Identifying unknown transformer voltages - Page 1
Forum section: electronics / beginners

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 23, 2016, 11:40:45 am
content: Hi all,I came across a beefy transformer today at a flea market and thought about trying my luck buying it.I brought it home but didn't try to power it up because I'm not really sure of the wiring it looks like an isolation transformer but the colors of the cables don't make much sense to me.As you can see from the picture it has blue/brown (neutral/hot) on one pin of the primary(?) and two blacks (hot again) on the other pin.On what appears to be primary again (no wiring on the other side) I see two green cables (gnd?!) and one blue.I get 0.005 ohm between the green and green and blue green and 0.000 between either black brown black/blue.I really don't know how to further test this without powering it up but since I'm a bit unsure on how to connect it to the mains I'd like to hear some opinions about it.I never had any problems with the color coding of mains cables on unknown transformers but this one is really all over the place and it has nothing written on its label to make matters worse.I was thinking to remove the screw terminals and their wires and just connect blue (neutral) and brown(hot) on the two pins where black/brown/blue are and just what voltages the other two pins are outputting (where green/blue cables are).

author: bktemp
author_rank: Super Contributor
timestamp: October 23, 2016, 11:51:05 am
content: I wouldn't trust any wire colours on this transformer.5mohms and 0mohms sounds wrong, except for some very low voltage windings. It is hard to guess the size of transformer from the picture, but I would expect >1ohm for the 230V primary winding for transformers <1kVA.

author: SeanB
author_rank: Super Contributor
timestamp: October 23, 2016, 12:01:49 pm
content: Most likely a step down transformer. Is there any text in those labels, as there you will find the info, but the picture has this all washed out.Looks to be around 100VA, so you can use the old trick of using a 40W incandescent lamp in series with the mains, or another 12VAC transformer ( not electronic, good old iron and copper) and apply to the one terminal with a series 21W car light bulb in series, and measure the actual voltages on the terminals and then you can work out both the ratios and which is the primary as well to use it.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 23, 2016, 12:04:55 pm
content: DC resistance won't tell you anything useful.Low voltage AC will give you ratios.Looking at relative wire diameters of windings will sometimes give some indication - primary will typically be the thinnest.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 23, 2016, 12:10:17 pm
content: ?he label is indeed washed out;it's not the picture.Wire gauges are the same all around.Looks like an isolation transformer to me but not sure of what should go where.I guess where the two blacks are should be the input voltage..Resistances are 0ohms and 0,5ohms not megaohms..

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 23, 2016, 12:16:37 pm
content: Sorry it's 5ohms and 0ohms.Picture next to a multimeter should give you an idea of the size.

author: SeanB
author_rank: Super Contributor
timestamp: October 23, 2016, 12:32:37 pm
content: Left side is primary, probably 220 or 400VAC, and the right side is 24VAC most likely, as this is a typical industrial voltage. So apply 220VAC to the left via a 40W light bulb as limiter, and it will probably have around 18VAC on the secondary.

author: bktemp
author_rank: Super Contributor
timestamp: October 23, 2016, 12:38:41 pm
content: Quote from: belzrebuth on October 23, 2016, 12:16:37 pm5ohms is more in the expected range than 5mohms.If you give me exact measurements of the transformer dimensions (size of the iron core) and the measured input + output voltage, I can give you a fairly accurate estimation of the power rating.

author: Ian.M
author_rank: Super Contributor
timestamp: October 23, 2016, 12:45:44 pm
content: I'm with SeanB. Use a series 40W (or possibly 60W) bulb. Which *MUST* be incandescent.If you find the primary (which *should* be the highest resistance winding, assuming its a step-down transformer) the bulb will be out or at least glowing so faintly that you cant see it in daylight, and you can measure the secondary voltage. If you accidentally connect to the secondary, it will saturate and the bulb will light. If that happens, disconnect and swap windings.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 23, 2016, 01:39:55 pm
content: Quote from: bktemp on October 23, 2016, 12:38:41 pmThanks!Dimensions are 12 x 10 x 5 cm.Guessing that the grey screw terminal is primary I connected a 23w lamp (all I had) in series with the mains to the one green/yellow and the other green/yellow.Lamp did not light so assumption correct!I get voltage only if I putonethe multimeter leads to one of the black cables.20vac but it varies slightly.Any idea why that happens?Blue and black give me about 0 volts ac..

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 23, 2016, 01:51:07 pm
content: I was wrong.One terminal of the secondary gives me about 16vac and the other about 25vac..With both terminals connected to my multimeter I get no voltage.I really don't think I've encountered such a transformer before..All I've seen so far had two leads for every of their secondary voltages..

author: bktemp
author_rank: Super Contributor
timestamp: October 23, 2016, 02:03:29 pm
content: Quote from: belzrebuth on October 23, 2016, 01:51:07 pmWhat was the second terminal where you measured those voltage? One of the primary connections?There must be something wrong with the measurements:If you measure 16V on one terminal and 25V on the other terminal, there can't be 0V between both terminals.What is the voltage across the primary?And is there any current flowing?

author: Ian.M
author_rank: Super Contributor
timestamp: October 23, 2016, 02:26:13 pm
content: Remove that crappy terminal block and wires so you can probe the transformer's terminals directly. As you remove it, please confirm that it was wired to SEPARATE terminals on the transformer Draw a diagram of the winding terminals and check the DC resistance to each of the other terminals. Mark which have continuity to each other. If you only find two terminals with continuity, its either a choke or its FUBARed because a winding has gone open circuit. If you find three or more with continuity, its an autotransformer. Its only if you find two separate pairs with continuity within each pair but not between them that it could be a conventional transformer, in working order.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 23, 2016, 02:33:00 pm
content: Removed terminal blocks and tested again.Transformer outputs 12.5vac.So all good!Thanks everybody.@bktemp If and when you have the time please let me know of the power rating.Dimensions are a few posts above.I measured the iron core.

author: IanB
author_rank: Super Contributor
timestamp: October 23, 2016, 02:58:35 pm
content: For future reference, the enclosed and shrouded nature of the grey terminal block on the left is a clear indication that it would be the mains (primary) connection. A second indication of the primary is that the winding for the grey terminal block is on the inside of the transformer (typically the primary is the inner winding and the secondary is the outer winding).

author: Tom45
author_rank: Frequent Contributor
timestamp: October 23, 2016, 03:07:59 pm
content: Anyone have an idea why the data plate was left blank?

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 23, 2016, 03:20:43 pm
content: Since I'm often on the hunt for transformers on flea markets and such it's not uncommon to see blank or missing labels.Attempting to clean some transformer labels with glass cleaning fluid or any other alcohol based mix might remove the ink.Other reason could be that the transformer is part of a bigger installation where every transformer is the same so the electrician or other person doing the installment knows what they're dealing with.Some OEM parts are also label-free ..I really don't know about this one but it's not uncommon to find a transformer without markings especially if you're shopping around in flea markets or buying stock from repair shops.I have a couple of transformers that came from Sony hi-fi systems that are also label free..

author: bktemp
author_rank: Super Contributor
timestamp: October 23, 2016, 03:30:58 pm
content: Quote from: belzrebuth on October 23, 2016, 02:33:00 pmAssuming an input voltage of 230V and a secondary copper resistance of 0.025ohms, the estimated power rating is around 200VA with an output voltage of 11.5V at full load.The core could be EI114-50. That matches the estimated power rating.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 23, 2016, 03:39:27 pm
content: Thanks!So to calculate the amps that could output @12V since it's a single phase transformer I just divide the VA/12 ?

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 23, 2016, 03:41:07 pm
content: Quote from: bktemp on October 23, 2016, 03:30:58 pmHow did you calculate the secondary copper resistance?

author: bktemp
author_rank: Super Contributor
timestamp: October 23, 2016, 05:45:56 pm
content: Estimating the power rating of a transformer is quite easy:There is no hard limit, you can draw any current as long as the transformer temperature does not get too high.So the typical power rating is the point when the temperature of the transformer increases by around 40°C without any additional cooling. I estimate the possible power dissipation based on the transformer dimensions.You get the highest power density when the power dissipation of primary and secondary windings are equally distributed. That is how typical transformers are designed.At 200VA the primary current is around 0.9A, so it dissipates 4.3W. The secondary current is 200VA/11.5V=17.4A, so the ideal copper resistance is 4.3W/(17.4A²)=14mohms. Since the secondary wire is quite thick and therefore needs a lot of space, I assumed a slightly higher resistance of 25mohms. But even with 15mohms the estimated power is only 230VA, so 200VA is in the right ballpark.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 23, 2016, 05:52:14 pm
content: Thanks for your input bktemp!Very informative.