Hello everyone, welcome to another video on the channel ElectroLab, have you ever heard of Snubber? RC circuit, filter? Snubber?
That's what we're going to see here, this is used a lot type of filter here in drive circuits by thyristors in actuation circuits with MOSFET switches and other transistors. It's a protection circuit, it's a circuit that avoids the burning of its element that is switching the circuit and I'll explain why. The big move of this video here, it will not only explain why, but how calculate the Snubber, it is generally rare that you find any literature that talks about the snubber calculation, we use usually some predetermined patterns.
Look this from here it works well with capacitors of both and resistors so much, but you don't know why they were chosen, right? So many questions here on the channel asking, look, how do I calculate the snubber. .
. . usually I answer, there are some standards, some predetermined values for applications in AC, for example, in dimmers, you already have values pre-calculated, use this one that works.
but why is that value there? Let's understand here how to do the calculation of an R and a C for the snubber circuit of a load inductive, okay? There are some other types of calculation, for example, for use with Thyristors, Triacs, SCRs, you have ready-made formulas to manufacturers that teach how to calculate the C and R for this circuit type.
Here we are going to deal more with the focus on inductive load. What inductive load is this? Can be one motor, it could be a relay, it could always be a load that there is some kind of winding, that is, that has a inductor characteristic.
And we're going to deal with that here. First, the great thing about snubber is just that, protecting the element that is switching. I put here a symbol of a key.
This switch can be a Mosfet transistor for example, ok? I put the key just to exemplify, here, I have my battery, the my source, a resistor that will limit the current, my load, it could be a relay here, it could be a motor here, ok? I I represented him as an inducer.
A key, that key, as I already said, it could be a mosfet, it could be a transistor. O snubber circuit here as optional. We'll see in theory and let's see in practice, because I set up a circuit for us see what happens when i have snubber when i don't have snubber, that it's very interesting for us to see it on the oscilloscope, OK?
So let's go there. First, starting with the theoretical question. Which it happens?
I don't have, let's ignore here that I don't have snubber huh. So when I close the circuit here, my transient closed, I start to circulate the current, my load is powered, my inductive load is powered, my motor starts working, my relay, docks it, OK? good the big The problem isn't when you turn it on, it's when you turn it off.
When I interrupt the current here, my mosfet opened, my key opened, the current stops flowing, correct? What is the main characteristic of an inductor? he resists sudden changes in current.
The inducer does not like to have the reduced or increased current. He will try to keep that constant current, always. So when I interrupt here, I cut the current, the inductor he will try keep sending that chain.
as he doesn't have way to circle, there will be a direct increase in the field electric here it will increase the voltage at this point, for what? To compensate for the lack of circulating current, so that the energy that is stored here in this inductor, will transform this energy here, increasing the tension to compensate until it collapses! But before the staff remembering that the PCB Way that makes the professional boards here for the channel is completing eight years of life, the anniversary of pcbway, with a series of promotions, instant coupons, just click as per your purchase, you have the automatic discount, also some sweepstakes and promotions with discounts of up to fifty per percent on the services they already provide.
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going back to our video. Notice that she will do this and will leave this extremely positive point here. By doing this, I will have a potential difference here in my very high switch.
Many times even I feeding here with the tension low, for example, I'm going to feed this here with nine volts, I can have hundreds of volts. We're going to measure it. Here, at this point.
And many times my mosget doesn't support it. O my mosfet can be for sixty volts, for forty volts, even to 100 volts, sometimes a pulse like that even if it's a pulse extremely fast, you risk burning the mosfet, precisely because this peak that happens here OK? What does the snubber do?
when this tension grows here and the key is open, the way to circulating the current will be through here, the capacitor will charge with the energy of the inductor and then it will dissipate through the resistor, that is, it will make a detour here. The value is important because I need to load with a capacity close to or similar to my inductor load. Then the calculation will be a function of energies.
The resistor in function of the current that will circulate here, that's why we do a calculation to have the best possible for that type of circuit, okay? So basically this, the snubber will protect by diverting this increase in voltage that will happen here when the switch is open. when the key has closed, this circuit here is transparent, it won't work anymore, it's transparent to the rest of the circuit.
The current will flow through the best way here, right? Go here through my, my mosfet, my transistor, that is, the device that is switching here, no longer through the snubber, when it opens it will bypass by the snubber, when increasing the voltage here in the inductor, OK? I think it was clear there, so let's go to the calculation.
How do I do this snubber, come on, I'm leaving of the energy principle, the energy stored in the reactive elements, who are the reactive elements here? O inductor and capacitor, right? So we know about physics that energy stored in a capacitor, energy in the capacitor, will have the following formula, the capacitance times the voltage squared, over two, it's pretty simple, right?
no there won't have complex calculation here not OK? And the energy this here is energy of capacitor ok? And the energy of the inductor?
The energy of the inductor energy of L will be the inductance the value of inductance times current squared over two, that is, they are very similar formulas, here it only changes to inductance here capacitance in the case of the inductor will be current and in the capacitor case will be voltage. well i made one experiment here, I measured a certain inductor and find out his inductance. In this calculation it is important that you know the inductance, you will need to measure this inductance to be able to make a more accurate calculation.
So I'm going to put here the inductance that I measured here of this inductor, it was sixteen point five mH, sixteen point five mH, okay? My Inductor has this value in the experiment what am I going to do, R1 it will have fifteen Ohms. Very good.
So having these two I will calculate mine ok? First thing I need to calculate the current that will flow here. When the key has closed I I will have the R1, the L I will have a chain here.
let's use a voltage of for example nine volts. Good the tension of nine volts for my current I, it will be the nine volts divided by fifteen ohms, okay? So my circuit current, this current that will circulate here, it will be zero point six amps.
I will always have to work with ampere, ok? I could speak here of six hundred milliamps, but I'll work with zero six amps, because that's how it's going to go into the formula. So this it's my chain.
I already have the chain to put here, already has the inductance, so I can know what energy it will carry in that inductor. The energy in the inductor is L, it will be mine inductance, inductance sixteen point five mH I have to convert this to Henries, this will give zero point zero one half five H, okay? the formula work it's Henries and amps, OK?
And my energy will be on joules, so zero comma zero one half five is mine inductance times current zero point six squared divided by two, OK? Okay, so let's make this account in the calculator. So it gave zero point zero one half five times zero six squared, gave this number here, divided for two.
That gave zero comma zero zero two nine seven. Zero comma zero zero two nine seven. can you round this here for comma zero zero three.
So the energy in my inductor is going to be zero comma zero zero three joules. Zero comma zero zero three Joules. This is the energy in inductor.
Now we are going to calculate the capacitor, so that it absorbs this energy that will be generated in the inductor. So I'm going to throw this energy here from the inductor to the capacitor in place of Ec in order to calculate the value of C. Well, but then you ask me, but who is V?
Who is V in this formula here? v go be the maximum tension that I will allow to get here to from there i need to have control of my snubber, so why example if I have here a mosfet that can work up to eighty volts, for security reasons, I can tell you the following, look, I don't want this peak to go beyond forty volts, I I want it to be cut off at forty volts. Then I go work with forty volts here.
I will put forty volts here and I will put here the energy that comes from the inductor and I will find the most suitable capacitor for this calculation, ok? So, let's do the calculation here. I already have here the my my energy, my energy is zero point zero zero three, it's going to be equal to the capacitance that multiplies.
Attention, remembering again, I'm going to choose forty volts. Why forty volts? Because 40V, because I'm estimating that my mosfet, by the datasheet, support, for example, eighty volts, for security I want that the peak here does not reach forty volts or reach the maximum forty volts.
So I'm going to throw forty volts here. This is a value estimated by you, depending on the key that you use here. You can put more, you can put a hundred volts there, you can put whatever you want, the problem is that you are leaving here that the value reaches values bigger.
So it's better to work with the tightest tension. within a safe tolerance. so i will put forty volts, that's squared, right, because V to square divided by two.
Doing this math, the two multiply up here and this forty squared divides, I'm going to go here multiplying, it's going to be two times comma zero zero three, divided that forty by square, will divide here. So just representing here to make it easier, it will look like this, , C equals two times zero comma zero zero three, divided by forty at the square, let's do forty squared right here, we you already know easily that it gives sixteen hundred. Divided by one thousand six hundred.
That's the value of C. So two times zero zero three, zero comma zero zero three of joules, gives zero point zero zero six, right? Divided by sixteen hundred goes give this big number here.
This number is in Farads, so I will put it in scientific notation here for example and he told me which gives three point seventy-five to the power of negative six Farads. Capacitance equal to three point seventy-five times ten to the power of negative six. Minus six is what?
Micro. sub-multiple of micro. So, it will give the most suitable capacitor, here it would be three point seventy-five microfarads.
OK? Three point seventy-five uF. That's the value now we find the value of C.
The value of R is very quiet, well immediate. OR will be a function of this has the same voltage as me I'm working here. About the current flowing.
THE current C which is zero point six. So I'll do the R here, oh. The OR will be the V over the I.
Ok? So the R will be forty, which is the voltage that I determined as the limit, over zero point six, which is the current that is circulating there, which I had already calculated before. So forty divided for zero six, that's sixty-six point seven ohms.
I'm going round here to sixty-seven ohms, it is not a commercial value, OK? It has a commercial value of sixty-eight, as well as three point seventy-five is not a commercial value, but you can put one here or below, like three point three here or above or four point seven, if you can, if need to make an association, OK. But at first it's not a extremely critical value, values close to these values here will work fine.
So basically this is the snubber calculation on an inductive load. I will now do in practice show the circuit with a capacitor like this one and a resistor like this one doing it with and without it, in a circuit like this one, ok? Nine volts, fifteen ohms and the sixteen point five mH inductor.
We will to practice then. Well, guys, here at I set up, okay? here the resistor of fifteen ohms this one is the inductor that goes simulate my inductive load, it I measured it on the LRC bridge and he gave sixteen point five mH ok?
What did we use in calculation. I got it, I put it, I put it here, but it's turned off now just so we can see what happens when turning on and off. In place of the mosfet I put a key.
That key will turn on and off that point that I showed in circuit, okay? In the key position there. That is, I will open and I will close the circuit.
I will feed from my source, I I will put nine volts, you will see the nine volts showing Here is? It's there. I'm putting nine volts here in the circuit.
So I wired the circuit here. my chain is approximately zero point six amps, which was that that we calculated. Here in practice is a little minus, zero point fifty-something.
Almost zero point six. I'm going to put my oscilloscope here, in the single trigger position, for what is a fast pulse, that high voltage pulse that the inductor will generate. So we can only see it in the single trigger function that it fires a flash when the current increase.
Well folks, now let's open the key. OK. Well, let's analyze it now with the open key.
Just take a look here. I'm ten X scale of keyed at ten X my probe, one hundred volts scale per division, one hundred volts per division! I keep my nine volts here, but take a look at it from here.
I will decrease here a little bit, I'm going to scroll down here to scale a little bit, so we can see where we've reached. One division, two, three, four. It peaked four hundred volts.
This very fast here initial. then he does this one that ringin, which is normal, is typical here in the damping, but this very characteristic pulse here, it has a hundred, two hundred volts. positives.
It still gives a hundred volt pulse negatives here. But this first climb here bordered the four hundred volts, something like three hundred and ninety-something here. It's almost here in the fourth division of the oscilloscope.
Notice, scale of one hundred volts per division, that's what the inductor generate. This peak here or mainly this other one here very robust, it can damage your key, be it a transient, be it a mosfet or whatever. Easily.
Mainly if it is not, or does not have, a feature that supports this voltage. Smaller mosfets, which support up to forty volts, fifty volts can easily burn out of this. hence the use of the snubber.
But now we're going to do the following, that snubber that we calculated, I put it here, there are two resistors here that I put in series to get the leg to that value of sixty-eight ohms, OK? And I will connect it. Oh detail, where is that I'm making this oscilloscope measurement, just to stay of course, ok?
The test tip is here. I'm measuring up of the key, okay? I'm measuring over the key.
is the point that I'm measuring. It's just like I'm measuring between the drain and the source of a mosfet, ok? It's what arrives at key, it is what arrives at my element that does the switching, OK?
That's where I do this voltage measurement. And where does the snubber go? In parallel with this key.
Remembering, folks, that in DC circuits, many times, what we see most there protection against this coil pulse. A diode, right? THE we even have a video here talking about the flyback diode, freewheel diode, that you use in parallel, right?
An inverted diode, parallel to the coil to avoid pulse, that this pulse damages the circuit. He this diode works very well, mainly in circuits of direct current and circuits that operate at frequencies casualties. When you work with high frequency signals or even with higher powers, you are more efficient use a snubber circuit, because most of the time it can be faster than a diode, especially if it's not a schottky diode here, for example, in that protection there.
a diode that has a longer delay, which can slightly disrupt the operation, delay for example tripping, relay, etc. so often for DC circuit, going back. where do i use one and where do i use the other?
Both can be used, but basically the diode is placed further in direct current circuit and circuits also operating at low frequencies and low tensions, okay? Higher voltages higher, higher frequencies is interesting you use the snubber ok? That's why it's also used against accidental firing of thyristors, which use obviously alternating current ok?
it works well there also against some triggering of a high transient frequency that can trigger the thyristor. So the snubber is preferred in this circuit, ok? So come on, I will now fit my snubber here in the circuit, just fit here the resistor leg the snubber now that is this capacitor these two resistors and it is in parallel with my switch and we are going to do the same thing, I'll call, let me reset the oscilloscope here.
Good, putting now a lower scale with the snubber connected, I will turn it off and we'll see what happens. got a pulse here, only he's a much smaller wrist, I'm twenty volts per division, so it was twenty, forty, fifty approximately fifty volts, the peak I have now turning on my much smaller key, right? four times smaller and within more or less within there of our expectation of limiting forty volts, right?
A little bit up there maybe there by tolerance. Shall we do it again? OK.
I had a pulse here try to approach here. The pulse here now was. .
. . I have twenty volts per division, so twenty forty little bit there forty.
Remember that our limitation was forty volts, look, that's what happened here. It's the pulse that I had when turning off the key. So protecting my trigger, my mosfet or my transistor from that voltage I had calculated, okay?
You can see that the pulse is now much smaller than than those nearly four hundred volts I had when I I didn't have the snubber connected. OK people? This is the application main snubber for inductive loads and this is the calculation based on the load energy of the reactive elements, inductor and the capacitor, which I will calculate so that it absorbs this energy and not allow it to be diverted by my element switch, thus protecting the circuit, this will avoid much headache in these circuits that apparently burn all every time a mosfet burns an IGBT all the time, it all the time burns a Triac or an SCR, OK?
The calculation for Triac and SCR is a little different, I can show in a video another video, right? because there is a slightly different calculation, a calculation recommended by manufacturers, right? There are some practical and direct formulas for you calculate the C and R of the snubber for thyristors.
This application here it is more focused for inductive load elements. Although in SCR you can also use loads inductive, OK guys? I hope you enjoyed it there assimilated some new knowledge, if you like it, don't forget to click on the like, to subscribe to the channel which is very important, mark the notification bell and see you next time video, a big hug.
