Let’s say spec calls for 2.08A. If I get a transformer they produces 2.08A even, is that fine, or should I leave some buffer? Get slightly bigger than what I need?
No that is not OK. You should overdimension a transformer certainly when a rectifier plus filter cap and load are connected. In general maximum load is then 0.8 times the rated current. That 2.08A are only possible to resistive loads without rectification/buffercap. In your case the transformer will be derated to 1.66A.
So you'll need a larger transformer anyway and then it is even better to add 30% capacity to that. The more or less standard 3.3A version (I take you use toroids) will be a good choice. If you use very large filter caps the 5A version will be a safe bet.
So you'll need a larger transformer anyway and then it is even better to add 30% capacity to that. The more or less standard 3.3A version (I take you use toroids) will be a good choice. If you use very large filter caps the 5A version will be a safe bet.
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Correct answer: It depends.
You also need to look at the source of the spec. Was it already de-rated? In other words, is the actual design load only an amp? Then the 2.08A is appropriate. As 2.08 seems to be an industrial control common size, maybe the designer correctly specified it. Maybe not.
It can get more complicated depending on average vs peak load and duty cycle. If building one, go big. If building 10,000, then thermal and current logging testing in situ worst case is in order as every penny saved counts.
You also need to look at the source of the spec. Was it already de-rated? In other words, is the actual design load only an amp? Then the 2.08A is appropriate. As 2.08 seems to be an industrial control common size, maybe the designer correctly specified it. Maybe not.
It can get more complicated depending on average vs peak load and duty cycle. If building one, go big. If building 10,000, then thermal and current logging testing in situ worst case is in order as every penny saved counts.
Yep, depends on the duty cycle. Transformers typically have quite a long thermal time constant (i.e. they take a long time to heat up or cool down), so the short term current capacity is typically much larger than the long term current capacity. Of course in any particular circuit the poor voltage regulation due to an over specification current draw may have other contraindications.
Don't forget, a little DC on the primary, like from dirty mains, combined with operating the transformer close to spec can quickly saturate the core.
As explained... the transformer is already underrated when used with rectifier and filter cap as in most cases. OP states the load specification to be 2.08A and the transformer to be able to deliver 2.08A. This is only possible when using a resistive load directly on AC. It is also bad designing as there is no margin for deviation in the load, variations in mains voltage and possible DC component etc. which do occur in real life. Even when using a purely resistive load adding 20% tot the power rating is normal. If things all are the usual so audio use/general use.
"Every penny saved counts" but not in correct designing where a few pennies more may be the difference in reputation of the brand. So the answer to OP's question and the thread title "Is it fine to use transformers that just meet spec?" is for both:"no it is not fine". It is both bean counting and dumb and hoping for the best which generally does not work out too well for reliability and performance. Would also be the rare case where in DIY audio "good enough" is chosen where "one could get away with it" to use the familiar terms🙂
"Every penny saved counts" but not in correct designing where a few pennies more may be the difference in reputation of the brand. So the answer to OP's question and the thread title "Is it fine to use transformers that just meet spec?" is for both:"no it is not fine". It is both bean counting and dumb and hoping for the best which generally does not work out too well for reliability and performance. Would also be the rare case where in DIY audio "good enough" is chosen where "one could get away with it" to use the familiar terms🙂
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I guess it depends why 2.08 amps is the “spec”. Is it 2.08 amps is the load current, 2.08 amps RMS secondary current drawn (taking PF into account), 2.08 amps will provide some margin to what it really needs, or is it 8.32 amps at a 25% duty cycle. Each of those “requirements” will lead to a different answer.
We can make it as complex as possible ... the answer still is that using parts that just meet specs is not a fine choice.
In DIY, we can afford to be very constative. When in business, we have to hit a price point. Every penny is important. Almost no one buys on solid engineering and reliability, they buy on price, advertising, or ego. Sad as it is, that's the way it is. A good engineer can hit that fine line. A poor engineer will either kill them for reliability or uncompetitive price. Either way the business fails. Marketing may tell you you have to use a blue LED not a red one, and the 5 cents has to come from somewhere. The engineer may do their job well, but marketing or production engineering may change it. Just reality. Not how it would be in a prefect world, but I for one do not livre in "Perfect".
The OP still has no actual requirement other than a spec passed to him from elsewhere. So for his project, the answer is unknown.
Three choices:
Trust the original designer specified the correct part and blindly follow.
Do the due diligence of the circuit analysis.
Apply a sledgehammer blindly and use a bigger one.
For laymen/DIY enthusiasts maybe it is a wise tip not to underdimension parts as trying to save pennies on stuff already costing way more than Far East produced ready built stuff is not the way to build good stuff. In this case likely a rectifier and filter caps will be used making the transformer wrongly chosen straight away. This has nothing to do with trusting a designer but with using the brains. Assuming a center tapped transformer plus full wave rectifier and 2 filter caps I think it is even a factor 1.41 (2.08 x 1.41 = 3A) that the transformer should be overdimensioned for correct operation (one will see that most brands dimension the various versions like that). Please note this is not true overdimensioning but textbook calculation. Anyway enough for now.
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In part it depends on final use/market.
PRO use? : Overspec at least 50% as suggested above, no doubt about that.
Home use?
Amp will probably be used at less that 25% of its capabilities 99% of the time, so .......
Same applies to heatsinking.
PRO use? : Overspec at least 50% as suggested above, no doubt about that.
Home use?
Amp will probably be used at less that 25% of its capabilities 99% of the time, so .......
Same applies to heatsinking.
It may be a flea power class A amplifier.....
"Those that want answers rarely deliver enough data" -> true again since we haven't heard of voltages.
"Those that want answers rarely deliver enough data" -> true again since we haven't heard of voltages.
Hi guys! Glad this sparked a lively discussion. I can provide some clarifying details.
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).
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 could have been included in the first post. You'll need a 12V 25VA transformer as minimum standard but a 30 or 35VA version won't hurt. With many a toroid manufacturer 30VA is a standard value.
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Sorry, I didn't realize there would be this much confusion. When I wrote the post I didn't have the booklet handy, then I went to bed.
Yes I also always tell what is wrong with my car after it has had service 🙂
Did you get the desired answers?
Did you get the desired answers?
I thought "spec" would indicate the value that was needed from the transformer after circuit calculations had been included. My manual here says "... so the heater power transformer must be able to sustain an AC 2.16A current draw". Which is where my post began.
Did I read "heater" in post #19? That is a resistive and easy load apparently without rectification/filtering and even then that designer counts 1.8 times the actual current draw. Bravo! Heaters are almost ideal loads except for the sometimes low cold resistance. Probably the designer has seen a quite low cold resistance of the heater(s) and advises to use a higher rated transformer.
BTW you answered the thread title question yourself and your answer is "NO" 😉
BTW you answered the thread title question yourself and your answer is "NO" 😉
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