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Old 1st Jul 2012, 4:12 pm   #1
dotsndashes
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Default Multi lead power transformer, how to identify windings.

I recently acquired a number of bits and pieces from an old Technics amplifier.

Included was a potted power transformer with mulicoloured leads. Stamped on the transformer are, SLT5M89 and ETP66H7E.

I have tried an ohmmeter across leads, no obvious indication which is primary.
The leads are coloured: GREY, BROWN, ORANGE, YELLOW, RED, BLACK, BLUE x2, WHITE x2 and
and a further lighter GREY.

Any help would of course be greatly appreciated.
Thanks for your time.
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Old 1st Jul 2012, 5:39 pm   #2
TrevorG3VLF
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Default Re: Multi lead power transformer, how to identify windings.

The first thing to do is to find out which leads are connected together.
If it is a transistor amplifier, then the secondaries will be the lowest resistance.
If it is a valve amplifier, then the primary will be very similar to the HT secondary but the secondary will have three leads i.e. two plus a centre tap.

The primary may have taps for 200 to 240V or may have two windings to be connected in parallel for 110V and in series for 220V.

To test, put the mains on the highest resistance winding with a low wattage light bulb in series and measure all voltages.

Happy detecting!

Trevor

Last edited by TrevorG3VLF; 1st Jul 2012 at 5:42 pm. Reason: spelin
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Old 1st Jul 2012, 5:44 pm   #3
Amraduk
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Default Re: Multi lead power transformer, how to identify windings.

If it was a solid state amplifier, then the primary winding should be the one with a significantly higher resistance than the others, it may also have thinner connecting wires than the secondaries. If a valve amplifier, then the heater winding will have a very low resistance. The mains and high voltage secondary windings will be much higher and probably very similar in resistance.

The mains winding will probably have some taps which will have a relatively low resistance compared to the full winding and it may have two windings to give 110/240V operation. The (or one of the) secondary windings may be centre tapped, so each half of the winding should be very close in resistance.

A Google search gives SLT5M89 as a Panasonic mains transformer. I didn't find any connection details but if you contact Panasonic, they may be able to provide you with that information.

Regards,

Dave.
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Old 1st Jul 2012, 6:29 pm   #4
GrimJosef
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Default Re: Multi lead power transformer, how to identify windings.

Quote:
Originally Posted by Amraduk View Post
If a valve amplifier ... the mains and high voltage secondary windings will be much higher and probably very similar in resistance.
Not necessarily though. On the Quad II valve amp, for example, the 250V mains winding is about 12 ohms end-to-end whereas half of the 315-0-315V HT winding is 60-odd ohms if I remember correctly.

Cheers,

GJ
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Old 1st Jul 2012, 6:47 pm   #5
Amraduk
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Default Re: Multi lead power transformer, how to identify windings.

True! I was thinking more along the lines of a single 250V secondary. Higher voltage and/or centre tapped windings will be of higher resistance than the mains primary winding, of course.

It also depends on the gauge of wire used in each winding. As the primary winding will probably be required to carry a higher current than the high voltage secondary, it will use a thicker gauge of wire, giving it a lower resistance per turn.

Regards,

Dave.
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Old 1st Jul 2012, 7:39 pm   #6
paulsherwin
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Default Re: Multi lead power transformer, how to identify windings.

While measuring the output voltages will tell you which winding is which, the off load voltage will be significantly higher than the on load voltage. A certain amount of experimentation is usually called for, especially if the transformer is to be used at anywhere near its maximum ratings.
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Old 1st Jul 2012, 8:06 pm   #7
glowinganode
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Default Re: Multi lead power transformer, how to identify windings.

Once you have established the extreme ends of the primary winding(s), mark the leads or tags, I use a dab of red paint.
This makes life easier in the future, a good habit to get into when stripping out old kit.
Rob.
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Old 3rd Jul 2012, 12:54 am   #8
Silicon
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Default Re: Multi lead power transformer, how to identify windings.

I would not connect mains voltage to a winding unless I was sure it was designed to withstand 360v (the peak voltage of the mains waveform).

I would use another transformer with a 6v output (or less) and a current limiting resistor and connect it to the winding that seemed to be the mains primary.

If I guess correctly, the other windings will output a safe low voltage which I can measure and work out the ratios of the windings.

If I guess incorrectly, one of the windings could have a very high voltage on it.

I recently tested some output transformers by feeding 2v ac into the secondary windings.
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Old 3rd Jul 2012, 10:55 am   #9
dotsndashes
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Default Re: Multi lead power transformer, how to identify windings.

Thanks Silicon for your input to my problem. Yes, that is precisely what I intend to do, somewhat as a last resort.

Thanks anyway, and hopefully will 'come up' with an answer soon..

Cheers!
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