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help driving relay

Discussion in 'Electronic Design' started by Paul Taylor, Aug 5, 2005.

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  1. Paul Taylor

    Paul Taylor Guest

    Hello to you all a quick question has anyone in the past driven a high
    powered relay 25A at 240v from a SSR which is being switched using TTL from
    some logic? im having problem getting the relay to reset itself when the ttl
    is removed from the SSR the relay basically stays latched I can hang a load
    on the SSR o/p and this sorts the problem I feel this isn't a very good fix?
    any ideas is much appreciated thanks in advance.

    Paul Taylor BSC (Hons)
    Electronics Technician
    School of Environmental Science
    University of East Anglia
    NR4 7TJ

    Phone: +44 (0)1603 592502
    Fax: +44 (0)1603 591327


  2. Ol' Duffer

    Ol' Duffer Guest

    Many SSR's don't like highly inductive loads (like your relay coil).
    High dv/dt (back emf) can make the SSR self-trigger. RC snubber
    networks are relatively common in this application. Your SSR's
    maker should have some application notes to help you sort it out.
  3. You'd do better with an SCR-based SSR (or possibly an alternistor
    type). These types are more tolerant of switching inductive loads,
    particularly the "off" part of switching.

    Kudos for mentioning that it's a high power relay (or contactor) and
    thus we can pretty much eliminate the possibility of the few mA of
    leakage through the snubber holding the relay on.

    Best regards,
    Spehro Pefhany
  4. Fred Bloggs

    Fred Bloggs Guest

    How are you measuring the off state, with a high impedance meter and no
    load? The SSR leakage will show the full voltage across the meter.
    Almost any kind of SSR will handle a relay coil drive, why don't you
    measure it with the relay in place, the meter should show much less than
    10% line voltage in the off state.
  5. I think the cause of the problem is the highly inductive nature of the
    load. Because of that, the load current is not in phase with the
    voltage, so at the moment of zero current (when the SSR tries to
    switch off) there is considerable line voltage that is being bucked by
    the inductive rate of change of current produced voltage. But at
    switch off, the rate of change of current snaps to zero, allowing the
    line voltage to appear, essentially instantaneously across the SSR.
    This exceeds its dv/dt limit, and the voltage swing capacitively
    couples into the SSR device gates, switching it back on.

    Adding a series RC across the SSR output will provide a path for a
    little current at that moment, to greatly reduce the dv/dt. You may
    get by with something like 100 to 470 ohms 1/2 watt and .1uF, but the
    capacitor needs to be rated for across the 240 volt line, since it
    sees this all the time the SSR is off.

    Prefab (resistor potted with capacitor in single case) snubber
    networks like this are also available.

  6. Question of the week:

    Why don't people put the snubber acroos the load to solve the snubber
    leakage problem? The turnoff dV/dT would be the same.

    I did it - solved most problems with small solenoids, and I never had any
    problem with powerspikes triggering the SSR's. When zero-voltage switching
    at turnon the inrush current is no problem either.

    Arie de Muynck
  7. 1) You're assuming there is no line inductance. This is the sort
    of thing that can work in one place, but possibly fail in some
    godforsaken windswept customer location without an airport,
    sushi, cappuccinos, fresh-baked nan or any of the other
    necessities of a civilized existence. A grim thought.

    2) The SSR maker typically has only two terminals to work with.
    This method does potentially aggravate the issue of transients on the
    line. You can always put supression in both places (across the load
    and across the line), but you have to deal with the unknown (and
    possibly low) impedance of whatever is creating those spikes, rather
    than the usually known (and relatively high) impedance of the load.
    No, but if it's triggered by the dv/dt or voltage of a spike it won't
    likely be at the zero crossing.
    On the plus side, for this method is that a failure of the snubber
    tends to put the load in a safer condition. Depending.

    Best regards,
    Spehro Pefhany
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