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Old 5th July 2006, 05:00 PM   #1
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Default Wal-Mart power supplies

hey guys I am looking to get a cheap powersupply for a Phono pre-amp project and I want to see if my current Idea is safe, or if I will be getting a threatening letter from the Moderators.

If I take a 120v to 12v transformer then take a 120v to 6v transformer, and turn it around, I have in turn made a 120v to 240v transformer, right? I just have to make sure the 12v, and 6v windings can take enough current, will this be safe? also, could I put the two transformers next to each other in order to help cancel-out some maganetic fields?

Thanks

Alex
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Old 5th July 2006, 05:28 PM   #2
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Hi Alexmoose

This is NOT ideal and far from being safe!

An alternative, that will cost you one more trafo, is to use a larger trafo for the 120V to 12V, and than two smaller trafo's connected with their 12V to the 12V of the first trafo. The 120V of these smaller trafo's are than connected in series, to get 240V. DO you understand what I am trying to say? I think this is way more safe (but I also risk a letter from a moderator).

I don't know if you thought about doing this, but you can use the 12V to feed the heaters (if the tubes are "standard" ones).

Erik
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Old 5th July 2006, 06:21 PM   #3
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In general you can not apply more than the rated voltage to any transformer winding. In other words you can't put 12 volts into a 6 volt winding. This will cause a higher than normal current to flow and the magic smoke will come out, and in this case it smells really bad!

There are 3 simple ways to do what you want.

Use two 12 (or 6) volt transformers back to back like you say, with a voltage doubler on the 120 volt winding.

Find a 12 volt transformer with a universal (dual 120 volt windings) voltage primary, and configure it for 240 volts for the second transformer.

The third method (and the one that I actually use) is two transformers. I use an isolation transformer that has a universal primary and a 115 volt secondary, hooked up backwards (apply 115 volts to the secondary, wire the primaries for 230 and use them as the secondary). I use the Triad N68X which is $11.20 from Mouser and rated for 50VA. This will power a small (10 WPC) P-P amp using a SS bridge. B+ is about 290 to 305 volts under load depending on your line voltage. For filaments, I use a Xicon 41FD030 (Mouser) which gives 6.3 volts at 3 amps for $6.98. Here is a safe practical way to power a small tube amp for under $20. What more could you want, $16 output transformers, Edcor XPP10-8-8K.

I have seen schematics on the web that missapply a dual winding primary by using half of it as a primary, and half as a secondary. This may allow the use of one transformer, but the windings were never meant to be used this way. This is not safe since there is insufficient insulation between the two windings. Please don't go there!
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Old 5th July 2006, 07:35 PM   #4
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Quote:
Originally posted by ErikdeBest
Hi Alexmoose

This is NOT ideal and far from being safe!

An alternative, that will cost you one more trafo, is to use a larger trafo for the 120V to 12V, and than two smaller trafo's connected with their 12V to the 12V of the first trafo. The 120V of these smaller trafo's are than connected in series, to get 240V. DO you understand what I am trying to say? I think this is way more safe (but I also risk a letter from a moderator).
Erik

I understand what you're getting at

these all sound like good, more reliablde solutions. which are only slightly less cost effective. for my pre-amp, I think I will try the voltage doubler, but the Isolation transfomer sounds like a great idea for an El84 PP amp!


How much current can I get out of using two 12v transformers B2B? is it simply conservation of energy 12v at 100ma = 120v at 10ma?
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Old 5th July 2006, 07:43 PM   #5
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Quote:
How much current can I get out of using two 12v transformers B2B? is it simply conservation of energy 12v at 100ma = 120v at 10ma?
Each transformer has a fixed loss based on the amount of energy required to magnetize the core ( core losses). There are also resistive and coupling losses that cause the efficiency of the transformer to be less than 100%. If you figure on 80 to 90% you should be OK.

So 12V at 100 mA is 1.2 watts. If the efficiency is about 80% you will get 1 watt, which is about 8 mA.
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Old 5th July 2006, 07:50 PM   #6
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Good point about dual primary transformers... some are actually bifilar windings, which would put only two thin layers of enamel between you and death if you depend on it for isolation.

Using back-to-back transformers is somewhat inefficient, since you use twice the iron, and get half the regulation (losses from two transformers). But if they're nearly free from a hamfest or surplus store, who cares! But if you have to buy new, separate heater and plate transformers are probably cheaper.
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Old 6th July 2006, 12:03 AM   #7
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Quote:
Originally posted by Tom Bavis
Good point about dual primary transformers... some are actually bifilar windings, which would put only two thin layers of enamel between you and death if you depend on it for isolation.

Using back-to-back transformers is somewhat inefficient, since you use twice the iron, and get half the regulation (losses from two transformers). But if they're nearly free from a hamfest or surplus store, who cares! But if you have to buy new, separate heater and plate transformers are probably cheaper.
The only time I would trust a dual primary transformer to offer good and safe isolation is when I can plainly see that the two windings are on separate bobbins. Such transformers are usually wound on a core that vaguely looks like a toroid. But instead of being round they tend to be square in shape. But realize that the power handling capability of such a transformer when using only one of the two primaries is 1/2 of what it's rated for. And that half is split between the load on the other "primary" and what load is on the nonimal secondaries.

If you want to keep the amount of iron in the equipment down, use a beefy AC output wall wart transformer. The wall wart would run the heaters and also power the "backwards" transformer to make B+. This also avoids having powerline or mains lines inside your equipment, a good thing for novices. Of course this only makes sense for preamps and other low powered stuff. For safety keep the wall wart secondary close to ground electrically (ie, don't make it ride on a hundred volts compared to equipment ground!)!
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