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Switching inductive loads

Discussion in 'Electronic Basics' started by Perion, Oct 27, 2004.

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  1. Perion

    Perion Guest

    Greetings fellow Earthlings,
    I know pretty much how to design circuits for zero crossover switching
    controllers for pure resistive loads but how is this done with inductive loads?
    How accurate an estimate for the inductance of the load is required for proper
    component selection?

  2. CFoley1064

    CFoley1064 Guest

    Subject: Switching inductive loads
    And greetings to you, fellow Earthling. Since Klaatu is busy (man, we could
    use him now), I guess I'll have to do.

    I'm assuming you're talking about switching inductive AC loads with TRIACs.
    This can be a bit of a problem because, at the moment of zero voltage crossing,
    current is still flowing which prevents the TRIAC from turning off.

    There are two approaches to your problem. One thing which is commonly done is
    to place an R-C snubber across the TRIAC to get the power factor down a little
    and help reduce the current going through the TRIAC at zero criossing. The RC
    snubber is also good for limiting dV/dT, which can be responsible for false
    turn-ons. This solution has limited usefulness, mostly because you have to be
    very aware of the load inductance, and you're wasting power while the load is

    The other approach, which is much more commonly used, and which will truly
    minimize the problem, is to use inverse-parallel-wired, or back-to-back SCRs,
    and trigger each of them on the appropriate half-cycle. Since an SCR blocks
    current when reverse-biased, it will turn off at zero crossing, and will stay
    off (assuming the voltage across the SCR doesn't exceed Vr(max). The
    difficulty with this is that it requires a more complex triggering system,
    because you have to keep track of which SCR is next to be triggered, and the
    gates are at different line potentials.

    ___ ___
    | |
    | A
    | -
    - |
    V |
    - |
    o--------------------/| |
    | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    One easy solution I've used successfully a number of times over the years is
    Teccor Electronics alternistors. These are TRIACs which have been specifically
    designed to switch inductive loads. They act the same as back-to-back SCRs,
    but only have one gate. They require a little more gate current to turn on, and
    only operate in quadrants I, II and III. But, you can use them with any
    standard TRIAC circuit where you've got a positive gate pulse with a positive
    voltage on the load (Q1), and a negative pulse when there's a negative voltage
    on the load (Q3). They're made to control voltages up to 800V pk., and
    currents up to 40 amps. Look up Alternistors in the Digi-Key catalog, and get
    a one-chip solution. Here's the data sheet: Data/Alternistor Triacs (

    For the standard newbie circuit of an RC charging circuit with a DIAC driving
    the gate of the TRIAC, all you have to do is remove the TRIAC, insert the
    alternistor, and you're good to go. (view in fixed font or M$ Notepad):

    | |
    | |
    .-. |
    | | |
    | | |
    '-' C|
    | Inductive C|
    R | C|
    .-. Load |
    | |<--. .-.
    120 VAC | | | | |
    '-' | | |
    o----o----. '-'
    | _|_ |
    | DIAC V_A _|_
    | | V_A
    C --- '---------/ |
    --- |
    | Alternistor|
    | |
    | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    It's still good to put a small snubber acrss the alternistor to help with
    dV/dT, say 0.047uF in series with 100 ohms as a starter.

    Good luck, and as always, be careful with line voltage
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