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PostPosted: 15 Aug 2008, 08:40 
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Guys,
I have 3 1240VA transformers that I bought before I knew any better. They ouput 42V after rectification.
What can I do with them?
Ri

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PostPosted: 15 Aug 2008, 12:53 
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Huge! I imagine it only has one secondary, no CT. With a single supply like that you are generally limited to solid state audio projects. Non audio you could make a huge variable power supply.

Not sure if this will work, but you could do some research on it. Take two transformers and connect the secondaries together. Now you have a 1200VA isolation transformer! Put them in a wooden box with an outlet or two. More than enough power to run all your equipment.

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PostPosted: 15 Aug 2008, 13:24 
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Bit more information
Primary is 120V 50Hz
Secondaries are 27.2V at 19Amps
Secondaries are 29.6V at No Load
Primary resistance 2x(.34+.13)
Secondary resistance 4x.065 ohm
Temp rise at 1240VA is 80 deg C
P/N is TI-043222B AU Iss 0 from Toroid International Ltd.
DIA is 165mm and Height s 98mm weight is 10.2kg

You are right that they would make a good isolation transformer, if its possible to wire them like you said. It would be the ultimate and last forever. Is there a voltage drop in there anywhere? Would I get 120 back out or do you think it would be more like 110? Would there be any resonances created that would heat up their cores?

But I would really like to make an amp that can use the available voltage. I dont know of any recommended chips or SS designs that use this kind of voltage.
Ri

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PostPosted: 16 Aug 2008, 00:51 
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The fact that they will give of some heat means that some voltage will be lost, but I doubt it will be much. Perhaps a volt or two.

I'm pretty sure you can do this, but you should search a bit to see if there are any downsides.

Cheers,
Gio

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PostPosted: 16 Aug 2008, 10:13 
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What a bizarre idea. I think it just might work. You will have to try it out and see. I searched and came up empty on this topic. This is far too simple, so I imagine it must have been done already.

The only issue I can think of is electromagnetic fields. In that regard, you will want to use a grounded metal enclosure. You will want to keep it away from interconnects and I like to use shielded power cables.

Yes, I imagine there will be some loss in power which will likely vary with the load. R is fixed, so as I increases so will power losses. As mentioned, The losses will become heat.


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PostPosted: 16 Aug 2008, 19:12 
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Well I found this:
Isolation transformer. This is very important for safely working on live chassis equipment like line connected switchmode power supplies (primary side). You can build one from a pair of similar power transformers back-to-back (with their highest rated secondaries connected together. I built mine from a couple of similar old tube type TV power transformers mounted on a board with an outlet box including a fuse. Their secondary high voltage windings were connected together to couple the two transformers together. The unused low voltage windings can be put in series with the primary or output windings to adjust voltage. Alternatively, commercial line isolation transformers suitable for TV troubleshooting are available for less than $100 - well worth every penny.

But I worry about resonance between the two. Guess its easy to try :)
Would I be able to reduce resonance with caps a resistors between? This way it would interrupt any difference of potential between the two toroids? If I am not making sense its cuz I really dont know what I am talking about but it kind of makes sense to me.

Also, I found that the Tripath TK2350 Chipset can handle 42V easily. However I have not yet found any chips for sale.
Ri

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PostPosted: 16 Aug 2008, 21:16 
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HI Ri,

Good stuff. This is something you will use for a long time so putting it in a metal case is a good idea. It would not hurt to add a fuse as well.

Let us know how you make out.

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PostPosted: 17 Aug 2008, 09:27 
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I forgot to ask, what sort of resonance are you worried about? Mechanical?

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PostPosted: 17 Aug 2008, 13:08 
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I wish I knew more about what I am worried about. I just remember seeing something, and Bruce mentioning something similar as well, about multiple transformers feeding one power supply leg and that some resonance could occur. This has me worrying about these transformers when there is little to no load and that maybe if they both have a certain amount of capacitance that they will start trading a tiny bit of electricity back and forth and cause some resonance. I am sure I dont know what I am talking about but anyway thats what has me a bit worried. I figured maybe a resistor between them would burn enough electricity to make that resonance not possible.
Am I way off base?
Ri

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PostPosted: 17 Aug 2008, 19:57 
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Well, I would expect that a toroid that large will vibrate mechanically especially if you have some DC offset in the mains supply. You can lay it on a piece of rubber and clamp it down to try reduce potential noise.

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