I completely agree. 400V was the minimum and most popular choice for
transient protection, of course a cap that size for "back emf"
protection has got it's work cut out for it.
I'm unfamiliar with those large caps you sent the link (and I'm not
going to search these threads for it) to before. What are they used for?
I was a little surprised at this back emf theory and Phil's fix. I can't
see where a low Q resonant circuit is going to be helpful, if anything
depending where it resonates at, and how it is turned off (say at the
breaker) it could add some dandy new problems. YMMV.
I'll leave you with this:
I learned long ago, never to wrestle with a pig. You get dirty, and
besides, the pig likes it.
George Bernard Shaw
I would think that if it were a surge that was initiating the
failures, it would have
to be a large one to breakdown the insulation on the transformer
In my experience, I haven't seen a transformer fail from what could
be attibuted to a surge. For example, after a lightning storm, I've
electronic gear that was dead and likely attributed to the storm. But
the transformers in the house, from the doorbell one to the plug-in
wart type, to even the transformer in the failed electronics were all
I've never had a transformer fail where I've had suspicion it could be
from a surge. IF it is a surge, it would almost surely have to be
coming from either the blower motor or the compressor.
An interesting experiment might be to rig up a small air gap across
the AC coming into the transformer. The size could be determined
so that it would take say 1000V to bridge it. I'm sure there must
be a chart or calculator online that would give you the size.
If he rigged that up
together with a fuse he could cycle the HVAC and see if any
spark results. Also, if the surge is the failure mechanism, then
the suggestions of adding fuses may not protect the transformer,
at least not for long. The surges will still arc across the
and eventually break it down so that even normal voltage will
start to short across.
As an alternate surge detector, he could use a MOV surge
protector connected across the transformer with a low
value, fast-blow fuse. If a big surge is there, good chance the
fuse will blow. Could get those parts at Radio Shack for a
few bucks. With either of those methods, if there is a
surge, then he could replace the MOV fuse and then proceed
to try to isolate where its coming from. IE, just cycle the
blower via the fan switch to check that. Then leave the fan
on constantly and cycle the AC compressor, etc.
On another note, the quality of these replacement parts in some
cases is very poor. I recently had experience with a Sears
dehumdifier that was about 4 years old where the blowr motor
died. So, I bought a new
motor from Sears. That lasted less than a year. Bought a
second one and that one failed in a couple minutes. And
I had verified that everything was correct. Fairly simple, it
was just two speeds, AC, with relays choosing which
winding to apply 120V to.
I then went online and looked at the feedback section at
Sears for NEW ones that were identical. Overwhelming
feedback that they were crap, with this fan motor failure
being the main culprit. People posting the same
experiences, with replacement motos failing in short
order. And mind you, this is 4 years
after the one I had. Obviously serious quality issues and
no one is paying any attention.
Another factor is that in the drive to save energy, meet
govt guidelines, etc, manufacturers today, in many
cases, are using lower power devices that are closer
to the margin of failure than they were 20 years ago.
On 4/16/2011 8:18 AM, email@example.com wrote:
I agree completely. Transformers are not fragile. Usually the
transformers I've seen fail were caused by long term thermal issues.
Undersized and overworked.
IF it is a surge, it would almost surely have to be
In which case it is a wiring issue because the line should absorb this.
That 1mF cap is only 2,000 ohms at 60Hz. I still can't buy into Phils
That has been suggested, or something very similar.
Yeah, but this transformer number 3, and near as we can tell they were
all different. The OP needs to take a knife and see what damage if any
lies in the wiring.
Always a good plan to see if you can track whether the problem is
common. I did a quick search for him and came up empty. Apparently not a
At least transformer based wall warts are going away. That is not bad.
I don't think this is a surge problem, it will probably be a wiring
problem, but it could also be something in the controller board. That
would surface by going through all cycles.
Oh my gawd, someone agrees with me. Looking at the picture says a lot.
Burnt on the primary side, looks like new on the secondary. And this
is the third transformer with an open primary!
I don't know about a the 1uF cap, seems way too high so I'll delete what
was below and pretend I didn't see it.
A capacitor across an AC supply??????????????? As a surge
Have not heard of that before.
A capacitor across the AC line would appear as a load - and could
form a resonant l/ci tank circuit, which would also appear as a low
resistance -causing high current to flow
There are 2 other POSSIBLE issues here though - - -.
Both are perhaps long shots - but mabee worth investigating.
The transformer primary APPEARS to be saturating.
Primary current on an unloaded transformer CAN, in some cases, excede
full load current. Possibly the transformer requires MORE load on the
secondary than it is getting.
An example is a microwave oven transformer. With no load on either the
high voltage or low voltage secondary, the primary will generally
saturate and overheat. If you remove the high voltage secondary and
add your own windings to make a "custom" transformer, it is not
uncommon for the primary to saturate at no/low loads - overheating the
Like I said - a long shot, but possibly worth investigating. adding a
small 24 volt pilot light across the secondary MAY solve that kind of
The other POSSIBILITY is a DC bias on the primary, which WILL cause
saturation on 1/2 cycle of the AC. Need a scope to check that
effectively - or something like a 10uf nonpolarized capacitor and a
100K ohm 1/2 watt resistor in series across the primary, with a DC
voltmeter connected across the cap. Make all connections BEFORE
turning on the mains power. You should expect to see readings of +/-
approx 25-35mv across the cap in a normal residential situation.
Lets say you read 275mv DC on the line, and the transformer primary
resistance is 2 ohms.. That will put a DC current of 137.5ma through
the primary - which when added to the normal AC current on the one
half cycle will greatly excede the saturation current of the primary.
A half wave rectified load on the same circuit could put a DC
component across the line.
An AC (nonpolarized) capacitor IN SERIES with the primary would
remove the DC component from the primary winding, but finding a
capacitor that would ballast the primary properly (allow full rated
primary current) while not causing a series resonance (which would
appear as a short circuit across the mains) is not something I would
try to calculate.
** Then your ignorance is showing.
The stated reason for the capacitor was in relation to the "blower fan"
inside the same unit as the small tranny.
The event the cap has to deal with is a back emf surge generated by that
fan when the AC supply is suddenly disconnected - for whatever reason.
** Draws 45mA continuously.
(snip absurd drivel)
** The primary appears to be EXPLODING !!
You ridiculous wanker.
Phil - your mamma should wash your mouth out with soap.
I SAID the other two scenarios were long shots - but so is everything
else that has been suggested. The windings of the trasnformer do not
APPEAR to be overheated - looks like just blackened at the connections
between the winding and the connecting wires.
It is definitely a strange failure - and I don't think it has been
properly analyzed to determine exactly what/where the problem is.
As Arthur Conan Doyle said, "after you have eliminated all the
possibilities, whatever remains, however improbable, must be the
** See the vaporised metal coating deposited on the plastic cover next to
That is a damn EXPLOSION !!
It happened very suddenly and made a loud bang too.
" 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."
If the insulation on the enamel wire of the primary is punctured by a HIGH
VOLTAGE SPIKE, effectively shorting out most of the primary - then the
120 AC supply ( no fuse exists remember ) will easily turn the two exposed
wire ends into metal vapour !!!
Cos they just became the fuses.
It just so happens that many small transformers made in China, India & Sri
Lanka etc are very prone to this sort of failure - due to bad manufacturing
So you are agreeing with what I said before - most likely problem is
cheap crap component.
We can be relatively sure it wasn't lightning 3 times.
If the back emf from the motor is causing the problem, the bad
connection to the motor should have made itself VERY evident by now.
Not saying it is NOT part of the problem - like you, I wasn't there
watching it fail - and neither you nor I heard the noise you speak of.
And IF the problem is what you say it is (and I'm not saying it is
not), then probing the old primary and getting to the winding beyond
where the solder joint "exploded" you should be able to measure a
significantly lower than normal primary resistance.
That measurement has not, as far as I know, been made and reported.
If it was mine, or if I had the transformer at hand, I'd have it apart
and analysed in no time. If the primary is shorted, I'd know, within
an hour or two of the failure..
And with that second primary, it would not be hard to determine if the
110 volt primary is shorted without even dissassembling the
At this point no-one has actually posted FOR SURE what the primary
configuration is. Is it a mult-tapped primary, set for 115, 208, and
230 volts, or is it 2 independent primaries, or is it 2 primaries that
need to be connected either in series or parallel depending on the
voltage (115 or 230)
I don't know this, and you don't know either unless you are
clairvoyant, because it has not, to this point, been established and
So we are all guilty of the same thing - making ASSumptions.
This one is a grasp at a straw. Was a magnet stuck on the transformer
at the time that it blew?
I realize that not too many people run transformers with magnets stuck
to them. But doing that does make their cores more prone to saturation.
- Don Klipstein ( firstname.lastname@example.org)
In addition to all the other observations and suggestions found in this
thread, there is one additional possibility to consider: Your Trane machine
simply hates you.
I've seen malevolent machines. I've been victimized by our metal overlords.
Tell the truth, have you never been tempted to shotgun a lawnmower?
<SNIP to malevolance of machines, not just furnaces>
Linked from http://www.watchitshred.com
That place even shows shredding of some engine blocks. They make
shredding of bowling balls look easy!
Is a computer or some other electronic device ticking you off?
They even shred shredders:
- Don Klipstein ( email@example.com)
Having an extremely warped mind, I thought some Hollywood special
effects guys should get together with a shredder company and make
a very special YouTube video of an Islamic terrorist being tied up
in a big burlap sack with a hog. The whole squirming mass would be
dropped into a monster shredder and all the sound effects, blood
and guts would be awesome. At the end, a disclaimer would read....
"No actual pigs were harmed during the production of this video"
Of course the reaction would be the same as the nut-jobs who were
upset about their Koran being burned and went around killing everyone
they could find who they believed to be a Christian.
Your idea tracks with one described by an acquaintance of mine.
He envisioned a web site, "stopthedesecration.com."
On the web site, a (masked) host is seen standing in a warehouse chock full
of Wahabi-sanctioned Korans. The site promises that for every outrage
committed in the name of Islam, some book (or a portion) will be desecrated!
There is also a "catalog" of outrages and consequences (if I remember
* Every anti-American or anti-western sign shown on TV = One page ripped out
* For every non-Muslim injured in the name of Islam = One entire Koran
* For every non-Muslim killed in the name of Islam = One entire Koran
shredded, mixed with pig blood and feces and flushed down a toilet.
Appropriate videos will be cataloged, each showing the offense AND the
sanction in graphic detail. A continuous scroll appears on the screen: "Only
YOU can stop the desecration! Do not permit your friends and family to harm
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