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Relay for 74HC-controlled outlet strip

Discussion in 'Electronic Design' started by Peter S. May, Sep 20, 2007.

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  1. Peter S. May

    Peter S. May Guest

    (Almost certainly overthinking this one, but another case where the
    wrong decision means more waiting for another mail order.)

    A couple of projects I have in mind would benefit from
    CMOS/microcontroller-controlled outlet strips, and I'm trying to make
    some decisions about what sorts of relays this might require. I'm
    assuming first that the contacts need to be rated for 125VAC (or more)
    and for well over the current I intend to drive (does P=IV apply to AC;
    e.g., is 10A right for 12500W @ 125VAC?). And I'm looking for either
    12V or 5V coils, whichever comes easier.

    The drive circuit I often see laid out is something like this:

    +------+------> +12VDC
    | |
    - @
    1N4001 ^ @ coil
    | |
    R |/ 2N2222
    signal >--------/\/\-----|
    |>---| GND

    I would assume that I should select R to drive the transistor to
    saturation. A relay I'm looking at has a coil rated for 120mA, 100
    ohms. As far as I know, that ought to work, since even the TO-92
    work-alike of the 2N2222 is rated for 500mA or so typically.

    Any flaws in my logic yet?

    Another matter concerns switch configuration. Would it be a better idea
    to leave one blade always connected (single pole) or do I need to
    connect and disconnect both at once (double pole)? I'm assuming that
    the third grounding prong is not disconnected under any circumstance.

    The IDEC RU2S-C-D12
    ( seems
    to do what I want. Am I totally off?

    One more thing: Say I'm running the microcontroller circuit and the
    relay from the 5V and 12V pins, respectively, of an ATX-style SMPS. Are
    there any special precautions that need to be taken other than the
    typical reverse diode across the coil, decoupling caps on ICs, and so forth?

  2. Why wouldn't you use a solid-state relay, which will work directly
    from a logic signal?
  3. mpm

    mpm Guest

    If you didn't want to mess with all those discretes, what about a
    ULN2003 or equiv.?
  4. Peter S. May

    Peter S. May Guest

    So far it appears that they're a lot more expensive for the same contact
    rating (about $4 vs. about $12) and high frequency response isn't so
  5. Peter S. May

    Peter S. May Guest

    If I were driving more than one, I'd probably use one of the ULN2803As I
    already have, but we're only talking one relay per device, so the splay
    of discretes really isn't an issue.

  6. whit3rd

    whit3rd Guest

    Instead of using your regulated power to run a relay coil, consider
    using a phototriac-output optocoupler to drive a 120VAC relay.
    It has most of the good features of a solid-state relay without
    the complication of sucking half an amp from a power
    supply intended for CMOS.
  7. Peter S. May

    Peter S. May Guest

    An interesting that could easily remove the complexity on the
    DC side of things.

    A 5VDC solid-state phototriac relay I'm looking at has the following
    phraseology in the data sheet: "No overvoltage absorption element is
    built in. Therefore, if the G3MC is connected to an inductive load, be
    sure to connect the overvoltage absorption element." I would guess that
    a coil relay is an inductive load. What would one use as an overvoltage
    absorption element?

  8. John Fields

    John Fields Guest

    No. The contacts will be rated for volt-amperes, not watts, and at
    125VAC, 12500VA, the contacts will need to be rated for at least
    100A if your load is resistive. (But, I suspect that 12500 was a
    trypo and you really meant 1250, yes?)

    For the same ampere-turn coil, a 12V relay will draw 5/12 the
    current of a 5V relay.
    A PN2222 has a beta of between 100 and 300 with an Ic of 150mA, so
    that would seem to be a good choice. However, in order to be sure
    that the transistor is saturated, it's customary to "force" the beta
    (Hfe) to 10. That's done by dividing the collector current by 10
    and pushing that current into the base.

    For a 120mA relay that would require 12mA into the base, which your
    micro might not be able to supply.

    However, there's a possible out. Let's say your micro can supply
    2mA into the base. In that case the transistor's beta will be
    forced to:

    Ic 120mA
    Hfe = ---- = ------- = 60
    Ib 2mA

    so it should still work. Try it. :)
  9. krw

    krw Guest

    Use X10. The life/home you save may be your own.
  10. Peter S. May

    Peter S. May Guest

    I did in fact mean 1250, not 12500, but I was talking about watts. No
    lightbulb, appliance, or computer PSU has ever been rated in VA. (This
    is odd to me, since from what I've read it's VA that applies to AC. Why
    are these things rated in watts?)

    I'm looking at using the relay-switched strip with a few combinations of
    small items. For example, a pair of wall transformers marked 25W and a
    toy lamp rated 17W. What's my current per watt at 125VAC?

    The single-pole configuration would be

    | |
    | O
    | \
    | O
    | |
    | |
    +--[] []--+

    while the double-pole would be

    | |
    O O
    \- - - - -\
    O O
    | |
    | |
    +--[] []--+

    Is there any reason to prefer one over the other? Is one more likely to
    arc than the other?

  11. Hal Murray

    Hal Murray Guest

    Because that's what you pay the power company for.

    If the load is a resistor, watts = V*A

    But consider a capicator or inductor. It takes current, but it
    gives it back on the other half cycle. No net power.

    Light bulbs have a horrible (10x) starting current when they are cold.
    That's nasty to relays.

    I'd add up all the power, divide by 125, round up generously, then
    look in the catalogs and probably round up some more.
  12. John Fields

    John Fields Guest

  13. Peter S. May

    Peter S. May Guest

    Agreed. Bringing X10 into the picture was a complexity and expense I
    wanted to avoid. A hard CMOS line is protocol-free, isn't as
    susceptible to radio interference (intentional or accidental), and would
    cost well under $10 to implement. As for safety, I'm not ignorant to
    any electrical and/or fire hazards inherent in this mod. The whole
    reason this idea seems reasonable is that outlet strips already have a
    big power switch, so what's the harm in inlining and/or replacing that
    switch with a relay of the same contact rating? Nothing significant, as
    long as I'm not careless.

  14. John Fields

    John Fields Guest

    Yes, I know, but relay contacts aren't rated in watts, they're rated
    in volt-amperes. Or horsepower, or tungsten loads in some
    Generally, they aren't. Look on their nameplates and you'll see
    that their operating voltage and current are listed separately. The
    reason for that is that they're reactive loads except for toasters,
    irons, and other appliances which present resistive loads to the
    If the transformers are truly just transformers, (as opposed to
    being AC to DC converters) then they're mismarked and should be
    rated in VA.

    If your mains are 125VAC and your loads are resistive, then:

    P 1W
    I = --- = ------ = 0.008A = 8mA
    E 125V
    I prefer the DPDT configuration since mains HOT and NEUTRAL are both
    switched. That way, even if the outlet strip or a load with an
    exposed ground and a non-polarized plug is plugged in backwards
    it'll be safe to touch when the relay isn't energized. Of course it
    can still kill you when it's on, but that's a different story. ;)

    BTW, you showed the outlet strip with outlets in series. I'm sure
    you meant something like this:

    .. mains
    ..| |
    ..O O
    .. \- - - - -\
    ..O O
    ..| |
    ..| |
    ..+--[] []--+
    ..| |
    ..| |
    ..+--[] []--+
    ..| |
    ..| |
    ... .
    ... .
    ... .
    ..| |
    ..+--[] []--+
    .. outlet
    .. strip
  15. Peter S. May

    Peter S. May Guest

    I'm only calling them "transformers" because "AC-to-DC adapter" is a bit
    long-winded and I've never been partial to the term "wall wart". But,
    in any case, that's what I'm talking about--black boxes with
    transformers, bridge rectifiers, and probably a couple of other things
    (never actually broken one open).
    What general rules are to be applied when determining whether a load is
    resistive or inductive? Having no formal training in such details, I
    tend to assume that something with a coil in it is inductive. By this
    logic, the AC-DC "transformer" (itself loaded with a small electronic
    device such as a router, cable modem, laptop, ...) is inductive, the
    motor part of the toy lamp is inductive, and the incandescent bulb part
    of the lamp is resistive. I get the feeling that I'm entirely off on at
    least one of these.
    That is not only what I meant but in fact what I thought I had said
    (abbreviated to a single "out" socket). My prediction of ambiguity in
    my illustrations leaves something to be desired...

  16. krw

    krw Guest

    X10 isn't only for "remote" control. You needn't drive the protocol
    or push buttons to use it.
  17. John Fields

    John Fields Guest

  18. krw

    krw Guest

    No, you buy one of their interfaces for $5 or perhaps less (perhaps
  19. John Fields

    John Fields Guest

  20. krw

    krw Guest

    Don't have one now, but I get ads for them all the time. So many
    that I throw them away (modules too). This stuff is *cheap*, if you
    don't buy it from RadioShaft (or Leviton) and there is a module to do
    just about everything.
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