:
1) Does it matter on which side of the capacitor/switch the resistance is?
2) Does the capacitor need to have voltage rating of exactly 50V or would it need a minimum rating of 50V?
3) Why not use the amp rating of the switch to calculate the snubber resistance as, if I understand correctly, the discharge of the capictor will flow through the switch back to the capacitor, and thus not affect any other components of the circuit?
4) As the energy of the coil is equated with the energy of the capacitor, does it mean that all the energy of the coil will go into the capacitor, and that when this happens there will be voltage difference of 50V over the capacitor untill the switch is opened again?
1) The snubber resistor and capacitor can be placed in any order; as they are placed in series it should not affect suppression of the transient voltage. It may be worthwhile noting that RC snubbers can be used with AC and DC loads. RC snubber networks (an actual component) are available and if you check a datasheet I expect you'll notice they are not polarised. I hope that answers your question.
2) Exact ratings of components don't really exist as they have a tolerance. If you wanted to limit to 50V then it might be desirable to have a capacitor with a rating of at least 50V and preferably more. 100V would be a good margin.
3) Not entirely sure that I have understood your question correctly -- sorry, it's probably me. The switch should have an amp rating to handle the expected current -- in this example circuit at least 1.2A -- but the calculations I go through are to limit the transient voltage. If I was using a switch rated for 50VDC / 100A it would not change my calculations. If you watch the video again you'll see that we simply use Ohm's Law to calculate a suitable resistor value, based on the known current and desirable peak transient voltage. You are right to think the capacitor will discharge through the switch when closed.
4) The voltage across the capacitor will be 50 V when the switch is opened. The voltage across the capacitor will be close to zero when the switch is closed.
@@PizzeyTechnology for Q3: I think I am a bit confused here, as I don't see what the calculated total current in the "main circuit" (battery, 10ohm resistor, inductor, switch, battery) has to do with the discharge current in the "snubber" circuit (capacitor, snubber resistance, switch, capacitor), as I feel that the current discharge in the snubber circuit will happen before the current in the main circuit will come "on". In addition, as we are trying to protect the switch from the discharge current, it would make more sense for me if the snubber resistance would be calculated as R_snubber=V_capacitor/I_switch. Part of my confusion probably also comes from the fact that I read in other sources to use the rated switching current for R_snubber (example can be found in the text under fig4 in: www.littelfuse.com/~/media/electronics/application_notes/reed_switches/littelfuse_magnetic_sensors_and_reed_switches_inductive_load_arc_suppression_application_note.pdf.pdf). Apologies for pushing, and thanks for the answer already provide.
Does anyone see when the switch is open, the current induced from the inductor is flowing into capacitor, then via 2 resistors in series; and the induced voltage is in series with the battery ? Also some energy is dissipated via the 2 resistors. After suffucient time that the current via the inductor becomes zero, the voltage across the capacitor equals battery voltage. Now if the switch is closed, there will initially more than 1.2A current flowing thru the switch due to discharge from the cap, and gradually goes down to 1.2A.
help a lot! very great and useful explanation!
I'm glad to help. Thanks for the feedback.
@@PizzeyTechnology you are my papa
btw, this can be the protection for mosfet or any transistor right?
Yes, but best in mind other options exist. For example, a free wheeling diode (reverse biased across the load) if using a DC supply.
@@PizzeyTechnology Yes I’ve use the reverse diode in the inductor.
I have to make a boost converter for the fast-charging capacitor circuit, therefore I can't dissipate all the energy on the diode.
Thanks 🇧🇷
Really clear and concise video thank you. Would love to know the simulation software being used.
Thanks. It's Circuit Wizard, a program widely used in schools.
@@PizzeyTechnology Will have a look ive heard of livewire and micro cap played with falstad but want something more polished and detailed. Ive used kiCad for basic schematics and pcb layouts.
Sir what about switching frequency(ex. 16kHz) incase of inverter
Please let me know
Great explanation ,How do I calculate resistor and capacitor for an inverter circuits between the drain of MOSFETs and ground
Abu, thanks for your appreciative feedback. Can you post a link showing the actual circuit?
thank you so much 💛😍
very clear explanation, new subscriber here : )
Thanks, I appreciate the positive feedback.
Will a snubber circuit recharge battery if it was switching a DC motor on and off? Making the whole system more efficient?also increase battery run time?
Hi could you help with a problem ...how to calculate a rc snubber for a bidirectional dc motor operating at14v that is damaging the switches controlling it ( power window circiut) thank you
Bravo
Does the current thru the circuit not depend on the total impedence of the circuit? Why do you use only the resistor value?
great explanation
Thank you so much for such a useful explanation. I have a 30A 12VDC relay that is drived with a bjt, a flyback diode and microcontroller. When I turn on/off the relay my circuit works properly but when it is connected to a motor or contactor after a while the microcontroller freezes. Is it good to add this circuit to the coil for solving the problem?
Have you added bypass capacitors on your power rails? You may need to do this to reduce noise on the power rails otherwise your microcontroller can be unstable.
Is this method appropriate to calculate the snubber for AC? Specifically a 230V AC contactor coil. Currently i use 100R (3W) and 100nF (440V X1 class) and it mostly solved the problem of EMI (IRQ false triggering and MCU resets) but i still occcasionally get false triggering of the IRQ line.
The AC contactor coil (230V AC) is controlled by a smaller 10A relay which has freewheeling schottky diode on it coil (5V DC). The snubber is installed in parallel to the contactor coil.
Good question but I'm always wary in replying to questions or making specific recommendations for circuits that relate to AC mains voltages. I hope that you understand. Have you tried probing the IRQ line with an oscilloscope to see what is going on?
really clear thanks a lot !
Hi (from Egypt). That's very informative. However, I have a solenoid (240 VAC, 6 watt, resistance across its contacts is 3.68 kohm) that I plan to control with a solid state relay. The solenoid has no data sheet available (I don't know the inductance nor the current). Is there a way to calculate an approximate values of R and C?
Concise!
Thanks. I'm glad you found it useful, and concise!
Does this have a time constant tau within which it will remove the spike voltage??
thanks for the explanation. I saw some circuit with R first and then C, are they going to have same response.
You can expect the same response. I'm glad that you found the video useful.
No they will not have same response. Please check dv/dt at midpoint of R and C when it is connected in RC and CR fashion. They are different. I am having the same query.
I'm not 100% sure, but I think the capacitor would discharge much quicker. Don't take my words for it. I'm a newb.
Y la potencia de la resistencia snubber, como la calcula?.