# Snubber circuit

1. Jan 31, 2015

### billy fok

i have did some research of snubber circuit. and i do know there is 3 types of snubber circuits:

• Capacitor (C)
• Resistor-Capacitor (RC) damping network
• Resistor-Capacitor-Diode (RCD) turn off snubber
can anyone explain snubber circuit for Capacitor??? thanks

2. Jan 31, 2015

### billy fok

when i do the research, it mainly tell me more about RC and RCD..

3. Jan 31, 2015

### Svein

I am not quite sure, but I expect that in a capacitor snubber, they rely on the resistance in the coil to absorb the energy.

4. Feb 2, 2015

### Mike_In_Plano

There's probably over 20 in all. There's all manner of R, C, D, and L. Then there are switched snubbers such as SCRs that switch to take the stress from other SCRs...

5. Feb 2, 2015

### Svein

My personal favorite is this circuit. There is no need for a separate snubber - the PNP emitter follower ensures that no voltage spike can occur.

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6. Feb 5, 2015

### meBigGuy

That's crazy! (as in cool). How come I've never seen that? But, how is it better than a diode?

7. Feb 5, 2015

### Svein

If you the coil is in the collector path of a transistor, the moment you turn the transistor off, it shows a high impedance to the coil. The coil reacts to the loss of current drive with a voltage spike that can reach twice the supply voltage value. That spike is the reason why we need a snubber circuit.

In my circuit, if you remove the current from the relay, the emitter presents a low impedance to the coil. If the voltage at the coil tries to get higher than a Vbe above the supply voltage, the PNP turns on and damps the current.

The advantage? For one thing, you do not have to calculate and implement a snubber circuit. If the PNP is able to drive the coil, it is also able to absorb the kickback from the coil.

8. Feb 5, 2015

### donpacino

to the original question, the capacitor in that case will be a decoupling capacitor. It will function very similarly to the RC scrubber.
look up decoupling capacitor.

9. Feb 8, 2015

### meBigGuy

The voltage must be higher that Vbe + Vr and will spike higher because of PNP turn on time.

But, that aside, you missed my point. How is that better than a reverse diode across the coil to the supply. In fact, a schottky will make the spike even less, and turn on very fast.

10. Feb 8, 2015

### Baluncore

Svein's circuit in post #5 must have an R value low enough to handle the base current of the PNP during flyback. Otherwise, the maximum collector voltages of both the PNP and NPN will be exceeded.One solution is to add a reverse diode in parallel with R.

I have considered circuits like that in the past but have always had a problem with controlling the PNP base voltage.
With R selected low enough to prevent over-voltage then the NPN must sink both Vs/R current and the PNP base current.

To turn off quickly requires a significant negative inductor voltage. A schottky diode with a series resistor can achieve that in a more controlled way than the emitter relay drive circuit of post #5.

11. Feb 8, 2015

### jim hardy

hmmmm

Seems to me when NPN turns off and the edit: PNP comes out of saturation, PNP's Vce starts to increase,
the current through Rupper would become Irelay divided by Beta of PNP .

So long as ratio of Rupper / Rrelay is less than Beta of PNP ,
the peak voltage at bottom of relay will not exceed 2 Vsupply?

Use something like a 2N3906 where you can count on Beta >50 and make Rupper 5X Rrelay ,
peak should be less than ~ 1.1 Vsupply.

Only trouble i see is relay's dropout time could depend on beta if that matters to the application.

Is my thinking straight?

Last edited: Feb 9, 2015
12. Feb 9, 2015

### Baluncore

Yes.
But beta is something I do not like to predict. Beta is usually specified as probably being in a wide ballpark.
On the other hand a schottky diode located at the relay coil, possibly with a series resistor, is much more predictable.
Today, a power MOSFET with low Ron would replace the complementary darlington pair which will always drop one Vbe.

13. Feb 9, 2015

### Svein

Oops - an emitter follower does not go into saturation...

14. Feb 9, 2015

### jim hardy

Well, you're right.. Bad wording on my part.

okay,

"..when the PNP's Vce starts to increase,..."

i think is more correct ?

thanks - as we choose our words better we reason better...

15. Feb 9, 2015

### Svein

Yes - but Schottky diodes traditionally have a fairly low breakdown voltage. A quick check - well, they have improved. You can now get Schottky rectifiers with 100VRRM. OK, I agree.

There may be a cost issue, but otherwise I agree.

16. Feb 9, 2015

### Baluncore

If you buy only the specifications needed, then the price of a MOSFET is now less than that of the NPN + PNP.
The problem of cost is when buying a superceded legacy part number that is no longer in production, or less than 1000 units.

17. Feb 12, 2015

Back to the OP.... In general I do not like to call a Diode in the Coil example a snubber - it is a free wheeling diode. Snubber - implies more of a suppression role - as when there is a V spike and the capacitor prevents excessive overvoltage, in many cases due to L * dI/dT at some level- ( ideally a FWD will prevent the V spike to start with)

As for why not always use a diode - there are many switching circuits where the inductance being switched off is not so "accessible" Consider l line rectifier or inverter. The snubbers often applied at the switch terminals and across the DC bus ( i.e. not on the load side) are not to counter the effect of the LOAD ( there is typically a Free Wheeling Diode for this) -- the snubbers used are to suppress the V spike generated in parasitic inductances. - often on the DC bus.

In the example below -- the snubber (RC) is not included to account for the V spike of the load (usually that is too much energy for this cap (in this case 0.1uF and 100ohm R) - but for all of the inductance between the Diodes and the load. When the diodes commutate some are turning off and basically every conductor in the circuit has some minute inductance - the higher the current, and the faster the switching the more this becomes apparent.

As for the resistor - in general you need the energy to go somewhere - otherwise you and up with a LC circuit ( agreed with very small R "always") - but without the R this tends to generate excessive ringing - or an oscillation that can be disruptive to the circuit or emitting unwanted EMI.

Note - in the circuit above, if the load is a large inductor(or long leads to the load) - you may still want to add a FWD in parallel to the RC shown, this tends to need to be a fast diode, and/or sized to handle a large percentage of the load current - adding cost. And the diode requires some time to switch on - the Capacitor is much faster.

18. Feb 12, 2015

### Baluncore

You show a single RC snubber on the DC output of a three phase bridge rectifier. Surely that is a job for the load. Any one phase will not take over conduction until the previous phase stops conducting. The output impedance is therefore always that of the load, it will self snub. At any point in time, only one line phase will not be in conduction.

I would have expected to see a delta of three RC snubbers on the input side, to prevent spikes on the line side of the bridge, where the line inductance of the non-conducting phase will generate transient switching spikes and noise back up the line when the diodes switch off. The snubber resistors are actually impedance matching diodes, AC coupled by the capacitors, to prevent transients on the line side, when each phase in turn ceases to conduct.

19. Feb 12, 2015