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Old 24th February 2006, 07:53 PM   #1
ash_dac is offline ash_dac  United Kingdom
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Default Calculating secondary transformer voltages

Hi,

I have an unmarked torroidal transformer with about 8 or 9 wires attached.

How can I calculate the output voltages on these secondary rails.
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Old 24th February 2006, 10:30 PM   #2
dnsey is offline dnsey  United Kingdom
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Why try when you can measure them?
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Old 25th February 2006, 12:33 AM   #3
Eva is offline Eva  Spain
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Measure DC resistance between each wire and the rest and make a table. This information will allow us to draw some hypothetic winding configuration.

The next step would be to inject a small known AC voltage, lets say 10V AC, into one of the hypothetic windings, and then measure the voltage induced in the rest of the hypotethic windings. The current drawn from the 10V source should be very small, otherwise a smaller voltage should be employed.

Be careful when measuring because those 10V AC put into one of the secondaries may as well produce 100V AC into the primary.

Once one or more primaries have been identified, testing them for 120V or 230V operation is as easy as connecting them to 230V with a conventional light-bulb in series. If the bulb lights, it means that the transformer is saturating and the voltage rating of the winding has been exceeded.
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Old 25th February 2006, 08:52 AM   #4
ash_dac is offline ash_dac  United Kingdom
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Quote:
Originally posted by Eva
Measure DC resistance between each wire and the rest and make a table. This information will allow us to draw some hypothetic winding configuration.

The next step would be to inject a small known AC voltage, lets say 10V AC, into one of the hypothetic windings, and then measure the voltage induced in the rest of the hypotethic windings. The current drawn from the 10V source should be very small, otherwise a smaller voltage should be employed.

Be careful when measuring because those 10V AC put into one of the secondaries may as well produce 100V AC into the primary.

Once one or more primaries have been identified, testing them for 120V or 230V operation is as easy as connecting them to 230V with a conventional light-bulb in series. If the bulb lights, it means that the transformer is saturating and the voltage rating of the winding has been exceeded.
Hi,

I've produced two tables. Hopefully my method of measurement was consistent.
Attached Files
File Type: zip tx measurements.zip (2.6 KB, 34 views)
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Old 25th February 2006, 10:05 AM   #5
AndrewT is online now AndrewT  Scotland
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Hi,
play safe rather than adventurous, because what follows is mostly guesswork.

1. wire up a mains light bulb in series with the mains power line as follows.
Plug top live pin to light bulb fitting pin1.
light bulb fitting pin2 to trailing mains socket live pin.
Connect neutral straight through from mains plug top to trailing socket neutral.
Connect plug top earth to trailing socket earth.
Insulate any exposed mains connected parts. Keep this for all future testing. Change the light bulb wattage to suit different equipment loadings.

2. plug the new light bulb into the mains: bulb should be off.

3. plug an appliance into the trailing socket:light bulb should glow or be bright indicating current flow.

4. disconnect appliance.

5. insert all the transformer wire ends into separate terminals of a terminal strip. This is to prevent inadvertant shorting and allow relatively safe voltage measurement.

6. using a short length of double insulated 2core mains cable,
connect purple wire to plug top neutral
connect red wire to plug top live.

7. plug your transformer plug top into the light bulb trailing socket and switch on. The light bulb should flash and go dim or go out. If it stays bright switch off and post us.

8. Carefully measure the voltage from purple to green, purple to brown, purple to blue, purple to red. You should get an increasing series of AC mains voltages. If any of them are above the incoming mains voltage then I figured wrongly.

9. measure the voltage orange1 to black, orange2 to black and orange1 to orange2. These will hopefully be low voltage with black as centre tap, but could be 110Vac so be careful.

10. Don't call us if it blows up.
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Old 25th February 2006, 10:29 AM   #6
Eva is offline Eva  Spain
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This is the winding configuration that I have derived from the values of your table:

Winding 1: orange - black - orange

Winding 2: red - blue - brown - green - purple

Your table also tells me that your multimeter measures 0.2 or 0.3 ohms even when both tips are shorted, am I right? Next time that you do any low value resistance measurement, you should find out this error value and substract it from the values obtained.

In order to find out voltage ratios, you should inject a small AC voltage between one of the oranges and the black wire, as I previously mentioned. Then you should measure the voltage obtained between each pair of wires in each winding and make another table.
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Old 25th February 2006, 10:43 AM   #7
ash_dac is offline ash_dac  United Kingdom
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Quote:
Originally posted by Eva
This is the winding configuration that I have derived from the values of your table:

Winding 1: orange - black - orange

Winding 2: red - blue - brown - green - purple

Your table also tells me that your multimeter measures 0.2 or 0.3 ohms even when both tips are shorted, am I right? Next time that you do any low value resistance measurement, you should find out this error value and substract it from the values obtained.

In order to find out voltage ratios, you should inject a small AC voltage between one of the oranges and the black wire, as I previously mentioned. Then you should measure the voltage obtained between each pair of wires in each winding and make another table.
Hi,

Yes it measures 0.3 ohms when the pins are shorted.

Would it be possible to pulse dc through the tx instead?
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Old 25th February 2006, 10:44 AM   #8
AndrewT is online now AndrewT  Scotland
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Too complicated.
Just build the light bulb tester.

I agree with Eva's colour order.
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Old 25th February 2006, 10:50 AM   #9
Eva is offline Eva  Spain
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Quote:
Originally posted by ash_dac


Hi,

Yes it measures 0.3 ohms when the pins are shorted.

Would it be possible to pulse dc through the tx instead?

Yes, It is possible as I use that technique for checking and measuring SMPS high frequency transformers, but you will have to build a pulse generator if you don't have it, and an oscilloscope would be also required to see pulse voltage and polarity through each winding.

However, for 50/60Hz transformer measurement it's much easier to use low voltage AC power provided by another transformer, or even from some wall-wart AC adaptor borrowed from any appliance.
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Old 25th February 2006, 10:58 AM   #10
ash_dac is offline ash_dac  United Kingdom
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Quote:
Originally posted by Eva


However, for 50/60Hz transformer measurement it's much easier to use low voltage AC power provided by another transformer, or even from some wall-wart AC adaptor borrowed from any appliance.

Hi,

Yes i'm looking around the house for a low voltage ac wall-wart but all I can find is dc !

I'm sure I've got one but I might go and buy one later (I suppose I could find a use in a cdplayer afterwards)
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