Author Topic: Isolation transformer  (Read 7320 times)

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Offline GathTopic starter

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Isolation transformer
« on: January 18, 2014, 07:47:49 pm »
Hey everyone!

I have been messing around with a isolation transformer I bought recently (https://www.distrelec.ch/ring-core-transformer-50-va-115-vac/noratel/aa-53129/354173/en), 230Vac input and 2x115V outputs (25VA each).
I simply put it in a box and added 250mA fuse and a mechanical switch on the primary side. As for the secondary side, I connected in series the 2 secondary windings, so that I can get a 230V outputs.

However, after doing this, I ended up with 270Vac output :o (check the images) ... and I am not quite sure why! I also check each individual secondary winding, which both provide 135Vac. As for my mains voltage, it is 234Vac.

So what is going on? Do you think this toroidal transformer is bust? I tried putting a resistive load, but that did not change anything.
Thanks your your input guys ;)
« Last Edit: January 18, 2014, 10:38:12 pm by Gath »
 

Offline IanB

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Re: Isolation transformer
« Reply #1 on: January 18, 2014, 08:33:32 pm »
What is the measurement of each secondary individually?

Also how does the voltage change if you add a load?
 

Offline denelec

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Re: Isolation transformer
« Reply #2 on: January 18, 2014, 08:47:41 pm »
You may have a short-circuit somewhere in the primary winding.
If the transformer is new, it could be a manufacturing defect.
I don't know what else could cause such an increase in secondary voltage.
 

Offline GathTopic starter

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Re: Isolation transformer
« Reply #3 on: January 18, 2014, 09:21:18 pm »
What is the measurement of each secondary individually?

Also how does the voltage change if you add a load?

I measure about 135VAC on each winding, which is consistent with the 270VAC I measure when both windings are serially connected.
As for the the voltage change, I tried with 100k (measured 270VAC, i.e 2.7mA) and with 10k (measured 266VAC, i.e. 27mA) resistors. Note: each winding is rated for up to 220mA.

You may have a short-circuit somewhere in the primary winding.
If the transformer is new, it could be a manufacturing defect.
I don't know what else could cause such an increase in secondary voltage.

Indeed a short circuit in the primary winding from manufacturing would explain this. Weird thing is, the transformer has a "passed" sticker on it for the manufacturer, So I assumed they actually tested the thing before shipping it out.
 

Offline IanB

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Re: Isolation transformer
« Reply #4 on: January 18, 2014, 10:14:13 pm »
Although it may indeed be faulty, I think I would try measuring the voltage with a 50 W bulb or similar load on the output, just to be sure.
 

Offline Len

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Re: Isolation transformer
« Reply #5 on: January 19, 2014, 01:33:21 am »
Weird thing is, the transformer has a "passed" sticker on it for the manufacturer, So I assumed they actually tested the thing before shipping it out.
I understand why you might think that, because that's how things work in Switzerland in my experience. But if the product was manufactured elsewhere, all that sticker means is that the unit passed by a guy who stuck a sticker on it.
DIY Eurorack Synth: https://lenp.net/synth/
 

Online Andy Watson

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Re: Isolation transformer
« Reply #6 on: January 19, 2014, 02:27:58 am »
The transformer will be rated for supplying the correct voltage at its stated load. Off-load, the transformer output will rise. Look-up transformer regulation. Typically, a 50VA transformer will have a regulation of 10-15% depending on manufacturer and design etc.. i.e., off-load, the output may rise up to 15% of its rated value. Smaller transformers are generally less well regulated.

 

Offline SeanB

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Re: Isolation transformer
« Reply #7 on: January 19, 2014, 09:53:19 am »
Try running it in reverse then, it will then give a lower output. Probably around 210VAC then.
 

Offline HackedFridgeMagnet

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Re: Isolation transformer
« Reply #8 on: January 19, 2014, 10:49:49 am »
Quote
The transformer will be rated for supplying the correct voltage at its stated load. Off-load, the transformer output will rise. Look-up transformer regulation. Typically, a 50VA transformer will have a regulation of 10-15% depending on manufacturer and design etc.. i.e., off-load, the output may rise up to 15% of its rated value. Smaller transformers are generally less well regulated.

What Andy Watson said.

If you look at the spec it 'seems to imply' 133.1v on no load per secondary.


 

Offline BravoV

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Re: Isolation transformer
« Reply #9 on: January 19, 2014, 10:51:22 am »
Try hookup with real load like < 20 watt CFL lamp and see if the voltage still within spec.

Offline Greg Robinson

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Re: Isolation transformer
« Reply #10 on: January 19, 2014, 11:07:25 am »
You're not taking into account regulation. Without a load, any isolated transformer will give a higher output voltage. You've got to consider the source impedance, which forms a voltage divider with the load. Larger transformer will have a lower source impedance, so will have better regulation.
 

Online Zero999

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Re: Isolation transformer
« Reply #11 on: January 19, 2014, 05:59:35 pm »
Quote
The transformer will be rated for supplying the correct voltage at its stated load. Off-load, the transformer output will rise. Look-up transformer regulation. Typically, a 50VA transformer will have a regulation of 10-15% depending on manufacturer and design etc.. i.e., off-load, the output may rise up to 15% of its rated value. Smaller transformers are generally less well regulated.

What Andy Watson said.

If you look at the spec it 'seems to imply' 133.1v on no load per secondary.
Yes it's the regulation and when the fact the input is 234V is taken into account it makes sense.
 

Offline IanB

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Re: Isolation transformer
« Reply #12 on: January 19, 2014, 06:11:26 pm »
To document the output voltage calculation for clarity:

From the datasheet the unloaded voltage is: U0 = 133.1 V

For both secondaries in series we have 2U0 = 2 x 133.1 = 266.2 V

But this voltage is with 230 V on the primary. If the input voltage is 234 V then the output voltage would be:

234/230 x 266.2 = 270.82 V

In the OP, Gath measured the voltage as 270 V. So it would seem the transformer is performing exactly to specification.
 

Offline tridentsx

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Re: Isolation transformer
« Reply #13 on: January 19, 2014, 06:23:05 pm »
Weird thing is, the transformer has a "passed" sticker on it for the manufacturer, So I assumed they actually tested the thing before shipping it out.
I understand why you might think that, because that's how things work in Switzerland in my experience. But if the product was manufactured elsewhere, all that sticker means is that the unit passed by a guy who stuck a sticker on it.

Its made in Sweden I think that it should be very high quality, all transformers I used that were Swedish made has been very good.
 

Offline GathTopic starter

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Re: Isolation transformer
« Reply #14 on: January 20, 2014, 08:50:23 am »
All right, so this is what this U0 means on the datasheet! Unloaded output voltage. Meh, I guess they could have been a tad more explicit on this in the datasheet for those who do not sue transformers on a regular basis :on Thanks a bunch all ;)

To document the output voltage calculation for clarity:

From the datasheet the unloaded voltage is: U0 = 133.1 V

For both secondaries in series we have 2U0 = 2 x 133.1 = 266.2 V

But this voltage is with 230 V on the primary. If the input voltage is 234 V then the output voltage would be:

234/230 x 266.2 = 270.82 V

In the OP, Gath measured the voltage as 270 V. So it would seem the transformer is performing exactly to specification.

Yep this totally adds up, and matches what I see. I'll test with a 200mA load later and let you guys know. Just for completeness of this thread.

You've got to consider the source impedance, which forms a voltage divider with the load.

This sentence pretty much sums this up. Real components are not ideal (duuh ...)

Weird thing is, the transformer has a "passed" sticker on it for the manufacturer, So I assumed they actually tested the thing before shipping it out.
I understand why you might think that, because that's how things work in Switzerland in my experience. But if the product was manufactured elsewhere, all that sticker means is that the unit passed by a guy who stuck a sticker on it.

Its made in Sweden I think that it should be very high quality, all transformers I used that were Swedish made has been very good.

Just like tridentsx says, When I see made in Sweden (and alike), and a "passed" sticker, I have a tendency of questioning myself first when a problem arises. These guys manufacturing the product probably know their sh*t better than I could. Now, when it's made in China, ... this is a whole different story...
« Last Edit: January 20, 2014, 09:00:30 am by Gath »
 

Offline GathTopic starter

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Re: Isolation transformer
« Reply #15 on: January 20, 2014, 08:57:01 pm »
Hey guys,

Here an update on on how the transformer I have is dealing with various loads.
I bought some light bubs (actually, they are halogen lamps fitted in a light bulb, rated for 30W, 46W and 57W), and a simple Eco lamp (fluo type, 5W).
I used them either alone, or in series to create various loads for the secondary of the transformer. This is what I measured :

Lamp(s)Output Voltage Vo (V)Io Output current (A)Power (W)Requivalent (Ohms)
No Load270.10.0000.0--
5W266.30.04812.85548
30W -- 30W256.70.10025.72567
30W -- 46W -- 57W254.60.10827.52357
30W -- 46W254.20.11830.02154
30W -- 57W253.30.12631.92010
30W250.30.14436.01738
46W -- 57W247.80.16440.61511
46W240.30.21150.71139
57W232.30.26260.9887
I have added in attachment the graph plotted out of these data, for those of you interested. It appears pretty linear :D.
Turns out I can compute (approximately) the expected output voltage for a given load current simply using Vo = (1000*(1.83-Io))/6.72

Just to remind quickly the conditions:
  • Transformer: Noratel AA53129 50VA, Prim: 230VAC, Sec: 2x115VAC, 133.1VAC on each windings with no load (from datasheet)
  • Both secondary windings are connected in series
  • and the measure voltage at the input (mains) is 234VAC@50Hz

Quite frankly, I still find this thing weird. I was somehow expecting to have something that would provide 230VAC over a given range of output currents.
I guess you need some output circuitry for this... Well I guess this will keep me busy for the next evenings ;)
 

Online Monkeh

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Re: Isolation transformer
« Reply #16 on: January 20, 2014, 09:06:33 pm »
Quite frankly, I still find this thing weird. I was somehow expecting to have something that would provide 230VAC over a given range of output currents.

Measure your mains voltage at different times of day.
 

Offline IanB

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Re: Isolation transformer
« Reply #17 on: January 20, 2014, 09:28:16 pm »
I have added in attachment the graph plotted out of these data, for those of you interested. It appears pretty linear :D.

That kind of linear output response seems somewhat typical. For comparison, here is a graph I plotted a couple of years back for a completely different transformer, one marked 12 V 2 A on the secondary:



Note that the open circuit voltage is about 25% higher than the rated voltage at the expected load.

The model for this would be a 15 V AC source in series with a 1 ohm resistor.
 

Offline HackedFridgeMagnet

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Re: Isolation transformer
« Reply #18 on: January 20, 2014, 11:58:42 pm »
Quote
Quite frankly, I still find this thing weird. I was somehow expecting to have something that would provide 230VAC over a given range of output currents.

I guess the good thing is that most 230v rated equipment doesn't expect or need a tight tolerance on the input voltage.

So normally it is not an issue.
 


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