What is fuse gonna do? Instead of blowing x-former,, blow fuses? Then still it is not right. Let;s go back from begining. How old is the system? When this blowing tranny started? From day 1 or some time(month, years after the system is installed? If tranny is hot to touch when in use, that is rad flag.
The block diagram does not show it, but a picture of a replacement board I found on the net seems to show 4 discrete diodes on the board
- which is what I have found on most control boards I have actually had my hands on. Being a 2 wire transformer secondary, the center tapped full wave scenario is a non-starter, leaving either a 4 wire full wave bridge rectifier or 4 discrete diodes as the only real options.
** The fuse will actually help you diagnose the problem.
If the fuse opens soon as AC power is applied - the tranny is being overloaded.
If the fuse opens after some time because insulation in the primary side has failed, replacement fuses will open immediately despite the secondary being disconnected.
This isunthinkab;e crazy idea but is the x-former being put in backward? Rgwew i a such thing as current limiting x-formers. One thig I'd try then I'd put in proper Wattage low value resister to lower the primary voltage.
Hi, 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.
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.
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? Crossovers?
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.
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 primary. In my experience, I haven't seen a transformer fail from what could likely be attibuted to a surge. For example, after a lightning storm, I've had electronic gear that was dead and likely attributed to the storm. But all the transformers in the house, from the doorbell one to the plug-in wall wart type, to even the transformer in the failed electronics were all fine. 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 insulation 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.
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 theory.
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 common problem.
No doubt.
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.
I've never seen it. In fact in my past life as a tech I rarely rarely replaced line driven transformers. Rewound quite a few switching transformers though.
FYI, for those following this thread (and perhaps thinking I've abandoned it): The only thing I have to report is that I was expecting the replacement transformer to arrive on 04/15 (Friday), but it never did. The last status on the UPS website (at 7:35am on 04/15) was "Mechanical failure occurred.", then
30 minutes later "Arrival Scan". From the UPS website:
"Mechanical Failure: This scan indicates that an exception has occurred due to a mechanical issue, which may cause a delay in delivery. The mechanical failure may have occurred within a UPS facility as a result of equipment breakdown, or while your shipment was transported by a tractor-trailer, airplane, train, or other vehicle."
Perhaps an air traffic controller fell asleep on the job.
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