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Choice for snubber cap?

Discussion in 'Electronic Basics' started by Neil Preston, Nov 9, 2004.

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  1. Neil Preston

    Neil Preston Guest

    Which type capacitor is appropriate for a triac driver snubber?

    I'm using a sensitive-gate triac (Teccor L601E3) with an optically coupled
    triac driver (MOC3011 equivalent) to control a solenoid valve coil that
    draws about 75 mA at 120V with a PF of about 0.15.

    According to the application literature and calculations, I plan to use a
    0.068 uF 400-600V capacitor and 3300 ohm resistor as a snubber for the triac
    driver. The app lit indicates that this should also be sufficient to protect
    the triac.

    I'm leaning toward the metallized polypropylene dielectric caps, such as the
    DigiKey BC1879 and/or BC2218. Would these be appropriate, or would a
    different type be better?

    Thanks,
    Neil
     
  2. Any type that is rated for across the line filtering should work. It
    looks like the first one you list (the only one I looked up) has a 630
    volt DC rating and and a 220 volts AC rating, so it should handle the
    occasional line voltage spike pretty well.

    I am wondering why you do not drive the solenoid directly with the
    MOC3011. It can carry about 150 ma of load current. The snubber you
    connect across the load will lower the total current a bit also (power
    factor correction).
     
  3. CFoley1064

    CFoley1064 Guest

    Subject: Choice for snubber cap?
    Hi, Neil. When you're choosing line voltage snubber caps, you really should
    pick caps that have been rated as "across-the-line" capacitors. These caps are
    self-healing -- that is, they're made so that in the event of an overvoltage
    punch-through, the short will fuse itself, preventing a short across the line.
    You can key in UL1414 as the standard to look for in the US, and CSA C22.2 No.
    1-94 for Canada.

    In the Digi-Key catalog, one good choice would be the Panasonic Interference
    Suppressor Caps on p. 903 of the current catalog (T043). One good choice might
    be DK P/N P10729-ND for an 0.068uF cap rated for 250 VAC. They're cheap ($0.20
    USD ea.).

    I'm kind of curious about the 3300 ohm series resistor, though. Don't you
    think that's a little conservative? You've got a TO-92 triac that can handle
    up to an amp, and you're only switching 75mA. Wouldn't something like a 330
    ohm 1/2 watt carbon comp resistor be a better idea? Your snubber will work a
    lot better and do a better job of protecting the logic triac if the cap can
    soak up energy faster.

    If you choose a snubber cap rated for across-the-line, then you can pick a
    series resistor small enough so the cap can actually accomplish something.
    Oh, yes -- don't forget the fuse.

    Triac Snubber Circuit
    FU1 LOAD
    ____ .-.
    L1 o-|_--_|--------( X )----o----.
    '-' | |
    3AG 1A | |
    | .-.
    | | |330 1/2W
    .--------------. | | |
    | | | '-'
    | | | |
    | | | |
    |Control Circuit _|_ ---
    | | V_A ---
    | | / | | 0.068uF 250VAC
    | o--| | |
    '--------------' | |
    | |
    | |
    L2 o------------------------o----'
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    Good luck
    Chris
     
  4. Neil Preston

    Neil Preston Guest

    Hi, Chris;

    This particular design from the Motorola app note is actually intended to
    protect the triac driver, which is triggering the gate of the triac at about
    3 mA. The triac driver sees the capacitor through a 180 ohm resistor. (See
    Figure 22 in the app note AN3008 at
    http://www.fairchildsemi.com/an/AN/AN-3008.pdf) (The Motorola app note is no
    longer available on line.)

    I'm also looking at the off-state current flow around the triac through the
    snubber, and trying to balance the two. Don't want the solenoid buzzing
    when it's supposed to be off.

    Thanks for the pointer to the capacitor. I hadn't run across that one yet.

    Neil
     
  5. James Beck

    James Beck Guest

    The problem I am seeing here is that the snubber might pass enough
    current to keep the solenoid pulled in once it is activated. The hold
    current and the pull in current can be vastly different. I have seen it
    happen. I would even hazard that the original value might still be a
    bit low, but testing can bare that out. The 330 with .068uF might even
    be enough to pull it in from power up.

    Jim
     
  6. CFoley1064

    CFoley1064 Guest

    Subject: Re: Choice for snubber cap?
    Hi, Jim. Check the impedance of an 0.068uF cap at 60 Hz. The cap limits the
    60 Hz current, not the resistor. The purpose of the series resistor in a
    snubber is to limit instantaneous current when the switch closes. The balance
    between rise in voltage from switching an inductive load off, and current
    inrush through the switch when switching on, is the whole art of choosing a
    snubber for inductive loads. And 3 mA shouldn't be enough to hold a 75 mA
    solenoid on. Generally you figure that less than 1/10 rated voltage or current
    is pretty much failsafe to turn off a solenoid off (although you might get
    something of a buzz -- if so, you might want to lower the capacitance rather
    than increase the resistor)

    From a later post, though, the OP says that the cap is actually protecting an
    opto-triac rated for 150mA. In that case, a 3300 ohm resistor makes more
    sense.

    Chris
     
  7. James Beck

    James Beck Guest

    Yep, you are correct.
    It works out to ~39K, (~3 - 4mA around the triac) so I doubt that would
    hold in the solenoid. That'll teach me to skip the math.

    Jim
     
  8. CFoley1064

    CFoley1064 Guest

    Subject: Re: Choice for snubber cap?
    You'll notice I didn't show the math, either! ;-)

    Chris
     
  9. Very good advice.
    Also excellent advice. The resistor is necessary to let the TRIAC
    turn on without exceeding its peak current capability, but only
    degrades the snubbing operation when the TRIAC turns off.
    If you put the snubber across the TRIAC, it will snub just fine, but
    does not lower the average current any, via power factor correction.
    It also allows a leakage current to pass through the load when the
    TRIAC is off.

    Putting the snubber network across the load, instead, lowers the total
    load current but might allow an occasional half cycle of fault
    conduction if a line spike occurs.
     
  10. Neil Preston

    Neil Preston Guest

    Hello again, John;

    I had played with the idea of putting a cap across the coil for the reasons
    you mention. I figure that a 1.6 uF cap would make the solenoid coil a
    parallel resonant circuit at 60 Hz, bring PF to 0 etc. But, I would also
    expect some ringing at shutoff..... Plus, caps of that value are large and
    expensive.

    In digging through a fairly extensive library of books (I taught electronics
    for 17 years) I found very little on snubbers except in some RCA and
    Motorola databooks. In all of them, the snubbers are placed across the
    triac. No mention is made of putting it across the load.

    Neil


    Very good advice.
    Also excellent advice. The resistor is necessary to let the TRIAC
    turn on without exceeding its peak current capability, but only
    degrades the snubbing operation when the TRIAC turns off.
    If you put the snubber across the TRIAC, it will snub just fine, but
    does not lower the average current any, via power factor correction.
    It also allows a leakage current to pass through the load when the
    TRIAC is off.

    Putting the snubber network across the load, instead, lowers the total
    load current but might allow an occasional half cycle of fault
    conduction if a line spike occurs.
     
  11. No need to achieve resonance to reduce the current a bit. Just reduce
    the resistor till the peak current possible (turn on at peak voltage)
    starts to get appreciable. If the cap passes 1/8th of the solenoid
    current, it can reduce the solenoid current by about that amount.
    This could get you well below the thermal limit that keeps you from
    driving the load directly with the isolator. Large pads on the
    isolator pins (without violating voltage rules) helps there, also.
    If the source impedance is essentially zero, it works either place,
    since it keeps the inductive load from producing a sudden voltage
    change when the TRIAC turns off at zero current but the source is not
    at zero voltage because of the inductive lag. The risk of putting it
    across the TRIAC is that if the capacitor shorts it keeps the load
    energized, and otherwise leaks current through the load any time the
    TRIAC is off. The risk ot putting it across the load is that if the
    cap shorts, it overloads the TRIAC, and if there is a line voltage
    spike, it goes through the cap and risks false triggering the TRIAC,
    though this is often inconsequential.
     
  12. rayjking

    rayjking Guest

    Neil,

    The reason for the snubber is to limit the dv/dt across the triac so it does
    not turn on ( either when the mains are applied or when the inductive load
    creates dv/dt from the residual current after zero current crossing ( which
    is not zero voltage crossing )). It sounds like your relay has such a high
    impedance that it cannot cause a very high dv/dt across the capacitance of
    the triac to need a snubber at all. Having said that you may need a very
    small snubber if the triac is a low dv/dt or have a low voltage rating. If
    you select a triac with a voltage rating much higher than required the need
    for a snubber diminishes.


    Ray
     
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