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snubber resistor power rating?

Discussion in 'Electronic Basics' started by Neil Preston, Nov 27, 2004.

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  1. Neil Preston

    Neil Preston Guest

    How might I ascertain the optimum power rating of the resistor in a triac
    snubber with an inductive load?

    Let's assume worst case PF=0. The last 90 degrees of the alternation dumps
    through the resistor into the capacitor. If we assume a large capacitor, the
    entire voltage appears across the resistor for 1/240 of a second.

    For example, using AC 120Vrms/170Vpk and 170 ohms restance with a
    sufficiently large capacitor (1 uF?) to make it negligible:

    If this were a continuously applied sine wave, the average power P=E^2/R
    would be 170W. However, this energy is only delivered to the resistor for
    1/4 cycle, so I figure we must consider the energy spike delivered to the
    resistor.

    Since power = work/time, W=P/T
    and since 1J=1Watt*1S
    and 1/4 cycle=1 second/240
    then the energy pulse w=P *T or w=P*1/240 = 0.71j.

    (I know, precise solution requires an integral. I rarely need to use calc,
    and have forgotten most of it. Be my guest. I'd like to see the proper
    solution.)

    (These values were chosen for simplicity. Actual application values are
    different.)

    So, how would typical resistors react to such a pulse? (In this application,
    it might occur as often as once per second.) Would a 1 watt resistor be
    sufficient?
    Would it be subject to internal arcing or other degradation?
    Would there be any performance/reliability difference in various types such
    as carbon comp, carbon film, metal film, MOX flame proof, wirewound, etc?
    Is there a rule of thumb for the ratio of the power rating to the intensity
    of the energy pulse?

    (I have not seen this issue addressed in any of the rather sparse
    application lit out there.)

    Thanks,
    Neil
     
  2. legg

    legg Guest

    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
     
  3. "Neil Preston" ...
    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
     
  4. Terry Given

    Terry Given Guest

    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 ****, 314kW. And a
    carbon-composition resistor solved the problem. HVR make some real good
    ones :)

    Cheers
    Terry
     
  5. (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.
     
  6. 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
     
  7. John Smith

    John Smith Guest

    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
     
  8. legg

    legg Guest

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

    Ken Smith Guest

    A basic question:

    Is this what we are dealing with here:


    R (load)
    --------------------/\/\/\/-----
    ^ !
    ! )
    ! ) L (load)
    ! )
    Mains !
    ! -----
    ! ^V Triac
    ! -----
    ! Trigger ckt----/ !
    V !
     
  11. Terry Given

    Terry Given Guest

    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
     
  12. Terry Given

    Terry Given Guest

    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
     
  13. John Smith

    John Smith Guest


    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
     
  14. Pooh Bear

    Pooh Bear Guest

    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
     
  15. Terry Given

    Terry Given Guest

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

    Cheers
    Terry
     
  16. CBarn24050

    CBarn24050 Guest

    Subject: Re: snubber resistor power rating?
    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.
     
  17. 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
     
  18. AAArrghhh...

    OK, here's the right drawing:

    "Arie de Muynck" ...
    L (mainly inductive load)
    ---------UUUU---------------------------
    ^ | |
    ! | /
    ! | \ R
    ! | /
    Mains | \
    ! ----- |
    ! A V Triac |
    ! ----- ___
    ! Trigger ckt----/ | ___ C
    V | |
    ----------------------------------------
     
  19. Rich Grise

    Rich Grise Guest

    The way I understand it, it doesn't. It turns off when the current through
    the whole loop becomes zero. I guess the snubber is there[0] to let the
    current continue to flow through the inductor for awhile, so that the
    resulting voltage doesn't make the triac conduct on the ensuing cycle.

    [0]You haven't shown the snubber in your diagram, but the way I
    understand it, it would be connected from MT1 to MT2 of the triac.

    Hope This Helps!
    Rich
     
  20. Ken Smith

    Ken Smith Guest

    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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