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solenoid switching

Discussion in 'Electronic Design' started by ludnough, Jan 16, 2004.

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  1. ludnough

    ludnough Guest

    Hi,

    What is it that causes electrical interference in solenoids switched with
    mechanical contacts?

    TIA
     
  2. Any device that produces magnetic field with a coil of wire has a
    property called inductance. Inductance is a sort of current fly
    wheel. That is, it produces voltage that keeps current from changing
    quickly. This is no problem when the power contacts close, because
    the inductance simple produces a voltage that cancels out the applied
    voltage momentarily, slowing the rate of rise of current. But when
    the contacts open and attempt to stop the current, the inductance
    produces enough voltage to blast an arc through the air between the
    contacts to keep the current from stopping instantaneously. This
    large pulse of voltage also produces radio waves that couple to any
    nearby electronics. In effect, it is a crude radio transmitter.

    Tell me whether the solenoid in question is being driven by AC or DC.
    The fix has to handle the normal applied voltage.
     
  3. When two mechanical contacts operate, they aren't perfect. For one
    thing, they are usually made of springy metal, so when they first start
    to make contact, they bounce. They don't do it once or twice, sometime
    they bounce multiple times. Each time, as the contact is made, then
    broken, then made again, a little spark happens. This spark has lots of
    high speed transients, and makes a lot of radio frequency noise. The
    early radio transmitters were just this type of spark gap, and that
    noise gets transmitted all through the area both through the air and
    through the power wiring.

    Also, the contacts may have a little (or a lot) of corrosion on their
    surfaces. As the contacts move across each other, this corrosion
    creates a varying signal that also generates noise in the circuit.

    There are various ways to mitigate this noise, such as using 'wetted'
    contacts with mercury or other liquids to buffer the connection, or
    placing filters across the contacts to counteract or absorb the energy.

    Charlie
    Edmondson Engineering
    Unique Solutions to Unusual Problems
     
  4. ludnough

    ludnough Guest

    First, thanks to you and Charlie for your helpful responses.

    The one I need to know about is being driven by 24VDC. For my own
    personal information, could you also tell me what problems and fixes are
    AC-specific?

    Thanks
     
  5. The most common suppression for DC coils is to put a diode across
    them, oriented such that the normal drive voltage reverse biases the
    diode, turning it off. When the drive contact opens, the coil voltage
    reverses, turning the diode on, so that the coil current transfers
    from the switch to the diode, and the coil voltage does not rise more
    than a volt or so, greatly reducing the voltage across the contacts
    (or other switching device). This low reverse across the coil does
    slow the decay of the magnetic field, and slows the release of relays,
    so if you want fast release, you have to add a resistor or zener in
    series with the diode, to allow the voltage to rise higher, while
    still controlling it. A small capacitor across the coil also reduces
    the rate of rise of voltage as the coil current charges up the
    capacitor. This is the method used on ignition systems to protect the
    points from arcing damage. It allows the contacts to get enough air
    between them before the voltage rises high enough to strike an arc.

    This is similar to what is done with switched AC inductive loads. The
    diode cannot be used, because the normal drive voltage will forward
    bias the diode and destroy it or something else with uncontrolled
    current. So the capacitor method is used, often with a low value
    series resistor to limit the inrush current when the AC is first
    switched on (in case switching takes place at the peak of the cycle).
    For instance, a 120 volt AC relay coil might have a .1 uf 400v film
    cap in series with a 470 ohm 1/2 watt resistor, if the relay is small,
    or a 1 uf cap and a 47 ohm resistor if it is a big contactor.

    I like to also put an MOV (resistor that varies with voltage, high
    resistance at low voltage and low resistance at high voltage) rated to
    switch on at a little higher AC voltage than that used to drive the
    relay. This limits how high the voltage can be pumped across the cap,
    making sure the spikes don't eventually damage the cap. But an MOV
    alone is not very good at eliminating the arc. It just reduces the
    strength and duration of it.
     
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