snubber resistor power rating?

Rule of thumb is to assume all cap energy is dissipated in the resistor. P = C x Vp^2 x f / 2, where f is the rep rate of the transient.

If there are two transients of differing voltage amplitude, as in phase-lagging quench, calculate for both independantly and add directly.

It has to be a pretty large capacitor at common triac frequencies before R begins to affect or dominate the loss. Large resistances also defeat the purpose of the snubber in limiting reapplied dV/dT.

Flame-resistant or fusible parts assist in safely providing for single fault conditions. If elevated resistor body temperatures are expected, try not to heat up the series capacitor indirectly, through proximity or track heat conduction.

At higher frequencies and peak currents, a suitable capacitor may be harder to find, and more expensive, than the resistor.

RL

Our three-phase, 460VAC, thyristor-controlled, DC and AC motor speed controllers had snubbers and MOV transient suppressors. All our controllers used the same snubber values. In two different sites (Denver and Puerto Rico), one of the MOVs would explode and the customer would send the unit back for repair. After noticing that the same unit had been returned more than once for the same problem, we got to looking into the cause more closely.

It turns out that the firing of the thyristors in conjunction with the snubber values and line inductance can _create_ a transient, and repetitive transients will destroy MOVs eventually. The controllers would operate fine for a day or three, then fail. Simulation showed that we needed to change the snubber values based on the controller model current rating.

We had no hard data showing that the line impedance in Denver and Puerto Rico was higher than in other places, but, as I recall, simulation showed that it most certainly could be the cause.

Pardon me for posting a little off-topic, but I thought it might be useful information.

John

(snip)

What entire voltage are you talking about? The line voltage? If that were true, the entire voltage would also appear across the TRIAC, and that is what the snubber is there to prevent. Ideally, there would be no voltage across the resistor, and the entire inductive energy would be transferred to the capacitor and then back, in a sinusoid oscillation. This process would control the rate of change of voltage to something less than the DV/DT self trigger limit of the TRIAC. The resistor is there to limit the capacitor inrush current when the TRIAC fires at peak line voltage. But that voltage lasts less than a quarter cycle, as the cap rapidly charges up to nearly the instantaneous line voltage.

At 60 hz, a 1 uf cap has an impedance of about 377 ohms. Put that in series with 170 ohms, and the total impedance has a magnitude of about

414 ohms, so the current is .29 amperes. the resistor power is 14.3 watts.

The net current phase leads the voltage by a bit, but there is resistor power dissipated at all times current happens, in either direction.

The RC time constant is usually chosen to be much less than the line cycle period, so the worst case can be approximated by assuming that the TRIAC fires at peak line voltage, and that the cap charging surge takes place before that voltage changes much (a DC voltage is present).

Quite possibly, especially if the resistive element is low mass, like a metal or carbon film. A bulk resistance (cylinder of carbon or metal oxides) handles such pulses better.

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John Popelish

"Neil Preston" ...

application,

such

intensity

I've once made the mistake of just calculating the power dissipation and using a film resistor (triac, inductive load, snubber use). They showed beautiful little sparks after a year of 1 Hz switching. Replacing them with (cheaper) carbon composite resistors solved the problem. Lesson: never ignore the PEAK dissipation and current that may occur, and check the datasheet of even a simple item like a $0.03 resistor.

Regards, Arie de Muynck

Thank you. I forgot to hit the reciprocal key on my calculator. (1/.377=2.65)

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John Popelish

The same problems occur in surface mount film resistors which can only take X10 power for short periods of time. Kamaya Ohm RPCNN and KOA SG73 series of resistors can withstand up to 10KW in SM2512 packages. Ohmite has an ox/oy series of leaded parts that can go to 80 Joules. Sure nice to see a Joule rating in resistor spec. They are a must in any power electronics designer's tool kit. Cheers, Harry

1 / ( 2 x pi x f x C) gives 2K65 as Z for 1uF.

A series 170 ohm resistor has little effect on 60Hz current, which will be 90mA @240VAC, producing 1.4W static loss on a non-switching circuit.

If this combination snubs a triac, firing at all possible line peaks, worst case loss due to the snubbing energy, if the circuit is damped effectively by the values used, is

C x Vp^2 x f / 2 = Pd

1E-6 x 340Vsqred x 120hz /2 = 7W

RL

I had similar problems in west Texas with some irrigation machines. You run into all sorts of things when you work with 3-phase power. I hated it.

John

A basic question:

Is this what we are dealing with here:

R (load) --------------------////----- ^ ! ! ) ! ) L (load) ! ) Mains ! ! ----- ! ^V Triac ! ----- ! Trigger ckt----/ ! V ! --------------------------------

If so, how does the get turned off with a current flowing in the load?

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kensmith@rahul.net   forging knowledge

You've decided on a value ?

Try a few and check temperature rise. Probably wise to err on the generous side.

Pulse ratings vary a lot on vendor for power film types.

Graham

Hell yes. I learned this very early on, when placing damping resistors in series with Y caps in big EMI filters. One day we noticed a flash of light when we switched the prototype on. We switched it off, pronto. A thorough set of diagnostics found no problem, but we weren't hallucinating so kept looking. And the 1R 2W PR02 resistor I had in series with the 100nF Y cap was open circuit. As were ALL of them, in all of the prototypes. We then asked the peak-power question, which was something like (400V*1.41)^2/1R = oh f*ck, 314kW. And a carbon-composition resistor solved the problem. HVR make some real good ones :)

Cheers Terry

A typical snubber (0.1uf + 100r) on a triac at 60hz produces a very small amount of power in the resistor. A half watt carbon resistor is more than adequate.

Ken Smith" ....

load?

No. It is:

---------------------------------------- ^ | | ! ) / ! ) L (load) R ! ) / Mains | ! ----- | ! A V Triac | ! ----- ___ ! Trigger ckt----/ | ___ C V | | ----------------------------------------

The triac turns off at the zerocrossing of the current through it. Since the mains voltage will be about maximum then, the snubber limits the slewrate, preventing the turnon by excessive dV/dt. The resistor provides damping of the turnoff efect. It also limits the current when the triac fires at turnon: I(pk) = Vmains(pk) / R and this discussion is about how a 2W 47 Ohm resistor likes that hefty spike...

Regards, Arie de Muynck

AAArrghhh...

OK, here's the right drawing:

"Arie de Muynck" ...

L (mainly inductive load) ---------UUUU--------------------------- ^ | | ! | / ! | R ! | / Mains | ! ----- | ! A V Triac | ! ----- ___ ! Trigger ckt----/ | ___ C V | | ----------------------------------------

the

Damn, someone else knows this :)

I am amazed at how often I see people "design" 12V FET gate drives using little wee 0603 parts - 4R7 for example. Other good parts re IRC's CHP series, and MMA0204/MMA0207 from several vendors.

Not all wire wounds are created equal either. peak pulse power handling capability for a WW is governed by the thermal connection from wire to ceramic. Shitty ones drop the WW mandrel into the slotted body and pour gunk overtop, creating air gaps underneath. A good pulse and flashes of red light can come out of the resistor - for a while. A good construction embeds the WW mandrel into gunk, then coats it, ensuring no air gaps. Vitrohm are great.

From a design perspective its quite simple though - look at peak and average power, for every part.

Cheers Terry

Hi John,

Thats a sneaky one. Heres a far less subtle problem:

I had a 2am call from Germany a few years back. One of our 400kW drives kept blowing up - the Germans built it into a machine, which they sent to Korea (IIRC), whereupon it promptly blew the hell out of the VDR board. They replaced it and boom. They replaced it and boom. They called me. Turns out Korea had some whacked form of 3-phase power distribution (open delta? I forget - I used to have a power systems of the world book) in which the phase-earth voltage was equal to, not 1/sqrt(3) of, the phase-phase voltage. Poor old mr VDR lasted a few seconds then pop. The solution - sidecutters and no VDR.

Cheers Terry

Most common of which was "component no longer there" type errors.

Cheers Terry

Ok got it.

So, the spike like current in the resistor is a current that starts equal to mains/R and then decreases very rapidly.

Does the triac get turned on only at zero crossings in this application or is it phase controlled? If it is turned on at zero crossings, there is a reduced requirement on the resistor. In the phase controlled case, the resistor can end up with 4 spikes of almost a big per cycle.

47 Ohms is a lot of resistance to solve this way but at lower voltages, I have made resistors to protect crowbar SCRs out just a length of hook up wire folded back on its self. The accuracy of the value isn't good but copper wire can take a huge spike with no trouble because the resistance is spread over a large volume and it is very thermally conductive.

I have had a lot of trouble finding any resistor that has a good pulse handling ability in surface mount. The ones I did find were very expensive and not very available. They were from one of the Tyco companies.

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kensmith@rahul.net   forging knowledge

Caddock does too:

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John Popelish