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-   -   Toroidal transformers (http://www.diyaudio.com/forums/power-supplies/90259-toroidal-transformers.html)

scottw 13th November 2006 05:40 PM

Toroidal transformers
 
Greetings,

Planning to build another GC. About the only thing I haven't tried is one of them new-fangled doughnut shaped transformers for the PS.

AnTek on ebay sells a variety of toroids(made in China), and through forum searches it seems most people that have used these have been happy with them. Any others wish to comment on these toriods? If it hums I have an expensive paper weight!

These toroids are dual primary, parallel for 115vac or series for 230vac (hope I got that right). So my question, if you had a toriod with dual 40v secondaries, could you series connect the primaries and run 115vac and get 20v from the secondaries? Would the VA rating decrease, would the tx be more prone to mechanical hum or run hotter? What size fuse would it take to light a 700va toriodal wired this way?

Thanks,

Scott

traw 13th November 2006 05:52 PM

i've had good luck with the anteks. they tend to bigger than similar rated offerings from other manufacturers. but always quiet and no mechnical hum for the 3-4 i've used. also email direct for quote and he may have something else in stock that's not listed.

haven't tried the 240 on 120 thing to know of drawbacks

AndrewT 13th November 2006 07:26 PM

Hi,
if you connect 115Vac to the 230Vac (windings in series) then you get half the VA rating.
A very expensive and heavy way to adjust your voltage.

scottw 13th November 2006 09:59 PM

Traw,

Thanks for the info, I'll probably give them a try.


AndrewT,

Doh, damn physics, I thought there would be something wrong with that idea. Thanks though.


Scott

CarlosT 14th November 2006 12:02 AM

Contact Avel Lindberg directly. I was pleasantly surprised at their friendliness and the the availability of very well priced in stock toroidals...actually kicks Antek in the a@@. Ask for Melanie.

CarlosT 14th November 2006 12:04 AM

Supposedly one of the advantages of a single primary 60hz toroidal is that it weighs a bit less and is smaller. Too bad the people expounding this had like a 13-week build lead and had nothing decent for under $65.

FastEddy 14th November 2006 12:41 AM

scottw: " ... These toroids are dual primary, parallel for 115vac or series for 230vac (hope I got that right). So my question, if you had a toriod with dual 40v secondaries, could you series connect the primaries and run 115vac and get 20v from the secondaries? Would the VA rating decrease, would the tx be more prone to mechanical hum or run hotter? ..."

* The VA RATING (power or wattage rating) always remain the same, no matter what voltage is applied to the primary(s). The power rating (Volt times Amps [VA] or Watts) is directly related to the wire sizes in the transformer, and wire sizes and manufacturing methods are the limiting factors for increasing heat, internal resistance, etc. *

Applying 115 VAC to the series primaries (normally in series for 230 VAC) will not harm the transformer at all, just reduce the output voltage on all of the secondary winding(s) voltage by half (1/2). Your Example: Series Primaries for 230 VAC to get dual (two) 40 VAC secondaries, then hitting the (series) primary(s) with 115 VAC will give you dual (two) 20 VAC secondaries.

Interestingly: the current rating (amperage) available from the 20 VAC secondaries would be doubled (!) ... as a result of the power (VA) remaining the same, and power = volts times amps ... in this case the voltage ratios would imply decreasing output voltage = 1/2, output current = 2 X ...

Torrid Transformers are generally more thermally efficient than other types = they generally run cooler ... that's one reason why they are appearing in modern home theater and higher end audio devices. Also torrids can be designed and manufactured with much lower levels of "hum" and internal noise, as a result of the circular core (the ferro-magnetic torrid ring) as opposed to open ended, box or rectangle cores.

Yes, you can run the series primaries at 115 VAC with an output at both secondaries of 20 VAC ... and yes, the torrid transformer will run cooler than most similar alternates ... and no, you will not notice nearly as much "hum" ... and no, you will not have more heat ... and the over all VA rating will remain the same ... and you can use a ~~= 7 Amp / 115 VAC / quick blow fuse ahead of a 700 VA XFormer IF you want it to trip just below or at the 700 VA power rating (This is advisable until you can actually measure the heat.) Any larger fuse and you should consult with a qualitied engineer.

:smash:

AndrewT 14th November 2006 06:26 AM

Hi Fast Eddy & Scottw,
I cannot get my reply past the school dictionary (it blocks the safety recommendations) so I will send a reply when I get home.
Sorry to keep you waiting, be patient.

peranders 14th November 2006 07:10 AM

I would say that the VA rating will become a bit lower if you plan to use a 230 VAC transformer at 115 VAC and this mainly because the windings have a bit more turns than necessary = more resistance.

It's OK to do that if this is the only alternative.

AndrewT 14th November 2006 05:09 PM

Hi,
Quote:

Originally posted by FastEddy
The VA RATING (power or wattage rating) always remain the same, no matter what voltage is applied to the primary(s). The power rating (Volt times Amps [VA] or Watts) is directly related to the wire sizes in the transformer, and wire sizes and manufacturing methods are the limiting factors for increasing heat, internal resistance, etc.
wrong, it also depends on the iron flux at currents upto maximum operational.
Quote:

Applying 115 VAC to the series primaries (normally in series for 230 VAC) will not harm the transformer at all, just reduce the output voltage on all of the secondary winding(s) voltage by half (1/2). Your Example: Series Primaries for 230 VAC to get dual (two) 40 VAC secondaries, then hitting the (series) primary(s) with 115 VAC will give you dual (two) 20 VAC secondaries.
correct.
But this next bit
Quote:

Interestingly: the current rating (amperage) available from the 20 VAC secondaries would be doubled (!) ... as a result of the power (VA) remaining the same, and power = volts times amps ... in this case the voltage ratios would imply decreasing output voltage = 1/2, output current = 2 X ...
is completely wrong.
Think about the Isquared R losses (the heat).
Double the output current and you quadruple the heat in the secondary. If you maintain the same current into the the primary (to maintain the same input VA) the the total heat has increased by 150%.
However trying to maintain the primary current has taken the ampere turns up to double what the manufacturer intended at full VA rating. The flux in the iron core will be pushed so far around the corner that saturation is very near (or maybe past it). The hysteresis in the core will absorb a very significant proportion of the input power at these high flux levels and the result is the core is now generating it's own extra heat. Meltdown is approaching.
This next part
Quote:

Torrid Transformers are generally more thermally efficient than other types = they generally run cooler ... that's one reason why they are appearing in modern home theater and higher end audio devices. Also torrids can be designed and manufactured with much lower levels of "hum" and internal noise, as a result of the circular core (the ferro-magnetic torrid ring) as opposed to open ended, box or rectangle cores.
is nearly right, but for the wrong reasons.
But this next bit
Quote:

Yes, you can run the series primaries at 115 VAC with an output at both secondaries of 20 VAC ... and yes, the torrid transformer will run cooler than most similar alternates ... and no, you will not notice nearly as much "hum" ... and no, you will not have more heat
is correct.
But again, this next bit
Quote:

the over all VA rating will remain the same ... and you can use a ~~= 7 Amp / 115 VAC / quick blow fuse ahead of a 700 VA XFormer IF you want it to trip just below or at the 700 VA power rating (This is advisable until you can actually measure the heat.) Any larger fuse and you should consult with a qualitied engineer.
is also wrong.
A fuse does not work this way.
At small currents the fuse lasts nearly for ever.
Current at fuse rating, the fuse will last for weeks and probably months, but will eventually fatigue and blow.
At currents above fuse rating the life of the fuse gets shorter as the current rises. At ten times rating it will blow quickly, at 100times fuse rating it will blow very quickly, but still not instantaneous.
Your advice on 7A fuse is based on erroneously maintaining the same VA at half voltage. For a close rated fuse you should be advising 350VA/115V if the unit uses that amount of power regularly.
To get the toroid to start without a soft start circuit one should increase the fuse rating by about 3times i.e. T10A and use a time delay (=anti surge).
If one uses a soft start then it depends on the ballast resistor (ot Thermistor) value, and may require some experimentation. You can often run with a fuse of less than the VA/Vac formula predicts due to spare capacity in the system and only occassional nuisance blowing occurs (if it is more often than once a month then consider using the next fuse up).


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