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Transformer question - I'm puzzled

Discussion in 'Electronic Basics' started by Doug Miller, Apr 12, 2007.

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  1. Doug Miller

    Doug Miller Guest

    Honeywell's published documentation for their R182C switching relay
    contains the following warning:

    IMPORTANT: The transformer [120VAC primary, 24VAC secondary] on the R182C may
    overheat when used with a series 20 thermostat if the total resistance of the
    thermostat circuit exceeds 2.5 ohms. If the measured resistance of the
    thermostat (including thermostat wire and thermostat contact resistance)
    exceeds 2.5 ohms, add a 100 ohm, 10 watt resistor between the W and R

    I don't understand that -- if the total resistance is, say, 10 ohms, clearly
    the current drawn will be *lower* than it would be at 2.5 ohms, or at 1 ohm.

    So how is the transformer in danger of overheating at a *lower* current? Seems
    to me that the greatest danger of overheating would occur with a circuit
    resistance of near zero ohms, i.e. a dead short across the transformer
    secondaries. Someone please explain this to me.

    [Please note that I *do* understand the purpose of the 100 ohm 10W shunt
    resistor, in reducing the total resistance of the connected load. What I
    *don't* understand is how a *lower* resistance avoids overheating the
  2. Chuck

    Chuck Guest

    I, too, am baffled!

    While I can't give you an explanation, I
    can at least pass along a link to a
    document that doesn't look like it was
    faxed from some distant galaxy. ;-) I
    couldn't make out the text on your link.

    60-2481.pdf (application/pdf Object)
  3. Only with a series 20 thermostat (obsolete?) it seems. Perhaps the resistor
    is helping to pull in the relay coils?
  4. jasen

    jasen Guest

    That makes sense, if it's an alternating current relay, if the relay wouldn't
    pull in due to the line resistance so the inductance of the relay would
    stay low, so current through it would stay high so it'd all heat up.

    I'm guessing the resistor does something to boost the power from the
  5. default

    default Guest

    It makes no sense, but Homer has an idea there. Is there a power
    contactor type of relay? Thingy where a laminated core has to pull in
    to move the contacts? If so, there's the possibility that the
    contactor's coil could overheat when it doesn't pull in.

    And there's a somewhat hinky type of low voltage transformer/relay
    that was popular in the '60's. I don't remember much about how it
    worked, but it was a sort of AC relay with extra winding on it. The
    winding acted as a transformer secondary and you were able to run a
    low voltage, isolated control pair out to a switch - the average
    homeowner could do it legally without an electrician. You short the
    control wires and it activates the mains circuit.
  6. ISTR Honeywell made those back in the '70's.
  7. Doug Miller

    Doug Miller Guest

    Probably earlier than that -- my house was build in 1955, and AFAICT, this is
    original equipment.
  8. I'm sure you are right. Some of the control equipment I worked on still used
  10. Sounds counter intuitave doesn't it? Might be a mistake.
    Maybe me it works somethiing like this, the load acts as a break, so
    drinking a a car with the brakes on a bit, that would make the engine do
    work? Or something like that?
    I do notice, transformers supplied with small devices alike modems or
    speakers etc
    tend to be hot even when the device is in active (drawing no current).

    And I think you need to be clear aout what is over heeating, the transformer
    or the load, with a low resistance is is likly that the thermostate would
    over heat,
    but you are concerned about the transformer.

    This is a sort of guess, but say you are putting Xammount of energy in,
    then X amount of energy mush also go out (sounds reasonable?) OK
    so the output energy can only be disapaited in the load and the transformer.
    Now if the load is ssmaller (higher resistance) then only place the rest of
    the eneeergy
    can go is into the transformer itself, so the lower the load (the higher the
    resistance) then the more energy must be disapaited in the tranformer
    I don't need to specify (or know) why this happens, its just basics maths.

    E(in) = E(transformer) + E(thermostat)

    So with a higher resistance in the thermostate it will use less energy
    of the rest of the input energy must go into the transformer itself.

    I don't know if that is right but it seems a line of thought which would
    make sense?

    It would follow that with no load *all* the input energy must be disapaited
    the transformer. Sounds reasonable?

    Anotheer car analogy, imagine the transformer is the clutch of a car.
    Now if you drove that car against an imovable object then the clutch would
    start to slip and burn loads of energy (and probably burn out) however
    with a low load (level road freewheeling) hardly any energy would would
    be spent iin the clutch (transformer).

    I assume a similar thing happens in a transformer.

    In short you are putting energy in and it has to go somewhere, so if its not
    going into the load it must go into the transformer, the mechanism of that
    process is academic but I will hazard a guess at it another time.

    Yours Sincerely
    Lord Turkey of Norwich(bird flu free zone

    Am I right?.
  11. ISTR one model which used a 120 volt to 24 VAC transformer with the relay
    coil in series with the transformer primary. You shorted out the 24 VAC
    contacts with the 'stat to pull the relay in.
  12. Looks as if the transformer needs a minimum load. This may be caused by
    a false or cheap contruction of the transformer itself. The transformer
    may get saturated driving light loads.
    On the other hand the real reason for this remark could be caused by
    other problems with won't sound better to the customer (i.e. overvoltage
    problems etc.)

    - Udo
  13. Ross Herbert

    Ross Herbert Guest

    My reading of the doc is that the instruction in the panel when using
    type 20 thermostats is WRONG.

    While it is a little difficult to make out the text for the center
    wiring arrangement in Fig. 3 it seems to refer to the type 20
    thermostat. Note that this arrangement shows a solid wire jumper
    between W and R which would apply for the maximum length of wire for
    any particular gauge of wire shown in table 1. Since the transformer
    is supplying operating current to its own contactor coil via the long
    cable run and the thermostat contact the maximum resistance of the
    cable run can not be allowed to exceed a certain value, otherwise the
    contactor coil will not operate reliably.

    What the instruction mean to say is that when the loop resistance of
    the wiring connecting the thermostat is LESS THAN 2.5 OHMS, then
    replace the solid wire jumper between W and R with a 100 ohm 10W
  14. Doug Miller

    Doug Miller Guest

    I'd considered that possibility, but I didn't understand how ...
    ...until now.
    Makes sense to me. THANK YOU.
  15. Lionel

    Lionel Guest

    The lower the current, the lower the chance of overheating. It's that
    simple, shit for brains.

    \___ Proud Cog #1 in the AUK Hate Machine
    _(AUK)====:: Do *you* think that you have the Right Stuff?
    /='='='='-, Apply TODAY by addressing a gratuitously cruel
    (O+O+O+O+O) flame to: "Uncle Fester", C/O & AUK.
  16. So I guess your one of those guys who think that if they call someone else
    stupid it makes them smarter!?!?! At least he has some brains...
  17. Lionel

    Lionel Guest

    On Fri, 01 Jun 2007 02:32:44 +0000, Lionel <>
    **** off, identity thief.

    \___ Proud Cog #1 in the AUK Hate Machine
    _(AUK)====:: Do *you* think that you have the Right Stuff?
    /='='='='-, Apply TODAY by addressing a gratuitously cruel
    (O+O+O+O+O) flame to: "Uncle Fester", C/O & AUK.
  18. mpm

    mpm Guest

    And neither one of you thought much about the physics involved.
    The question was lower resistance, and the answer dealt with lower
    current. Huh?!

    The question of harmonics wasn't even addressed, so let's do that now.
    Assuming this is 120/240 1-Phase, and a 5:1 step down, let's run this
    transformer at rated power, rated current, rated heat rise, etc...

    Now, let's take that same transfomrer input current value and convert
    it to harmonics of 60-cycles (even or odd, it really won't matter for
    this simple example). Convert it all.

    ....The transformer expected lifetime is left as an exercise for the
    And remember, we're still supplying rated current. (value)

  19. Marra

    Marra Guest

    The life depends on how hot it gets in normal running.
    The expansion and contraction of the windings will eventually make the
    resin around them break down and short out.

    The bigger the temperature change the shorter the life.
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