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Old 15th September 2002, 08:00 PM   #1
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Default unknown transformer capabilities

I have a very large transformer I got from a friend but cannot find any spec on it, and I did try the manufacturer and online.

It is not a toroid, but it was free and a good startng point for a first amp project, if I can figure out what it does...
I know I can figure out a pin out for it easily enough, but is there any way to determine which would be the primary? from there on I could measure the voltage... Would anything really bad happen if I hooked a secondary winding to AC accidentally, or wold it just work as a step up transformer with teh real primary winding showing greater than 120?
Any help / suggestions would be appreciated... And no, I am not going to ust plug 120 to this thing.. tho I may plug 20 AC from a train transformer and measure the diferent winding values..
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Old 15th September 2002, 08:36 PM   #2
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The transformer that you have is probably a custom part from an old stereo system or amplifier (lots of these are custom designed for the system). But that is not worry just that it could be very difficult to cross reference as you have found. If you place AC on the secondary, the transformer will simply run backwards being a step up as you guessed. The best way to find the primary is that is should be would with the smallest AWG wire because it is carrying the lowest current. Good Luck!!

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Old 16th September 2002, 10:12 AM   #3
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NO NO NO!!!

Quote:
Would anything really bad happen if I hooked a secondary winding to AC accidentally, or wold it just work as a step up transformer with teh real primary winding showing greater than 120?
If you connect the secondary to the mains you're transformer can blow up!!! (I did it once) To find out which is the primary I use a simple ohm meter, the primary has the largest resistance because there are more windings.

Quote:
Any help / suggestions would be appreciated... And no, I am not going to ust plug 120 to this thing.. tho I may plug 20 AC from a train transformer and measure the diferent winding values..
this will also work, consider to use series resistor to reduce current when switching on.

HB.
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Old 16th September 2002, 04:22 PM   #4
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The wording of the question is a little ambiguous. If you hook a secondary winding to the AC line while a primary is also hooked up to the AC line, things would suck for you. But if you simply hooked the transformer backwards it would run backwards. That is keeping the respective windings on respective sides. The resistance measurement is accurate also.

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Old 16th September 2002, 04:43 PM   #5
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Default On unmarked power transformers

At this site there are two articles that may be of help -

Low Resistance Adapter for a Digital Multi Meter.
Deciding how much current you can draw from
that unmarked power transformer depends on
knowing the resistance of the windings.
These values can be as low as 0.1 ohm.
--------------------------------------------------------------------------------
"How to Use those Unmarked Power Transformers"
Here is a program you can download for free that will
do the tedious, although simple, calculations to help
you make informed estimates of how much current each
transformer winding will deliver without burning out.

http://www.angelfire.com/electronic/...htubes/#lowres
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Old 18th September 2002, 09:43 PM   #6
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thanks, I dont want to build that whole circuit.. what a PITA for just one transformer. All of the windings on this beast are below 1 ohm tho. I think I remember it having 2 primaries as well. It definatel has 12 guage windings tho. some 14 too, on one random extra winding.
I will do the train transformet thing and measure the output voltages on all the windings, and then just scale that to 120. My only issue hen is finidng the current ratings of the windings...
Is it safe to just pull more and more current until it starts to get warm (gradually, in order to stabilize at each load) and if I did this, anyone know how warm their transformers get?
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Old 18th September 2002, 10:52 PM   #7
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Default If you don't have a variac

if you have an audio generator, a 1kHz signal is going to see a lot of impedance and the losses arent' that great that they will screw up the result. You load the transformer with a 0.1 volt, 1 kHz signal into ANY winding and determine the turns ratio by measuring the voltage on the other sets of leads.

You can't blithely just hook the thing up to the AC line and see what happens. Let's say you accidentally hook 120 VAC to the 6.3VAC secondary. First off, you are going to have an approximate 20:1 relationship to the primary winding. On whatever happens to be the HV secondary leads the output ratio is even higher. At this extreme of voltage the insulation is likely to break down and you will be a very unhappy (or dead) camper.

You can approximate the current carrying capacity of a transformer by calculating the cross-sectional area of the core. It's in one of the ARRL handbooks, and is hardly exact ...

Always and everywhere, safety first.
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Old 19th September 2002, 02:41 AM   #8
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If you hook up 120V to the secondary (assuming the secondary is a lower voltage winding) it will try to draw a lot of power, saturate the core, and blow a breaker. It will not just run backwards. I would just put it in series with a washing machine motor or other large inductive ballast when first applying power to test, so it would protect the transformer (and the mains) if it was hooked up wrong.
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