Another question, does he keep buying same x-former over and over or
something different little havier one? Along with fuse I'd put in series
a low value proper Wattage resistor to lower the primmary voltage little
bit. Crazy thinking but hope the x-former is not put in backward.
Measure both AC and DC current through the secondary of the transformer.
If putting a multimeter into DC current mode gets a reading around or over
50 milliamps (,05 amp), then something is wrong with the load.
If DC secondary current is near or over 200 milliamps, then,
"Houston, we have a problem"! Probably with one of the recently-mentioned
discrete diodes in a board's bridge rectifier.
Ideally, DC current through transformer windings should be zero. With
exception to specialty cases, typically involving special transformers
that use DC-handling means such as gapped cores.
If this tranny is running into problems related to DC, look for DC
through the secondary. If that turns up substantially, then the
already-mentioned 4-discrete-diode bridge rectifier on the board is
likely to be the culprit. Look for solder joints there that need touching
up, or else replace the bad diode if one is found to be bad (or all 4 of
them), or the whole board.
Depending on ability and willingness to use a soldering iron and to
troubleshoot and repair a board to component level, even with pointers
to a suspect identified set of components on the board...
It may be more practical to get a replacement board if the existing one
causes substantial DC to flow through the secondary of the transformer
that is prone to failing.
- Don Klipstein ( email@example.com)
** Simply fitting that DAMN 1/4 amp s-b fuse will tell you immediately if
any such problem exists.
Significant DC in the secondary will causes high current to flow in the
primary ( due to core saturation) and BLOW the fuse !!!
If the 1/4 amp fuse holds and the tranny does not get stinking hot - all is
I have been (sort of) following this conversation. I assume this
transformer also drives the contactor for the AC compressor.
Maybe there is a short in that circuit and the transformer
secondary gets shorted when the thermostat calls for cooling.
It also could be powering a thermostat. Some of them will take power
from the transformer circuit. For example, I have a Honeywell
that has an optional 24V connection to power the thermostat, with
batteries then being the backup. Doing it that way allows the display
to be backlit 24/7 too.
Also, it's not unusual to have humdifiers tied into the 24V circuit
Again, he needs to start doing some basic current measurements,
starting at the transformer and find out how much current the whole
thing is drawing and then if it's high, work to isolate it. If the
is normal, then I'd put in a 1.5 amp fuse in the secondary
Contrary to the suggestions to use a fast blow, I'd probably just use
a regular one, as whatever is capable of destroying a transformer
should be most capable of opening any fuse.
That's a good idea too.
I don't see how he could have a waveform that is so out of shape that
burns up this one specific transformer, yet there are no apparent
complaints of any other problems in the house. Or how you'd get
such a badly distorted waveform in the house in the first place.
this is a long shot but...
I noticed on the Trane wiring diagram that you posted that the
connections for the IGNITION circuit are shown to be near to the
connections for the primary of the transformer.
If there is some insulation problem on the ignition wires and the VERY
high voltage from the ignition circuit is sparking or jumping over to
the transformer primary circuit, this could cause the symptoms you are
Make sure the wires that are part of the ignition circuit are not even
close to the anything else. and inspect them (with the power off) for
any sigh of cracks or other faults. These have very high voltage
like in a car spark plug and can jump several inches if there is an
insulation problem. And the problem would be intermittent. The
transformer primary voltage would look perfect except when the high
voltage spark jumps over to it and this could easily damage the
insulation on the transformer which is not designed for very high
If you are an electrical novice, you may want to think about calling
in for some help at this point.
http://www.trane.com/webcache/un/furnaces%20%28furn%29/product/22-1666-07_04012009.pdf ** From the damage to the tranny visible in the photos - the primary has
developed an internal short ( due to insulation failure) and since there is
no fuse link in series, the resulting current was high enough to make an
exposed wire leading to the terminals explode.
An internal short could develop due to heat alone because of an overload on
the tranny - but this requires a fault to exist on the secondary side which
seems not to be the case.
High voltage spikes on the primary could also cause insulation failure
leading to the damage seen in the pics - lightning does this sort of thing.
So also could back emfs from the blower fan if the is a bad connection in
the AC supply feed.
I suggest you provide the next replacement for that vulnerable tranny with
some "protection" - firstly an in-line fuse of say 1/4 amp AND a
capacitor wired across the primary of say 1uF rated for continuous use
across the AC supply.
If there is an overload on the tranny, the fuse will blow.
The 1uF capacitor should suppress spike voltages enough to save the tranny
1 uF sounds a little high.
I would think .01 uF at 400V would be a better bet. Or an MOV (metal
oxide varistor) or three (across the line and then from each side to
I don't think this is a voltage spike problem though. Voltage spikes
take out other components first.
I thought you had simply misspoke and that this was an honest error.
The reactance of a 1uF cap at 60Hz is: 2652 ohms (1/(2*pi*F*C)
V^2/R = W
Assuming primary, as why would you put it on the secondary:
120^2 / 2652 = 5.43 W
Does that not seem wrong to you?
If not then go buy a 200V non polarized 1uF cap. It is no easy chore.
** Film caps rated at 600VDC will generally last a fair while wired across a
120 volt AC supply ( but not with a 240VAC supply ) - but is it far better
to use a purpose designed and agency approved "class X1" or "classX2"
Such a cap dissipates no energy at all.
What planet do you come from ??
** 1uF caps for use across the AC supply are cheap and plentiful.
Typical examples are metallised polypropylene " class X2 " types and sell
for $1 or $2 each.
What planet do you come from ??
** What sort of know nothing JERK thinks that capacitors dissipate energy
Then proceeds to calculate the reactive impedance and treat it the same as
Zero out of ten.
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