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Old 16th September 2009, 04:10 PM   #1
Atilla is offline Atilla  Norway
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Default Funny traffo specs

I've got a funny transformer at hand and I wonder if I can use it for something - power up a tube heater, do a small adjustable bench supply for opamps, something. However, it's got weird ratings:

It's fused with a 100mA fuse on the primary.

It's got three secondaries labeled:
1 - 14V 1.3VA
2 - 14V 0.3VA
3 - 23.8V 2.5VA

However, the secondaries are actually:

1 - 16-0-16, Center tap (33 when measured on the ends)
3 - 31-0, no center tap.

That is measured with bridge rectifiers, caps, some leds and a few ICs attached (it's still on a board of some old telecommunication equipment).

What this suggests to me is very crappy regulation, therefore really low VA ratings, but the labeling makes no sense. Two different ratings on the same center-tapped winding? 2.5 or even 0.3 VA ratings? Do they mean amps maybe? It's a regular-sized encapsulated EI transformer, It should be able to deliver at least about 10VA total.

If those ratings are real I don't think there's any way of me adjusting the thing for a decent application of any sort. Any ideas that would make it useful?
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Old 16th September 2009, 04:27 PM   #2
AndrewT is offline AndrewT  Scotland
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I think the 100mA tells you the total VA is between 20VA and 30VA.
I think the labeling is A not VA.
14V * 1.3A = 18.2VA
14V * 0.3A = 4.2VA
23.8V * 2.5A = 59.5VA
Total 82VA. Seems far too high.
What are the wire diameters of each of the tappings? That is a really good way to estimate the current capacity of any winding.

Try measuring the AC voltages from each of the windings with 220Vac/230Vac on the input.
A centre tapped with different wire in each half is possible, but it's then not a centre tapped, it is simply two different windings that are series connected.

Finally the regulation of a small EI is high, expect 15% to 30%.

Safety note.
Insert each tapping into an insulated terminal block and screw the terminal block to an insulating board.
Helps stop you killing yourself.
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Old 17th September 2009, 06:49 AM   #3
Atilla is offline Atilla  Norway
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I'll take it off the board to see the wire diameter. They are fairly thin however and I don't have a gauge. What could I use to measure them precisely? I'll try to take a mark of it on paper or something like that..

The measurements I posted there aren't the DC, they're the AC on each of the windings with 220V on the primary. The second winding then has as high as 50% regulation if the rating is correct, which seems to be a little bit too much. I can try to load the DC side with a power resistor and see how much it'll drop.

Last edited by Atilla; 17th September 2009 at 07:02 AM.
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Old 17th September 2009, 12:20 PM   #4
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Quote:
Originally Posted by Atilla View Post
.........That is measured with bridge rectifiers, caps, some leds and a few ICs attached (it's still on a board of some old telecommunication equipment). ............
remove the transformer from all this indeterminate loading. Measure the AC voltages with the secondaries open circuit.
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Old 17th September 2009, 05:26 PM   #5
Atilla is offline Atilla  Norway
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There, measurements with 220V at the primary, Transformer completely unloaded:

"24V" rated winding - 32.8Vac

"14V" rated windings - 17.2Vac to the center tap, or 34.7Vac when measured relative to each other.
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Old 18th September 2009, 08:02 AM   #6
AndrewT is offline AndrewT  Scotland
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seems a bit odd that one over reads by 36% but the lower voltage only over reads by 23%.

Assume for the moment that the regulation is 23%.
The 24Vac becomes a real 26.6Vac winding.

I wonder if the 2.5A rating is a clue to why it is rated @ 24Vac. It could be that they are deliberately over-rating the current and this produces an extra 10% of voltage drop in this winding. It will run hotter but if the client only intended using 50% of all the ratings the temperature would remain safe.
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Last edited by AndrewT; 18th September 2009 at 08:04 AM.
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Old 18th September 2009, 08:28 AM   #7
Atilla is offline Atilla  Norway
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I'm thinking the same thing, the bigger winding should have behaved better than this, considering its rating. I think what you said is right, the second winding is expected to have more current draw in order to drop to its rated voltage. Probably that's why they decided they need a separate power circuit on that board, otherwise the +/- form the other winding would do the same job.

I think I'll try to use it in a dual supply for testing small circuits. I can regulate the voltage from the centre taps fairly well if the current draw is not too big.

Or I guess I could use the second winding for a voltage multiplier for a hybrid headphone amp design I want to try. The ripple will be horrible in a normal setup, but the current draw on that rail will be very low so I can get away with it. At least, it should be ok for testing on a breadboard.

I'll put a load rated for 100-300mA to see how it'll behave for those applications, I think I have a few LED arrays with a CSS around that are consuming values in that range. Or I'll just drop a power resistor across and see what happens.
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