Slightly off topic. One of the pieces of gear that came in a few months ago was an eight channel noise gate.
It had no AC mains fuse! Instead it used an “impedance protected” power transformer. One of the diodes in the bridge rectifier had failed by shorting.
The power transformer smelled quite burnt and no longer worked. Opening it up to see if it had a thermal fuse, showed it didn’t. Peeling back the insulating paper showed a clear burn though of the primary coil wire.
Suffice it to say the unit was tagged unrepairable and discarded. It did not go for recycling on the chance it might be saved and repaired.
Every few years I do see transformer failures. But never before a melted primary or a lack of any fuse.
I do see wall wart power supplies that are “impedance protected” also without a fuse. Pretty much everyone discards these upon failure.
I did peruse the internet to see if college level transformer design is offered. Turns out around here a few places offer them as an option after the basic courses.
When I got my degree they did have a bit in the main curriculum, but dropped that a few years later and it became an unpopular option. (Owing to the lack at that time of a textbook still in print.)
It had no AC mains fuse! Instead it used an “impedance protected” power transformer. One of the diodes in the bridge rectifier had failed by shorting.
The power transformer smelled quite burnt and no longer worked. Opening it up to see if it had a thermal fuse, showed it didn’t. Peeling back the insulating paper showed a clear burn though of the primary coil wire.
Suffice it to say the unit was tagged unrepairable and discarded. It did not go for recycling on the chance it might be saved and repaired.
Every few years I do see transformer failures. But never before a melted primary or a lack of any fuse.
I do see wall wart power supplies that are “impedance protected” also without a fuse. Pretty much everyone discards these upon failure.
I did peruse the internet to see if college level transformer design is offered. Turns out around here a few places offer them as an option after the basic courses.
When I got my degree they did have a bit in the main curriculum, but dropped that a few years later and it became an unpopular option. (Owing to the lack at that time of a textbook still in print.)
That would suggest the design is to intentionally open the primary in case of a secondary fault. I doubt it met UL or CSA standards. The expression " cheap Chinese crap" comes to mind. I think I would avoid that brand of equipment.
We are on the third page here, and still the OP has not done the analysis to know if the specified transformer was correctly specified or not.
Analytical approach 🙂
Post #15 says:
The spec I have does account for some of these factors. The current load for the circuit is 1.2A@12.6v, and the spec states that the power transformer must be able to provide 1.8 times that current draw, or 2.16A (I incorrectly stated 2.08).
That is 27 VA so a 25 or 30 VA will be the right size according the given specs. There is already a factor 1.8 calculated and this with a purely resistive 15W load, no rectification and no filter cap. OP will not sleep bad when using a 25VA transformer.
Post #15 says:
The spec I have does account for some of these factors. The current load for the circuit is 1.2A@12.6v, and the spec states that the power transformer must be able to provide 1.8 times that current draw, or 2.16A (I incorrectly stated 2.08).
That is 27 VA so a 25 or 30 VA will be the right size according the given specs. There is already a factor 1.8 calculated and this with a purely resistive 15W load, no rectification and no filter cap. OP will not sleep bad when using a 25VA transformer.
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Actually made in California. Not a cheap line, but one aimed at recording studios. Not a brand I use. Current version uses a wall wart power supply.
lots of variables to work on, lots of transformer core samples that are each requiring different approaches and considerations, there is no one size fits all solutions, but there are a lot of options out there to choose from ...and so from one universal transformer formula, there are thousand ways to apply the formula since the variables of voltage, frequency, flux density, core area and turns leads to thousands of permutations, you just choose what is important to you....myself leans of overbuilds since i make my own traffos and profits were never a concern, i never sold transformers as an item, but i sell my built up amps...
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i am trying to retrieve that article written by Norman Crowhurst wherein he said that it is okey to to draw an ampere more from a filament winding in a power transformer rated for say 3A, i am quite sure there is a context wherein this was mentioned...
i read this article maybe 30 years ago....and since the filament winding mentioned was a a part of a bigger power transformer that had several other windings and bigger cores, so this is the context....so say that this leads to burnout is missing it imo...
i read this article maybe 30 years ago....and since the filament winding mentioned was a a part of a bigger power transformer that had several other windings and bigger cores, so this is the context....so say that this leads to burnout is missing it imo...
Somewhat off topic, I have a fancy five-bulb chandelier that I had consistent trouble with those two-pin Halogen bulbs, the ones that run pistol hot...they were fifty watt, but they were burning up the leads going to the small ceramic plugs.
After burning up almost all the ceramics, I yanked it off the ceiling for a good "go thru"...perusing the stores I found a seven watt LED bulb about the size of a fingernail with the same pins to plug into the ceramic socket. The fancy brass "center" of the thing housed a monster six-inch (300VA?) 12V toroid ..so now, new ceramic sockets all & the transformer loafing along at supplying only 35 watts worth instead of 250, should last "forever".
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Rick...
After burning up almost all the ceramics, I yanked it off the ceiling for a good "go thru"...perusing the stores I found a seven watt LED bulb about the size of a fingernail with the same pins to plug into the ceramic socket. The fancy brass "center" of the thing housed a monster six-inch (300VA?) 12V toroid ..so now, new ceramic sockets all & the transformer loafing along at supplying only 35 watts worth instead of 250, should last "forever".
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Rick...
We made transformers, you would not be surprised with burned primary, funny thing is most failures are mechanical, the core not properly insulated and running high secondary current (battery charger) the 50/60 Hz can be damaging since mechanical vibration shave through the isolation, and it shorts permanently and obvious now is a different animal altogether. Core may heat beyond the limits and the thing starts smelling, varnish runs out (if vacuum impregnated) some older stuff used wax, even worse. The Primary winding shorts on a sharp edge and the whole chassis can become live. I am not absolutely sure about 110VAC, but 240VAC will most certainly kill you!
There is sometimes no isolation between core and wire, a single winding of tape does not constitute safety of any degree, fuses and cutouts may be ineffective sometimes far too slow reacting. Core buzzing is a sure sign of early failure, vacuum impregnation is not to isolate windings but stop them vibrating, the transformer now becomes a speaker. Nothing but wall warts offers poor only isolation against electrical failure, most direct coupled stuff should be double isolated, i.e. box within a box.
Incorrectly specified plastic will ignite when a flashpoint is reached, and the problem now is your house burns down. Insurance does not cover it by saying it was wilful damage, since there should not be any flammable item nearby. Wall warts are the by the first culprits in causing house fires in S.A
Should a fuse blow don't think of replacing it is smart, it went for a reason, find this reason before even looking any further. Some wall warts have no earthing connection but may be to the PCB ground plane and earth could be wired to the neutral as easily as the live since there is no index.
These are no safety warnings, it is common sense, as I always say - nothing is as it seems especially things from China, corner cutting and material savings can occur within any manufacturer as well as in the production process. Don't trust a sticker saying QC by Jim, look for the international markings - are they trustworthy - not really.