So, what I'm taking away from this:
1. You should always include a margin above what the circuit is "supposed" to draw.
2. In the case I presented, the designer has already done this, and thus the 2.16A figure represents the proper transformer value.
1. yes
2. yes
3. P = U x I so 12 x 2.16 = 26VA. Since it was already overdimensioned a 25VA would work out OK and a 30VA would simply be a good choice.
2. yes
3. P = U x I so 12 x 2.16 = 26VA. Since it was already overdimensioned a 25VA would work out OK and a 30VA would simply be a good choice.
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transformers are designed around wire size of 500 circular mills per ampere, that is how they get that 2.08A, but even so, nothing stops you from drawing 3 amperes and things still look good....
what happens is with more currents drawn current density is higher than 500 CM/A, losses a tad higher, the reverse is also true is just 1A is drawn.
my take, compute your total load and make your choice on that basis...
So the higher the transformer capacity is relative to the load the less loss there will be? So you could chose to overspec a transformer by a large amount in order to achieve a higher level of efficiency?
of course you can, i tend to over spec my designs...it can be done, an over spec design leads to a cooler running traffo...
jean-paul,
My observation of these correspondence types of web sites allow the free expression of opinions. Unfortunately some of the opinions have little basis in reality.
Transformer design and specification has been around so long that it is not covered in state of the art education and literature.
The last time I had a magnetics based problem, I had a nice chat with a well informed fellow at the item’s manufacturer. He had a 2 year technicians degree and an amazing amount of on the job education.
One magnetics bit I have been amused about is that in the U.S. most small power transformers are designed to run into a bit of what I call soft saturation. (A bit of a rounded B-H loop for the core.) This improves the apparent voltage regulation, although it does lower the efficiency. The added heat from the loss is small enough most folks don’t notice or concern themselves with it. Of course it does provide a small cost savings.
Amusingly enough most of the U.S. power transformers in catalogs are clearly rated for either 110 or 120 volt primaries and few notice the AC mains voltage is actually higher these days, but the secondary voltage measured with peak reading RMS calibrated meters is as rated.
The usual load of a single or bridge rectifier does produce the expected voltage, as the actual RMS voltage has little influence on the final DC voltage. Typical AC mains voltage at the consumer’s outlet can easily have 5% distortion.
I think the unusual opinions rise from folks thinking real small power transformers model theoretical transformers as well as most passive components. In reality they are made to mimic theoretical parts at the expected operating point.
Thus the problem with understanding load ratings for complex loads. To make matters worse, there are some serious common misunderstandings, one of my most noticed is the term “RMS power!”
My conclusion is that you have offered correct and valuable advise as requested and some others’ opinions are and will result in much smoke!
One U.S. manufacturer (Signal) does offer a nice paper on applying transformer ratings to reality.
My observation of these correspondence types of web sites allow the free expression of opinions. Unfortunately some of the opinions have little basis in reality.
Transformer design and specification has been around so long that it is not covered in state of the art education and literature.
The last time I had a magnetics based problem, I had a nice chat with a well informed fellow at the item’s manufacturer. He had a 2 year technicians degree and an amazing amount of on the job education.
One magnetics bit I have been amused about is that in the U.S. most small power transformers are designed to run into a bit of what I call soft saturation. (A bit of a rounded B-H loop for the core.) This improves the apparent voltage regulation, although it does lower the efficiency. The added heat from the loss is small enough most folks don’t notice or concern themselves with it. Of course it does provide a small cost savings.
Amusingly enough most of the U.S. power transformers in catalogs are clearly rated for either 110 or 120 volt primaries and few notice the AC mains voltage is actually higher these days, but the secondary voltage measured with peak reading RMS calibrated meters is as rated.
The usual load of a single or bridge rectifier does produce the expected voltage, as the actual RMS voltage has little influence on the final DC voltage. Typical AC mains voltage at the consumer’s outlet can easily have 5% distortion.
I think the unusual opinions rise from folks thinking real small power transformers model theoretical transformers as well as most passive components. In reality they are made to mimic theoretical parts at the expected operating point.
Thus the problem with understanding load ratings for complex loads. To make matters worse, there are some serious common misunderstandings, one of my most noticed is the term “RMS power!”
My conclusion is that you have offered correct and valuable advise as requested and some others’ opinions are and will result in much smoke!
One U.S. manufacturer (Signal) does offer a nice paper on applying transformer ratings to reality.
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i have been designing and building power transformers used in my builds for almost 40 years now, i have not had a single traffo burn out on me... https://www.facebook.com/media/set/?set=a.4107400259281577&type=3
small transformers have lower regulation due to the fact that you are severely limited in the size of wire you can put into the winding bobbin due to large number of turns required..
core temperature rise depended on the flux density your core is asked to operate on, higher flux higher temperature rise....
small transformers have lower regulation due to the fact that you are severely limited in the size of wire you can put into the winding bobbin due to large number of turns required..
core temperature rise depended on the flux density your core is asked to operate on, higher flux higher temperature rise....
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how did you intent to use the transformer any way? as a rectifier psu or simply heater supplies?
RDH4, chapter 5, page 237 contains all information about transformers, free to download...https://ax84.com/archive/ax84.com/static/rdh4/chapte05.pdf
RDH4, chapter 5, page 237 contains all information about transformers, free to download...https://ax84.com/archive/ax84.com/static/rdh4/chapte05.pdf
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The one I mentioned will be for a heater supply. There's a second 230v 90mA transformer for the rectifier, also specified as 1.8x the required current draw.
a separate traffo for heaters and B+ is an excellent idea imho, been there done that too...
Hammond and Sowter has a page dedicated to power supplies...
https://www.hammfg.com/electronics/transformers/rectifier
https://www.sowter.co.uk/rectifier-transformer-calculation.php
Indeed, but since window area is a prized and scarce commodity ,increasing wire diameter forces designer to choose next larger lamination size (talking classic EI transformers here) so problem is sort of auto-solved.
Not so sure about that, and depending on career.
I studied Engineering starting in 1969 and it was pretty deeply covered and studied, of course Engineers were expected to be able to fully design stuff from the ground up.
Often not made "in house" but designer provided custom supplier with full specs and blueprints which then had to be followed to a T. Period, no ifs or buts.
Then came a semi-lazy era where amp designer ordered a transformer by general "black box" specs: winding voltage and currents, total VA, desired size, but let OEM supplier deal with the actual build details.
Still acceptable.
But nowadays "Electronics turned into Computing" if you catch my drift, and some Engineers became "Sales Engineers", hopefully can spec the final product but buy it from Chinese suppliers who designed it all by themselves, just check it roughly meets specs before applying the "Label of the Day" to its front panel ... or something that amounts to that.
One sad result is that when somebody asks for schematics or service manuals, "Factory" or to be more precise "Brand owners" don´t supply them.
Are they hiding Service data? ..... no! THEY HAVE NO CLUE!!!!!
Best they can do is offer full board replacement (IF available), full unit replacement, or plain abandon you.
That said, Electrical or Electromechanical Engineers DO study Magnetics very well, it´s at the core of their Job!!!
If anything and leaving Audio aside, their everyday fodder is Power and Distribution Transformers, Electric motors, etc.
Read TVGeek's answer, he is absolutely correct. Unless there are facts on the table, no engineer, DIYer, Audiophile will guess or p[redict an outcome objectively. That is the problem here, not a lack of knowledge, but a lack of facts. The engineer may be Nelson Pass or Jack the Ripper. The engineer may have designed to a specification not by guesswork, you don't know if it is a commercial design or some audiophile hocus pocus.
Yes that is an extra reason not to use parts that just meet specs as that certainly does not work with transformers as I tried to make clear. Choosing a part that is correctly rated (again, it is not overspeccing) has a higher chance of success, certainly when the mentioned facts are lacking but do occur in real life. There we had the factors for.
As this is a hobby site one maybe should keep it simple as simple things tend to be memorized. "Always oversize the transformer" although being in fact hampered/incorrect/partly true it will still lead to a higher chance of correct operation. Then the ridiculous real oversizing by using a 100VA transformer on a 6VA load....Still no harm done versus the opposite 🙂
All beautiful theory by people that already know their stuff aside... please see how fellow hobbyists select transformers. Then realize "Always oversize the transformer" is the least problematic. If I would get a penny from every person that uses what is in stock or just something that vaguely resembles to work out OK I would be millionaire.
As this is a hobby site one maybe should keep it simple as simple things tend to be memorized. "Always oversize the transformer" although being in fact hampered/incorrect/partly true it will still lead to a higher chance of correct operation. Then the ridiculous real oversizing by using a 100VA transformer on a 6VA load....Still no harm done versus the opposite 🙂
All beautiful theory by people that already know their stuff aside... please see how fellow hobbyists select transformers. Then realize "Always oversize the transformer" is the least problematic. If I would get a penny from every person that uses what is in stock or just something that vaguely resembles to work out OK I would be millionaire.
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In general, whichever current of the transformer is smaller will be defined as the rated current if comparing the saturation current and temperature rise current of the inductor. The size of the current changed depending on the time that current is applied.