# Transformer question - I'm puzzled

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

1. ### Doug MillerGuest

Honeywell's published documentation for their R182C switching relay
http://customer.honeywell.com/techlit/pdf/95-0000s/95-6762.pdf
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
terminals.

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
transformer.]

2. ### ChuckGuest

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

Chuck

http://customer.honeywell.com/techlit/PDF/60-0000s/60-2481.pdf
60-2481.pdf (application/pdf Object)

3. ### Homer J SimpsonGuest

Only with a series 20 thermostat (obsolete?) it seems. Perhaps the resistor
is helping to pull in the relay coils?

4. ### jasenGuest

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
transformer

5. ### defaultGuest

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. ### Homer J SimpsonGuest

ISTR Honeywell made those back in the '70's.

7. ### Doug MillerGuest

Probably earlier than that -- my house was build in 1955, and AFAICT, this is
original equipment.

8. ### Homer J SimpsonGuest

I'm sure you are right. Some of the control equipment I worked on still used
tubes.

10. ### Lord Turkey CoughGuest

Sounds counter intuitave doesn't it? Might be a mistake.
Maybe me it works somethiing like this, the load acts as a break, so
imageing
drinking a a car with the brakes on a bit, that would make the engine do
more
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
thermostate
resistance) then the more energy must be disapaited in the tranformer
itself.
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
(V*V/R)
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
in
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
;O))

Am I right?.

11. ### Homer J SimpsonGuest

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. ### Udo PiechottkaGuest

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 HerbertGuest

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
resistor.

14. ### Doug MillerGuest

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

15. ### LionelGuest

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

--
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16. ### Jon SlaughterGuest

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. ### LionelGuest

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 soc.singles & AUK.
~^^^^^^^^^~~~^~^^~'~~^'^~~~"~~'"~^~'"~~^~"~'~^'^~^~^^~^~"~^~"'~'"~^~~

18. ### mpmGuest

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)

Enjoy.
-mpm

19. ### MarraGuest

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.