Autotransformer wiring - Page 1
Forum section: electronics / beginners

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 14, 2016, 01:12:28 pm
content: Hi all,I've been given an autotransformer from a friend that is supposed to convert 220v to 110v.Friend got it from god knows where and never used it.On the transformer body there are two labels.One that says 220-110V 1500watt and another 2x110v.This is definitely an autotransformer because I get continuity in all cables except the ground lead so I won't be using it with any hot chassied equipment.One plus is that it has a GND connection so I can wire two plugs (EU and US) with ground connectors .The problem is that I'm not sure about the wire colors.Yellow cable gives me 0ohms to the transformer chassis and infinite resistance with all the other cables so that's GND.The rest are as follows:BLK-BROWN 0.3 ohmsRED-BLK 0.3 ohmsBRN-BLK 0 ohmsI would expect black to be neutral and common between the 220v input and 110v output but I would also expect a pair of leads to have higher resistance than another but black and brown pair has the exact same resistance as the red and black pair.Following an older color scheme one would except red to be 220v hot but I don't know the age of the transformer so I can't be sure.I tried powering the transformer with a lightbulb in series with the red cable and black as neutral ;the lightbulb did not lit but I got no output against brown and black..edit:I forgot to mention that these four leads are the ONLY connections the transformer has and they come out in the front as seen in the picture.

author: Seekonk
author_rank: Super Contributor
timestamp: November 14, 2016, 02:33:43 pm
content: BLK-BROWN 0.3 ohmsBRN-BLK 0 ohmswhat about YELLOW. I assume that this is just a center tapped transformer. Kudos on using a lamp. It is easy to see the saturation voltage as the peak flat tops on a scope if you have enough voltage without damaging it.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 14, 2016, 06:03:04 pm
content: YELLOW has only continuity with the transformer chassis and infinite resistance(not connected) with all the other leads.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 14, 2016, 06:06:35 pm
content: Quote from: Seekonk on November 14, 2016, 02:33:43 pmThe 4 wires you see in the picture are the ONLY connections the transformer has.There is no 220V in in the other side so I don't think it's a center tapped like 110-0-110.That was my first thought too when I saw the 4 wires and the 2x110 label but no other connections indicate that's something else.

author: Seekonk
author_rank: Super Contributor
timestamp: November 15, 2016, 08:27:30 pm
content: Quote from: Seekonk on November 14, 2016, 02:33:43 pmAren't those the same too wires? You have three transformer winding wires and ground. What am I not seeing. RED BLACK BROWN, one of those has to be a center tap.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 15, 2016, 09:08:35 pm
content: Yes I posted the same pair of wiresred-brown are 0.3ohms and red-black are also 0.3ohms.black and brown are a dead short.(0ohms)Center tap is the 110v output?CT should have exactly or close to half the resistance of the other two right?Makes sense to be 0ohms if the other two are 0.3 but what about neutral?I mean whatever the brown is the black should be as well since they're shorted.That only leaves me with one input (maybe the red) and no clue what the brown/black are..The last time I checked the transformer I replaced the bulb I was using with a halogen lamp and the lamp unfortunately lit in the two or three instances I tried.Since we have four wires and two of them are a dead short that leaves two wires.One is GND (yellow).So there only one left!I tried black as neutral ,red as 220v input and the brown left unconnected.Withe the lamp and a switch in series with the hot the lamp lights up and I get no voltage on the brown (some millivolts maybe)I also tried without the lamp and I tripped the mains..Also tried brown as the input and red as the output keeping black as neutral with the same results..Maybe I don't understand something correctly or the transformer is faulty somehow..

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 15, 2016, 09:53:18 pm
content: Update:It seems like the transformer works but I've yet to figure to out how to do it properly.I tried connecting only the hot to the black.No lamp.I get 100 volts ac between the brown and the yellow.I also get 100 volts between the red and the yellow.So that's the 2x100 on the transformer label.I don't know what to do with the mains neutral though.When I tie it to the transformer yellow I get 220V on the brown and the red..

author: helius
author_rank: Super Contributor
timestamp: November 15, 2016, 10:29:32 pm
content: An autotransformer has all its inputs and outputs connected together, just at different phases. Since you want to convert down from 220 to 110, you will be splitting the phase angle into two parts, 180° apart:The 220V input is on 'A' and 'B': say that 'A' is 220V hot and 'B' is input neutral.The output is on the same wires, but uses the center tap to split the phase angle. So one of the 110V outputs hot is connected to 'B', with neutral at 'C' (note well that the "neutrals" of input and output are not the same potential! You need to use an isolating transformer if these can be connected together through cables). The other 110V output hot is connected to 'A', with neutral at 'C'. The potential between the "hot" pins of the two 110V outputs is 220V: they precisely split the phase of the input.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 15, 2016, 11:36:57 pm
content: Thanks for the explanation helius but I still don't quite follow what's happening on this particular transformer regarding the neutral.I have another autotransformer (220 to 110) and it works like this diagram.It only has one output though and I've found it much easier to connect to mains.This one though is a bit trickier to understand because of the 2x110 out and the neutral configuration.To my understanding this particular transformer needs to be completely isolated from the mains neutral and ground in order to work.If I connect the neutral to the transformer yellow lead (which is disconnected from everything else except from the xformer chassis) I get the same potential the neutral has to the hot (220v) to all three leads (brown,red and of course black which is already on the mains hot).Leaving the mains neutral unconnected and only the hot connected I get the 110 on the brown and the red when in respect to the yellow lead.So somehow the mains neutral seems to "cancel the transformation" because it shares its 220 potential against the mains across the transformer?I do have an isolation transformer but I wouldn't want to tie it with this transformer just to connect its "isolated neutral" to mains neutral.From what I've gather so far and in order to wire this particular transformer I should leave my mains neutral unconnected?And then wire two US outlets with brown/red as 110 hot and the yellow as shared neutral between them?My concern is that the transformer chassis in this configuration is not earthed and the transformer input is also floating in respect to the mains ground AND neutral.

author: jonovid
author_rank: Super Contributor
timestamp: November 16, 2016, 12:26:10 am
content: my autotransformerhere is a 1000w microwave oven transformer the primary winding taps at 120v 220v 240v . the secondary windings LT 18v winding. and the deadly HT 1KV winding - I will cut this off. my plan is to use it as a step down transformer. add ground tabs and input fuse. I plan to use a old computer ups power supply case for a box. tapping the primary 0v and 240v as input & also tapping the same primary 0v and 220v as output. this transformer also can have a US 120v outlet too if I need one. does anyone think the re-use of microwave oven transformers is a good idea? if the earth leakage test is ok. may just add a 200amp 5v winding for spot welding maybe as a multifunctional unit. ?

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 16, 2016, 03:07:53 pm
content: Any advice on how to wire the input and output power plugs on this transformer would be greatly appreciated.

author: jonovid
author_rank: Super Contributor
timestamp: November 16, 2016, 04:29:10 pm
content: Quoteif you do not know the plugs and sockets in your country or region get some onequalifiedto do the work for you.always use three pin plugs and sockets never two pin . as the transformer is a through-put so the E or earthing pin for the end appliance.use Three-pin AC output socket and a Three-pin AC input plug.Your transformer has 4 wires ? yes. 110v 110v and a CT =center tap the 4th wire is earthing. the case.so if I get it . your transformer110v and CT= 110v110v to the other 110v = 220v

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 16, 2016, 04:45:17 pm
content: Thanks for the diagrams but I actually do know how to wire a plug.I don't really know about this particular transformer though.You certainly can't just follow standard wiring because interconnected inputs and outputs and a common neutral make things a little more complicated.I still don't understand what I should do with my mains neutral without using an isolation transformer.I guess just leave it unconnected.I'm thinking for the input plug to have only my mains hot 220 connected to the BLACK and wire the YELLOW of the transformer as the neutral for both the US plugs keeping BROWN and RED as 110v hots.Of course I have to mark the outlets and plug them only with the correct polarity otherwise this won't work.Mains ground ,as I see it , cannot be connected neither to the input nor output plugs.Transformer body is tied to the YELLOW cable which acts as neutral for the 110 hot so grounding it will raise the voltage to 220..My concern is that since there is no "real" ground in any part of this ,transformer could prove dangerous if left unattended.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 16, 2016, 04:56:44 pm
content: Quote from: jonovid on November 16, 2016, 04:29:10 pm

author: jonovid
author_rank: Super Contributor
timestamp: November 16, 2016, 05:06:57 pm
content: did an update to the other postQuotelooking at you autotransformerit says 110 x2 so that's 110v plus 110v total 220vthe neutral is the CT 3 wiresthe 4 wire is ground

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 16, 2016, 05:19:45 pm
content: The transformer outputs 120v to two leads(red/brown) in respect to the transformer ground(yellow) when the 4th lead(blk) is connected to the 220 hot.So yes 4 wires.The CT is the input for the 220v or the output for the 120?I don't get the :120v and CT= 120v120v to the other 120v = 220vIf I measure voltage between each of the two 120v wires and the "hot 220" I get 0 volts so I guess they're in phase?

author: jonovid
author_rank: Super Contributor
timestamp: November 16, 2016, 05:33:34 pm
content: have you considered the autotransformer has a fault? 3 wires must be linked for the transformer to be working.if you only have two wires linked. the 3d wire is faulty.phase? its a single phase transformer. get a 2nd opinion. on that one.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 16, 2016, 05:48:50 pm
content: I'm not sure if it's faulty or not that's why I started this thread.In order to verify its operational state."Faulty" is a general term without actually specifying where the problem lies,right?At this point I can't confirm if it has a fault since I haven't fully understood how to connect it to the mains.I've had a couple of autotransformers before and none had put me in so much trouble but none of them was like this one.If you've understood how each wire should be connected (in this particular case not in a autotransformer in general) could you maybe it share it in order to confirm if it has a fault and where?

author: Ian.M
author_rank: Super Contributor
timestamp: November 16, 2016, 06:12:31 pm
content: So far, it seems likely that it has a shorted winding. Normally a 2:1 / 1:2 autotransformer has two windings of equal resistance with one terminal commoned, so taking the three terminals in pairs, you get two pairs with resistance X between them and one pair with 2X. Neutral in == Neutral out (anything else is unsafe) and must connect to one end of the pair of windings. The live connections are to the center tap and the far end of the windings, and the center tap live is the lower voltage one (so for bucking 220V to 110V, its the output).Take the end bell off the transformer and inspect the windings for signs of overheating. If you are lucky, you'll find the short and fix it. If not, it may be possible to determine the center tap by inspection. However if you cant find any pair of wires that has DC continuity and can be placed in series with a 60W incandescent bulb powered from the mains without the bulb lighting significantly, its either FUBARed or *NOT* a mains autotransformer.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 16, 2016, 06:21:24 pm
content: Okay.I'll try to take it apart and see what I find.*Update;Look at the state of this thing!Looks like's been set on fire..If I decide to keep it after all I think I'll have to do something with all that burned paper.The enameled copper wire seems okay ;although the leads where the cables were connected were pitch black and almost carbonized.Anyway,I cut the molten wire mess inside and the ground lead and I'm left with three leads coming straight out of the core.The center tap looks like two wires joint together.I used big solder blob to melt the enamel and now I have three tinned leads with resistances:Left -Right 0.7ohmsLeft-Center 0.3 ohmsRight-Center 0.3ohmsThat seems correct for a CT transformer.Connecting mains in series with the lamp to the far end terminals (far left and right) lights the lamp very dimly ..

author: Ian.M
author_rank: Super Contributor
timestamp: November 16, 2016, 11:30:39 pm
content: What's the other side look like? If the scorching is superficial due to arcing between the leads, with no signs of overheating the other side, it may be salvageable, but if the whole winding is cooked, its unsafe to use.Ignore jonovid - you already know the phasing is correct because it passed the dim-bulb test. If the winding isn't cooked, a soak test for a couple of hours with a fused supply to the winding end via a thermal trip in contact with the core and a 1200W (80% of nominal max) 115V resistive load (A 1KW 230V bar fire element is 250W @115V) on the CT followed by stripping the burnt insulation and rebuilding it with fishpaper, phenolic glue and transformer varnish would be the way forward.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 17, 2016, 12:00:53 am
content: The other side looks promising.I peeled some of the paper insulation off while trying to remove the bell (one screw is still stuck in there so I forced a bit to open ).I plan to replace the outer insulation since the inner is between windings so nothing I can do there.I think most of the black color on the leads side is on the paper and not on the wire.I went through the core with one probe of a meter with the other attached to each lead and nothing is shorted so it's indeed superficial.On the back side I accidentaly scratched about 30-40mm of the insulation/coating one one wire but it's on the outside and only on that wire so the outer insulation should take care of this.I'll try to source the fishpaper,varnish and glue and some kind of heavy load to test the transformer.BTW I saw some transformer tapes on eBay.Since I never did anything similar may I ask if one of these could suffice for this job?Some kind of tape could be a tighter fit and easier to handle but I don't know about proper airflow and how safe that could prove.edit:Prices on the bar fire elements are a bit on the high side I think..And I don't think I can find any used ones.What about 10x100w incandescent lamps in series ?

author: jonovid
author_rank: Super Contributor
timestamp: November 17, 2016, 12:10:43 am
content: it looks a cooked to me . do the sniff test

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 17, 2016, 12:16:23 am
content: The smell was awful when I first opened the casing but I did the even closer sniff test and I think it's mostly the paper.Cooked transformers have a different kind of smell ;the one similar to burned chips and exploded ceramic caps have..Will see after testing..I don't think I'm ever going to use this with loads more than 500 watts total since I don't have many devices that run on 110v but if it's actually proven okay it'll be nice to know it's there..

author: Brumby
author_rank: Unknown
timestamp: November 17, 2016, 02:12:55 am
content: For the moment, I suggest you forget about 'Neutral' and 'Hot' connections for the moment - and justcategoricallydetermine the configurationand stateof the transformer.Since you have pulled the cover off, I take it you have confirmed the yellow was, indeed, connected to the case. That is good. We can forget about that one for the moment.We now have only 3 connections to worry about - and from theprintedlabel, it is clear we should expect an autotransformer inside. Thehand writtenlabel noting 2 x 110V is a bit misleading - and while I could describe how someone might put it there, it is just going to muddy the discussion ... so let's just ignore that for the time being.However .... about those 3 connections.....Quote from: Ian.M on November 16, 2016, 06:12:31 pmThis is something I have felt ever since this comment:Quote from: belzrebuth on November 15, 2016, 09:08:35 pmThat just isn't right for a CT transformer with two equal halves. You were absolutely right when you said:Quote from: belzrebuth on November 15, 2016, 09:08:35 pmBut then you added:QuoteNo. It doesnotmake sense. Just try drawing two resistors joined at one point to give the measurements you have observed. (I think I can see where your statement comes from - and if I do, you have swapped a couple of things around.)To me, the only configuration that makes sense of those readings is if there is a dead short across a whole half of the winding (ie between two connections.)Now that you have opened the can and we can see the transformer windings, the physical construction would strongly suggest the inner and outer connections are the ends of the winding with the one in the middle being the centre tap. That being the case, if I were to label the connections as I, C and O for inner, centre and outer) we should have:RIC= ROCRIO= RIC+ ROCHowever, at these low levels, I am hesitant to rely on such resistance measurements with a regular DMM. What I would suggest is applying a voltage - with some current limiting - across the inner and outer terminals and take somevoltagemeasurements. You are going to get much better resolution trying something like this - and then doing a bit of arithmetic:

author: Zero999
author_rank: Super Contributor
timestamp: November 17, 2016, 10:54:46 am
content: Quote from: jonovid on November 16, 2016, 12:26:10 amStart your own thread, rather than hijacking someone else's.

author: SeanB
author_rank: Super Contributor
timestamp: November 19, 2016, 10:10:06 am
content: Now it is open the wiring is easy. The innermost connection is neutral for 115VAC and 230VAC, the middle one is the 115VAC output and the outer is the 230VAC input.Smell and well warmed are kind of typical for these, they tend to be left on for decades and almost always have a mild overload on them, thus they run hot and the paper slowly cooks. A coat of transformer varnish ( plain polyurethane varnish and dunk it for an hour, then let air drip dry for 24 hours followed by a bake in an oven at 70C overnight - note do not do this in an oven you ever will want to cook in again, you never will get the smell out, get a cheap small oven on sale and use it) and it will work well again.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 19, 2016, 10:33:16 am
content: Isn't that varnish too flammable for this purpose?Also ,what about its thermal properties?I've seen whole power supplies (some old commodore 64 ones) and transformers dipped in epoxy but it said that epoxy (once cured) has thermal dissipation qualities but I don't know about plain varnish..

author: SeanB
author_rank: Super Contributor
timestamp: November 19, 2016, 11:10:03 am
content: The wire insulation is pretty much the same polyurethane, and all transformers are coated in it. The solvent is the flammable part, and once you evaporate it all off ( that is the 24 hour air dry) you can bake it to a hard finish in the oven.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 19, 2016, 11:15:36 am
content: Okay!Thanks for the clarification.I will try that once I find an oven cheap enough for the job.

author: Brumby
author_rank: Unknown
timestamp: November 19, 2016, 11:23:53 am
content: Quote from: SeanB on November 19, 2016, 10:10:06 amThat's how I would suggest it gets connected - but, please, CHECK the thing.I seriously feel there is a short. If there is one, an oven isn't going to help.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: November 19, 2016, 11:33:36 am
content: I already connected the transformer as suggested above and it works as expected so no shorts.The short described in my earlier posts and the associated confusion caused by a molten wire knot that acted as a strain relief for the cables leaving the tranformers' windings ends inside the bell of the transformer.The transformer probably heated to an extreme and the caused the insulation of the big wires to melt so they shorted.Once I cut the wires leaving the transformers' leads, the windings' resistances read fine.

author: Brumby
author_rank: Unknown
timestamp: November 19, 2016, 11:35:24 am
content: AH - Good news.