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Snubber network question

Discussion in 'Electronic Basics' started by [email protected], Mar 1, 2004.

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  1. Guest

    Hello all,


    3 weeks ago, I had the thermostate of my hot water boiler replaced because
    it generated sparks right before switching on the circulating pump.
    (sparks to be heard via the radio as crackles).

    After 1 week of good working, the new thermostate now also sparks.

    Could this be solved by placing a snubber network (R en C in series) over
    the switching contact of the thermostate ? And if so, how should I
    dimension this RC network, knowing that the mains voltage here in Belgium
    is 220V/50Hz. Personaly I was thinking of a 1k resistor in series with a
    capacitor so that the RC value falls in the range of 10 kHz, i.e. a 1 k
    resistor in series with a 15 nF cap.

    But is this effective ? And what should the voltage of the cap be ? I
    would guess, since the pump motor acts as a coil, that the voltage can
    rise up to as much as then times the line voltage, so about 2000V would be
    in order. Is this correct ? Should the resistor have specific features ?

    Come to think of it, now that I do the math for the resistor, maybe it
    should be 100k, question of limiting the current.

    Anyhow, I'm a little (or a lot) lost here, and I'd be grateful for any
    suggestions.

    TIA

    Danny
     
  2. CBarn24050

    CBarn24050 Guest

    100nF + 100R
     
  3. CFoley1064

    CFoley1064 Guest

    Subject: Snubber network question
    Good morning, Danny. Looking at your post, I'd first like to ask a couple of
    questions. When the first thermostat switch was replaced, was radio
    interference the only problem, or was there something else? Water heaters
    usually don't come with any guarantees that they're not going to generate RFI
    when they turn on. I'm assuming the first switch failed, and that the RFI is a
    symptom you picked up investigating along the way. I'd also like to know if
    possibly the thermostat is driving the motor directly, or it's driving a relay
    that's driving the motor. Also, if the RFI is occurring on turn-on or turn-off
    of the motor.

    One thing you might want to look at is a partial short in the relay coil or
    motor coil, and the possibility of a partial coil short to ground. That can
    jack the current way up, and cause contact arcing where you normally wouldn't
    have any. Check the wiring, and ohm things out. It might help to find out the
    typical resistance values of any coils or motors in the circuit, and measure
    them to check. Measure to ground too. The standard principle is that, if it's
    designed to work well and something doesn't work right, you should look for
    something that's broken first. You might have to temporarily disconnect things
    to make measurements.

    Many control circuits will use a transformer to generate a lower control
    voltage which is switched by the sensor. This is especially common in the EU,
    because of different wiring standards. That is used to drive a relay coil,
    which drives the line voltage motor. Before you make any mods, you need to
    know the electrical wiring, and exactly what's being switched where. You
    mentioned that the problem occurs on turn-on rather than turn-off. That might
    mean something's going on you're not aware of. Get the wiring diagram, look at
    it.

    When a contact opens across an inductive load, the voltage across the inductive
    load being turned off will rise (Lenz' Law). Since the mechanical contacts are
    moving away from each other slowly, a spark will be initiated across the
    contacts, and will continue until the contacts are far enough away to
    extinguish the arc. Without a snubber, the inductive kick of the load is being
    absorbed by the energy dissipated by the spark. Not a good way to do things,
    because it chars and pits switch contacts (and generates RFI). At worst, the
    arc can eventually spot weld the contacts together on an intermittent make of
    the contacts. A snubber will absorb some of the inductive kick power when the
    switch opens, keeping the voltage across the contacts low enough for long
    enough that a spark doesn't form across the contacts while they're separating.
    In other words, the rise in load voltage remains lower than the arc-over
    potential. The rest of the power of the inductive kick will eventually be
    absorbed by the coil itself.

    Now you can look at specifying a snubber. A snubber is an R in series with a
    C, and is placed across the load (or across the switch contacts, if it's not
    possible to put it across the load). The snubber will conduct normally when
    the load is on (or off, if you've put it across the contacts). The current is
    limited by the cap rather than the resistor (which is good, because it means
    you can use realistically sized resistors).

    Find the rated switching current of the switch. Choose a resistor which would
    provide the rated current across the switch if placed in series with the line
    voltage. For a 24VAC control voltage and a 1A switch, that would be 27 ohms.
    For 240VAC at 1A, that would be 270 ohms. A 1 watt minimum resistor should be
    used here for 240VAC, 2 watt for 24VAC. A 3 watt wirewound is always OK --
    just make sure you don't use carbon film, they are unable to handle the power
    of current spikes. Now look at the cap. You need to get a self-healing AC cap
    rated for more than the AC control voltage. For a 24VAC circuit, you would
    like a cap rated for at least 50VAC; for 240VAC you should choose 350VAC or
    400VAC rated. To calculate the capacitance, you need to characterize the
    opening and closing time of the switch, with distance over time, as well as the
    inductance and resistance of the motor. Too much here, especially for a
    hobbyist or do-it-yourselfer. It might be easier to start with an 0.1 uF cap
    for 240VAC, or 0.47 uF if you've got 24VAC, and just try it. If you're still
    getting sparking across the contacts, bump up the capacitance just until there
    isn't a visible spark across the contact. Assuming you've got a low voltage
    control transformer switching a relay, your snubber circuit might look
    something like this (view in fixed font or use M$ Notepad):

    Thermostat Control Circuit

    Thermostat
    SW1
    _/
    -. ,--------o/ o-----o---.
    220VAC )|(24VAC or | |
    )|(12VAC .-. |
    -' '----. | | |
    | R | | C|
    | '-' C|RY1
    | | C|
    | | |
    | --- |
    | C --- |
    | | |
    '-------------o---'

    Summary -- play safe with line voltage, look for something that's broken before
    you make mods, select the resistor based on switch rating, choose cap based on
    ability to quench the spark and don't pick a bigger cap than you need.

    Good luck
    Chris
     
  4. Jamie

    Jamie Guest

    get into the thermostat and put a ceramic disc cap across the contact
    points.
    a .01 uf should be ok but a .1 uf would be better. (one that can
    handle at least 400 Volts.
     
  5. Guest

    Hi Chris,

    thanks for your response.

    As for your questions, the thermostate switch switches the mains directly,
    and as for the sparks, they are not generated in the boiler itself as I
    hear the switch sparking. I don't know if I can find the specs of the
    motor coil. But the system works fine up to now. I'm only afraid for the
    pits on the contacts (which you also mention) which may weld them together
    or destroy them entirely.

    Your remark about placing the snubber over the load or the switch has me
    wondering: is it really possible to place the snubber over the load ? I
    thought a snubber was something typical to be placed over switches (triacs
    etc) ?

    Anyway thanks for your reply already.



    Danny
     
  6. Jamie

    Jamie Guest

    i am sorry, that was intended for the originator.
     
  7. CFoley1064

    CFoley1064 Guest

    Subject: Re: Snubber network question
    Snubbers can be placed either across the load or across the switching
    mechanism. For solid state switching, it usually makes more sense to put the
    R-C snubber across the load, because that's where the circuit board is. The
    snubbing action of the R-C is essentially the same. For a snubber across the
    load, it's conducting as an RC element when the power to the load is on. For
    an RC snubber across the switch contact, current will flow through the snubber
    when the switch is off.

    If you're making a mod to an existing piece of equipment, you probably want the
    snubber to be across the load, to avoid epithets, swearing and general bad
    karma from the poor guy who's repairing the circuit next time. If you have
    voltage (even through an RC snubber) across the load when the switch is off,
    and he doesn't know about it, he may have a bit of a surprise when he touches a
    wire that's supposed to be de-energized. Of course, the source will be
    current-limited, and it probably won't be fatal, but it probably won't be the
    highlight of his day.

    Remeber that, if you make unauthorized mods to a circuit, mark it off. I've
    always believed there's a rather toasty sub-ring in a very warm place (probably
    fifth or sixth circle) reserved for the afterlife of those who make mods to
    circuits or code without leaving very plain notice for unsuspecting future
    tinkerers. Probably they are forced to repair each other's work over and over
    for eternity, with each rat's nest and indigestible block of spaghetti code
    getting worse and worse and worse.... ;-)

    Good luck
    Chris
     
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