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Old 20th September 2007, 10:45 PM   #1
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Default running a transformer backwards(anti-hi-jack)

I've found the "running a transformer backwards" thread on the tube forum very interesting, but not wanting to hi-jack the thread, I thought I'd start another...
I happened upon a large, unusual toroidal transformer on the shelf of a local electronics store that carries surplus gear several months ago. Finding the thing a distraction on each and every trip into the store, I finally gave in and brought it home. It is unusual (at least to me) in that the label indicates dual primaries (100V and 20V) and dual secondaries (2 x 120V) with 1000VA total.
The unit was manufactured by Toroid and I verified their lead/color scheme for primary/secondary wiring and it certainly appears correct. Cool? Yeah, I figure that this is as good a candidate as possible for reversing. I paralleled the secondaries, applied 120V and pretty much as expected, 20V and 100V on the primaries. Any reason to think I wouldn't get the full 1000VA @ 20V ?


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Old 21st September 2007, 04:20 PM   #2
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bhueller?
anyone?


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Old 21st September 2007, 04:35 PM   #3
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Default backwards transformer

The whole transformer should handle 1000VA (it doesn't much care which way the power is flowing). However, it would be helpful to know how the power was distributed to the 20V and 100V windings. In a pathological worst case, let's say nearly all the power was intended to go through the 100V winding and the 20V winding was just a small sense winding. It would be wound with skinny wire, and if you tried to pull 1000VA through it, there would be way too much voltage drop and the winding would heat up, maybe even burning out.

So the key is what are the resistances of the windings. You can measure this and estimate voltage drops. Don't forget that if you are driving a cap-input filter that the rms current is much higher than the average current, due to the high current spikes from this kind of filter. The rms value is what is used to calculate heating effects. It is usual to derate a transformer by a factor of 2 or 3 when using a cap-input filter.

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Old 21st September 2007, 07:16 PM   #4
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John,
Thank you very much for the reply.
I just metered the windings...
120V (sec) .7ohms individually, </=.4 parallel
100V (pri) .4ohms per
20V (pri) </=.1ohms per

I don't believe My meter provides the resolution or accuracy necessary to be 100% sure, but if I'm reading the results correctly, I should be in fine standing.
Am I mistaken?

thanks again,
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Old 21st September 2007, 08:00 PM   #5
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I suspect that the primary use designed to provide 1000 VA when wired to a 100 Volt supply or a 120 Volt supply. So the primaries should handle 10 Amp. 10 Amp and 20 Volts is 200 VA.
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Old 21st September 2007, 08:41 PM   #6
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Quote:
I suspect that the primary use designed to provide 1000 VA when wired to a 100 Volt supply or a 120 Volt supply
Actually, the primaries are (2x)100V/(2x)20V with (2x) 120V secondaries, so IF the transformer will meet the 1000VA output @ 100V OR 20V, wouldn't the pair of 20V primaries be rated @ 25A ea. ?
Old dog learning...


thank you for your time!

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Old 22nd September 2007, 12:33 AM   #7
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Kevin,
Surely you are correct and the most likely scenario is that the tranny is intended to be run @
100V or 120V 50/60 hz and that the 20V winding was not intended to run solo.

Thank you.

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Old 23rd September 2007, 06:09 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
the two primary wires are taps to allow operation on either 100Vac or 120Vac. Each will take the same current and use the same wire diameter.

When reversed they will output the same maximum current. An earlier poster got it right by suggesting 10Aac from each winding.

Take care to protect the back to front transformer from physical damage. The primary is now likely to be on the outside and you don't want to come into contact with an exposed mains winding.
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Old 23rd September 2007, 07:49 PM   #9
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Andrew,
Thanks for the reply. I went on to check the winding wire gauge/ampacity tables and I concur.


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Old 5th October 2007, 06:41 AM   #10
chibimo is offline chibimo  United States
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Default Seventenths

Seventenths,
I believe I met you last week over at "the local electronics recycler" on the eastside. you turned me on to chipamps.
I bought one of the toroidal transformers you reccomended. Could you please contact me? Admin wont let me e-mail yet!
Brian
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