First note that the transformer is specified as a 115/230Vac
Use that to convert the 120Vac fed transformer to find the open circuit output voltage. The 26.1*115/120 gives almost exactly 25Vac.
So we have a 115/230:25+25Vac transformer on open circuit voltage. If you needed you could calculate the turns ratio, but that does not help us here.
Now look at the 7.7Aac paralleled output. That is 3.85Aac/winding when the output voltage has dropped to 24.8Vac. If that were the maximum rated output current then we can calculate the transformer regulation.
regulation = open circuit secondary voltage / rated secondary voltage = 26.1/24.8 = 1.052 or 5.2%.
Now apply that regulation to the transformers rated voltages. 25/1.052 = 23.76Vac (loaded to 3.85Aac) The likely maximum current is around that 3.85Aac figure, but could be slightly more or slightly less.
Let's for this example assume that the rated output current is 3.85Aac.
The VA rating then equates to 3.85 * 23.76 * 2 = 183VA
We have a 115/230:23.76+23.76Vac 5.2% regulation 183VA transformer, but based on the assumption that 7.7Aac in paralleled configuration is the maximum rated current.
The arithmetic is not complcated, but you do have to know in waht order to apply the conversions and corrections. In other words a beginner is very likely to be confused and end up ordering the wrong transformer.
Remember I said turns ratio was not important.
Well, when it comes to predicting the absolute maximum voltage obtainable from the transformer, it is crucial to know that worst case voltage (ratio based on mains input voltage) to safely select the smoothing capacitors that the transformer feeds.
I repeat again,
Antek do not tell any lies. They specify how they measure the various voltages and currents.
Their transformers just like everyone else's transformers give out a variable VA depending on the mains voltage fed into the transformer.
Use that to convert the 120Vac fed transformer to find the open circuit output voltage. The 26.1*115/120 gives almost exactly 25Vac.
So we have a 115/230:25+25Vac transformer on open circuit voltage. If you needed you could calculate the turns ratio, but that does not help us here.
Now look at the 7.7Aac paralleled output. That is 3.85Aac/winding when the output voltage has dropped to 24.8Vac. If that were the maximum rated output current then we can calculate the transformer regulation.
regulation = open circuit secondary voltage / rated secondary voltage = 26.1/24.8 = 1.052 or 5.2%.
Now apply that regulation to the transformers rated voltages. 25/1.052 = 23.76Vac (loaded to 3.85Aac) The likely maximum current is around that 3.85Aac figure, but could be slightly more or slightly less.
Let's for this example assume that the rated output current is 3.85Aac.
The VA rating then equates to 3.85 * 23.76 * 2 = 183VA
We have a 115/230:23.76+23.76Vac 5.2% regulation 183VA transformer, but based on the assumption that 7.7Aac in paralleled configuration is the maximum rated current.
The arithmetic is not complcated, but you do have to know in waht order to apply the conversions and corrections. In other words a beginner is very likely to be confused and end up ordering the wrong transformer.
Remember I said turns ratio was not important.
Well, when it comes to predicting the absolute maximum voltage obtainable from the transformer, it is crucial to know that worst case voltage (ratio based on mains input voltage) to safely select the smoothing capacitors that the transformer feeds.
I repeat again,
Antek do not tell any lies. They specify how they measure the various voltages and currents.
Their transformers just like everyone else's transformers give out a variable VA depending on the mains voltage fed into the transformer.
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Comparing Antek's method to a couple of others:
Toroid Corp 732.182
rated 320VA, dual 18V secondaries, rated for 117VAC input, specs are given for 117VAC input
19.6V unloaded
18V @ 8.89A
18 x 8.89 x 2 = 320VA
Signal DP-241-8-20
rated 100VA, 20V center tap, rated for 115VAC input, specs are given for 115VAC input
??? unloaded
20V @ 5A
20 x 5 = 100VA
So it does seem that Antek is "unique" in their specs. I agree with the original poster that I should be able to expect a transformer will meet both nominal voltage and current ratings simultaneously, and at the same input voltage that the VA rating is specified.
Toroid Corp 732.182
rated 320VA, dual 18V secondaries, rated for 117VAC input, specs are given for 117VAC input
19.6V unloaded
18V @ 8.89A
18 x 8.89 x 2 = 320VA
Signal DP-241-8-20
rated 100VA, 20V center tap, rated for 115VAC input, specs are given for 115VAC input
??? unloaded
20V @ 5A
20 x 5 = 100VA
So it does seem that Antek is "unique" in their specs. I agree with the original poster that I should be able to expect a transformer will meet both nominal voltage and current ratings simultaneously, and at the same input voltage that the VA rating is specified.
Good with mine in a DIY phono preamp. I was looking for cheap and no noise. Got it. Antek is fine by me... and is about 30 miles from where I live so 1 day ship; it's all good.
Cheers,
Bob
Thank you for solving the mystery. The 200va transformer (secondaries in parallel) is really a 125va transformer (secondaries in series, like when used in a normal split rail supply).
Somehow I almost managed to buy the right transformer from them. I wanted a 50v 4 ampere transformer for split rail supply. So, I got the 25v 8a (secondaries measured in parallel) transformer. It is unexpectedly huge in size. Almost right? Yes, I'd wanted it for parallel TDA7294, got more voltage rather than less and had to change my project to parallel TDA7293.
I'd agree that for transformers of <1 kVA or so I've never seen any exceptions (until now!) to the nominal voltage being the one you get with full resistive load.
However, I did see transformers of the ~10kVA size being rated in such a way that nominal output voltage meant the one at 80% of full power resisitive load.
But for power distribution transformers and huge industrial sizes, as SoNic_real_one said, nominal voltage is the no-load voltage without exceptions. Short circuit impedance (which is dominated by leakage reactance) is specified so that voltage drop can be calculated. So actually, there isn't that much difference in the way the rating is specified because the resistive-full-load voltage will be very close to the no-load one.
One important difference is that for transformers less than a couple of kVA or so, resistance usually dominates the short-circuit impedance while for the huge ones it's leakage inductance. Also, in power grid applications, the resistance of wires and windings can usually be neglected in voltage and power flow calculations without appreciable errors. For power loss calculations on the other hand, they are of course important.
In any case, the load phase angle matters for the output voltage of a transformer. The output voltage will be different at nominal load current depending on if the load is inductive, resistive or capacitive.
For a typical toroid transformer of the size we are used to, the output voltage will be close to the no-load value for an inductive or capacitive full load.
The power distribution kind will, on the other hand, have close to no-load voltage for a resistive full load but for an inductive load it will be lower. For a capacitive load at rated apparent power the secondary voltage will be higher than the no-load value, how ever strange that may sound!
However, I did see transformers of the ~10kVA size being rated in such a way that nominal output voltage meant the one at 80% of full power resisitive load.
But for power distribution transformers and huge industrial sizes, as SoNic_real_one said, nominal voltage is the no-load voltage without exceptions. Short circuit impedance (which is dominated by leakage reactance) is specified so that voltage drop can be calculated. So actually, there isn't that much difference in the way the rating is specified because the resistive-full-load voltage will be very close to the no-load one.
One important difference is that for transformers less than a couple of kVA or so, resistance usually dominates the short-circuit impedance while for the huge ones it's leakage inductance. Also, in power grid applications, the resistance of wires and windings can usually be neglected in voltage and power flow calculations without appreciable errors. For power loss calculations on the other hand, they are of course important.
In any case, the load phase angle matters for the output voltage of a transformer. The output voltage will be different at nominal load current depending on if the load is inductive, resistive or capacitive.
For a typical toroid transformer of the size we are used to, the output voltage will be close to the no-load value for an inductive or capacitive full load.
The power distribution kind will, on the other hand, have close to no-load voltage for a resistive full load but for an inductive load it will be lower. For a capacitive load at rated apparent power the secondary voltage will be higher than the no-load value, how ever strange that may sound!
Antek transformers are likely not manufactured in the US. The country of origin is not stated anywhere, and emails asking this are not answered. A friend who sopke to John on the phone said that the owner was Cambodian, so the transformers may be of Asian origin. No their specs are rather unique.
I have used Antek toroids to power tube amps for several years now. They represent the BEST value in VA for your $$$ by a considerable margin. It also should be noted that the physical size of the transformer may NOT always match the published spec, and sometimes the data sheet is just flat WRONG. I have 3 AN4TK400's and they are all wound differently. All were early designs and I guess each batch was different until they got the recipe right.
I got an early AN2T230 (no longer made) and it was larger than spec such that it wouldn't fit the amp already designed. I ordered an AN1T230 which would be overloaded but fit. It has been running fine for 2 years and doesn't get as hot as a correctly sized Hammond transformer.
Their tube OPT's are however not so good. THe published specs show high frequency rolloff, and users on this forum report the rolloff is worse than spec.
The same owner also runs Par-Metal. Their cabinets are nice and below the price of most others.
No affiliation...blah...blah........
It is good quality materials, and unexpectedly large for its ratings. Personally, I shop by voltage drop, simply doubling the drop listed in the datasheet. I need to choose an unloaded voltage that doesn't drive my transistors all sketchy, so there's a high limit. Of course the low limit is clipping. For example, reduced audio quality if TDA7294 is over-volted or even run sketchy. The TDA7294 runs best under-volted but with least sag.
Comparing Antek's lineup and looking for the highest voltage, lowest drop small size inexpensive transformer for TDA7294, I get: The 22+22vac AN3222. There's enough current and just right voltage too. Only 1v drop at 6a for $36? It is fantastic!
Antek does have some really great selections with lovely specs. . . but some models like the 24v don't have excellent specs. All do have quality construction and run quietly. Sometimes the size is unexpectedly huge, more like a CD canister size, such as the good 25v AN4225 and after you succeed (or fail) to lift it, the means for low voltage drop is obvious for that one.
Comparing Antek's lineup and looking for the highest voltage, lowest drop small size inexpensive transformer for TDA7294, I get: The 22+22vac AN3222. There's enough current and just right voltage too. Only 1v drop at 6a for $36? It is fantastic!
Antek does have some really great selections with lovely specs. . . but some models like the 24v don't have excellent specs. All do have quality construction and run quietly. Sometimes the size is unexpectedly huge, more like a CD canister size, such as the good 25v AN4225 and after you succeed (or fail) to lift it, the means for low voltage drop is obvious for that one.
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