Isolation transformer - Page 1
Forum section: electronics / beginners

author: Gath
author_rank: Contributor
timestamp: January 18, 2014, 07:47:49 pm
content: 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(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

author: IanB
author_rank: Super Contributor
timestamp: January 18, 2014, 08:33:32 pm
content: What is the measurement of each secondary individually?Also how does the voltage change if you add a load?

author: denelec
author_rank: Regular Contributor
timestamp: January 18, 2014, 08:47:41 pm
content: 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.

author: Gath
author_rank: Contributor
timestamp: January 18, 2014, 09:21:18 pm
content: Quote from: IanB on January 18, 2014, 08:33:32 pmI 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.Quote from: denelec on January 18, 2014, 08:47:41 pmIndeed 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.

author: IanB
author_rank: Super Contributor
timestamp: January 18, 2014, 10:14:13 pm
content: 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.

author: Len
author_rank: Frequent Contributor
timestamp: January 19, 2014, 01:33:21 am
content: Quote from: Gath on January 18, 2014, 09:21:18 pmI 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.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 19, 2014, 02:27:58 am
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: January 19, 2014, 09:53:19 am
content: Try running it in reverse then, it will then give a lower output. Probably around 210VAC then.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: January 19, 2014, 10:49:49 am
content: QuoteWhat Andy Watson said.If you look at the spec it 'seems to imply' 133.1v on no load per secondary.

author: BravoV
author_rank: Super Contributor
timestamp: January 19, 2014, 10:51:22 am
content: Try hookup with real load like < 20 watt CFL lamp and see if the voltage still within spec.

author: Greg Robinson
author_rank: Regular Contributor
timestamp: January 19, 2014, 11:07:25 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2014, 05:59:35 pm
content: Quote from: HackedFridgeMagnet on January 19, 2014, 10:49:49 amYes it's the regulation and when the fact the input is 234V is taken into account it makes sense.

author: IanB
author_rank: Super Contributor
timestamp: January 19, 2014, 06:11:26 pm
content: To document the output voltage calculation for clarity:From the datasheet the unloaded voltage is: U0= 133.1 VFor both secondaries in series we have 2U0= 2 x 133.1 = 266.2 VBut 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 VIn the OP, Gath measured the voltage as 270 V. So it would seem the transformer is performing exactly to specification.

author: tridentsx
author_rank: Regular Contributor
timestamp: January 19, 2014, 06:23:05 pm
content: Quote from: Len on January 19, 2014, 01:33:21 amIts made in Sweden I think that it should be very high quality, all transformers I used that were Swedish made has been very good.

author: Gath
author_rank: Contributor
timestamp: January 20, 2014, 08:50:23 am
content: All right, so this is what this U0means 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 allQuote from: IanB on January 19, 2014, 06:11:26 pmYep 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.Quote from: Greg Robinson on January 19, 2014, 11:07:25 amThis sentence pretty much sums this up. Real components are not ideal (duuh ...)Quote from: tridentsx on January 19, 2014, 06:23:05 pmJust 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...

author: Gath
author_rank: Contributor
timestamp: January 20, 2014, 08:57:01 pm
content: 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)IoOutput current (A)Power (W)Requivalent(Ohms)No Load270.10.0000.0--5W266.30.04812.8554830W -- 30W256.70.10025.7256730W -- 46W -- 57W254.60.10827.5235730W -- 46W254.20.11830.0215430W -- 57W253.30.12631.9201030W250.30.14436.0173846W -- 57W247.80.16440.6151146W240.30.21150.7113957W232.30.26260.9887I have added in attachment the graph plotted out of these data, for those of you interested. It appears pretty linear.Turns out I can compute (approximately) the expected output voltage for a given load current simply using Vo= (1000*(1.83-Io))/6.72Just 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 seriesand the measure voltage at the input (mains) is 234VAC@50HzQuite 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

author: Monkeh
author_rank: Super Contributor
timestamp: January 20, 2014, 09:06:33 pm
content: Quote from: Gath on January 20, 2014, 08:57:01 pmMeasure your mains voltage at different times of day.

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2014, 09:28:16 pm
content: Quote from: Gath on January 20, 2014, 08:57:01 pmThat 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.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: January 20, 2014, 11:58:42 pm
content: QuoteI 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.