Modding a 120VAC isolation transformer for 220VAC - Page 1
Forum section: electronics / projects

author: erikbrenn
author_rank: Regular Contributor
timestamp: July 25, 2018, 10:37:58 pm
content: I messed up buying a 120VAC isolation transformer from the US, my mains is 220V. It's a Trip-lite HG 250VA, rated at 250W/2.2AI want to modify it so it can be used with 220V. I have reverse engineered the circuit and in short:- Circuitbreaker is marked with the rating "250V/2.2A"- Caps that couple input and output are all X1 type and rated for 250 VAC- The 2-pole switch at the back is rated at 16A/250VThe transformer inside has a sticker saying 120VAC input and 123VAC output.Question:Is there anything in the transformer itself that make it unsuitable for 250V/50Hz? Can I swap the circuit breaker with a "250V/1.1A" to use it with 220VAC, and to stay within the 250W limitation?Images:(Cap values in schematic are wrong, should be 2.2uF as in photo)

author: ArthurDent
author_rank: Super Contributor
timestamp: July 25, 2018, 10:58:55 pm
content: You could use a 220VAC transformer on 120VAC without much to worry about but there is no way to use the 120VAC transformer you have as an isolation transformer on 220VAC. The only possible use I can think of is to use it as a auto stepdown transformer by connecting the windings in phase across 220VAc and get an output of 120VAC out between the neutral and the center tap of the two windings. The output capacitors could remain across the 120VAC output part of the winding but I would increase the voltage rating on the input cap that is across the line in to around 400V and also fuse the output at 2.2A just to be on the safe side.If you choose to try this make very sure that the windings are properly wired in phase or they will buck and instantly blow the fuse.https://qph.fs.quoracdn.net/main-qimg-2f810c218ff19e863d3c3ab4220f7042

author: erikbrenn
author_rank: Regular Contributor
timestamp: July 25, 2018, 11:32:40 pm
content: Thanks for input....> but there is no way to use the 120VAC transformer you have as an isolation transformer on 220VACProbably a silly question but what in the transformer windings/design makes it unsuitable for 220V? I mean it's a 1:1 transformer, so if I put e.g. 80VAC on it using my autotransformer, won't I get approx 80VAC out? If so, why won't it work for 220VAC as well..?

author: german77
author_rank: Regular Contributor
timestamp: July 26, 2018, 12:10:47 am
content: Quote from: erikbrenn on July 25, 2018, 11:32:40 pmOutput voltage it's not the problem. It's the insulation thickness if it's rated for 120VAC It's probably going to arc at 220VAC resulting in a catastrophic failure. You could probably rewind the transformer and double the insulation layers between the coils and use a wire with the correct insulation thickness.For cost and time I will buy the right one.

author: erikbrenn
author_rank: Regular Contributor
timestamp: July 26, 2018, 12:30:17 am
content: Thanks, that makes sense.Since I can't use it I might even test that arc failure theory in my garage, by using my autotransformer to gradually increase voltage to 220V while looking for smoke and measuring temp using my IR meter . I coiuld use a 220V/7W led spotlight as a load....Anyhow, so a 220V transformer would have thicker insulation between primary and secondary, and thus would have to be somewhat larger?

author: David Hess
author_rank: Super Contributor
timestamp: July 26, 2018, 12:35:20 am
content: Quote from: erikbrenn on July 25, 2018, 11:32:40 pmThe self inductance of the transformer limits the circulating current to a level below saturation so if a significantly higher voltage is applied, the transformer will saturate because the inductance and maximum supported flux is too low. The solution would be to either double the number of windings (1) or use a larger core or both. You could for instance buy a second one of the isolation transformers and wire them in series across the line so each only sees 120 volts although I do not suggest this.(1) Doubling the number of windings doubles the number of amp-turns making saturation worse but also quadruples the self inductance cutting the circulating current by 4 times so twice the voltage can be supported with the same peak flux.

author: sourcecharge
author_rank: Regular Contributor
timestamp: July 26, 2018, 12:50:00 am
content: I don't think the transformer is the problem although you might find that it is less efficient.The problem is the caps are only rated to 250V AC, usually, that's not 250Vrms AC that's 250Vp AC.The windings of the transformer should be able to hold 1a and 240Vrms, but the caps might pop if you raise the Vp AC above the rated voltage.This unit is designed to be a medical grade unit. Can't you just return it and get the proper one?If not, then if you do change out the caps, mark it permanently that it has been changed and that it is NOT a medical grade unit.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 26, 2018, 12:52:09 am
content: "It's the insulation thickness if it's rated for 120VAC It's probably going to arc at 220VAC..."That is not the problem here. A transformer has a primary winding and a soft iron core. the primary winding energy is converted to magnetic energy in the transformer core. If a transformer has, say, 480 turns on the primary, and when connected to 120VAC magnetizes the core completely, the transformer will be operating properly. If you increase the voltage on the this same primary winding the core will start to saturate causing the transformer to hum, generate heat, and the output on the secondary winding will become distorted. The current will rise rapidly and it will either blow a fuse or perhaps burn out.The insulation on the copper wire used will withstand a reasonable high voltage and in the 480 turn example I gave, on 120VAC input there will be a turn-to-turn voltage of .25VAC which isn't a problem. The insulation between primary and secondary could possibly withstand 1000V-2000V.

author: David Hess
author_rank: Super Contributor
timestamp: July 26, 2018, 01:27:25 am
content: Quote from: sourcecharge on July 26, 2018, 12:50:00 amTry applying 240 volts AC to a transformer configured with a 120 volt AC primary. The problem will become apparent quickly and possibly immediately.

author: Circlotron
author_rank: Super Contributor
timestamp: July 26, 2018, 03:18:53 am
content: A given transformer will only tolerate a maximum area under the curve for each half cycle of applied voltage. That is to say, if it is okay with 120 volts for 10mS half cycle then it will only be good at 240 volts for a 5mS half cycle. In other words, if you wanted to double the *rated* applied voltage you would also need to double the supply frequency. Failure to do this will mean the transformer will saturate at slightly more than one half cycle at 240V, pull way heaps of current and quickly burn out, even with no load on the secondary.(Simplified explanation not taking into account that magnetising current and flux lags 90 deg behind supply voltage).

author: TerraHertz
author_rank: Super Contributor
timestamp: July 26, 2018, 12:32:58 pm
content: As others are pointing out, it's not the transformer insulation that is the problem. It's the ratio of winding turns to amount of iron, and the transformer saturating.For OP, their explanations using the term 'saturation' may not really explain, unless OP googles the meaning of 'saturation' in magnetic systems.In any wire coil, 'inductance' is a measure of the coil's opposition to changes in the current in the wire. This opposition occurs because the current creates a magnetic field in whatever material (air, iron, whatever) occupies the space in and around the coil. If the current changes, the magnetic field changes too. But that changing field induces an opposite EMF (electromotive force) in the coil, opposing the direction of change of the current.You can think of inductance as like a resistance to alternating current.However this only works so long as the medium surrounding the coil is able to carry a magnetic field 'current' in proportion to the applied current times turns of the coil. Iron can support a lot more magnetic field than air, because the iron atoms can align their nucleus magnetic poles to the field. (Not _really_ an explanation, I know. Better just accept that bit.) Anyway, a consequence of how iron carries a magnetic field is that there's an upper limit. Once all the iron atoms are aligned, increases in magnetizing force (current x turns on the coil) don't achieve proportionally more magnetic field. Effectively the incremental inductance becomes more like air, ie pathetic. And so the opposition of the coil to further increases in current falls to a very small value. The coil is said to be 'saturated', and coil current increases to very high values. Now limited mostly by the plain DC resistance of the copper winding, which will be just a few ohms.All mains transformers are designed to operate with a good safety margin below saturation of the iron, over the full range of output loading. Doubling the applied input voltage will almost always widely exceed that margin, the transformer iron saturates, input current becomes very high, and either a circuit breaker trips or the transformer rapidly overheats and burns out.However... ha ha, I notice the manufacturer's name on the transformer label is "YOUWON". Which means it's likely made in China. They produce for worldwide markets, so maaaaybe it's actually a 240VAC transformer, labelled for US market. But they also like to minimize costs (less iron and copper) so it might just as well be under-rated even for 120VAC.You know what? Why don't you just try plugging it in? It has its own circuit breaker... See if it trips. Listen for a heavy hum. Measure the input AC current if you can. If it keeps running, is it getting hot with no load?If it isn't saturating, then you did win. Otherwise you have wasted your money.Incidentally, the 60Hz/50Hz difference is a potential problem too. At the lower 50Hz a transformer will reach a higher magnetic field in the iron than for the same voltage at 60Hz. The difference can be enough to exceed a 60Hz rated transformer's designed operating margin below saturation, and for that reason alone the transformer saturates and overheats.Anyway, if you are lucky and it runs OK, you could omit the capacitors, they are superfluous. An attempt to slightly improve mains power factor (I vs V phase) and transient damping. The mini circuit breaker - yes, change it to a lower current one if available.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 26, 2018, 12:36:26 pm
content: If they're cheap enough, get a second one and connect the primaries and secondaries in series.

author: sourcecharge
author_rank: Regular Contributor
timestamp: July 26, 2018, 01:27:27 pm
content: @34:30Isolator Series Dual-Voltage 115/230V 600W 60601-1 Medical-Grade Isolation Transformerhttps://www.tripplite.com/dual-voltage-transformer-medical-grade-isolation-115-230v-600w-6-outlets~IS600HGDVDouble check your schematic:@5.30

author: erikbrenn
author_rank: Regular Contributor
timestamp: July 26, 2018, 08:44:51 pm
content: Thanks a lot for the answers to all of you, especially TerraHertz’s long answer…I found the explanation about transformation core saturation intuitive, especially after remembering that an inductor has a magnetic field that opposes changes in current. So obviously when this field is saturated it can no longer oppose anything, so you basically end up with a short.I’m hoping that the transformer actually supports 240VAC as well, as suggested by TerraHertz. I have 1000w variac so I guess I will give it a go.As with regards to your last posting “sourcecharge”, I did see Todd’s video some time back, and thought the problem was just the groundwire from the wall input to the ground on the output sockets, which defeats the purpose of an isolation transformer. So I was planning to remove that.However, looking at the video again I saw, to my surprise, that the N output of the transformer (white wire) is earth referenced at the output connector (arced green-yellow wire in photo below). That made no sense to me at first. But after some research I have learned that the US, and most of the world, uses a TN mains ground configuration, where neutral-N and protective earth-PE are the same (split from PEN leader coming from power supplier). So doing the same on the output of this unit makes sense in that context. But where I live (norway) 80% of houses have local grounding system (IT mains grounding configuration), and N is hot (approx 130V ). Not thatthatwould cause a problem if I connected the unit to my mains. AS the N output is isolated it just means that the N output would be referenced to my mains ground.Anyhow, here is what I plan to do, comments would be appreciated- remove the ground connection from the chassis to the output connector (input connector will still be grounded to chassis of course)- remove the ground reference from the N output connector (as in Todds video)- remove the X1 cap on the input and the two X1 caps on the output as they're voltage rating is insufficient. Also, I suspect their purpose is simply to satisfy HG rating, which is not important for my hobby lab.- Connect the unit to my variac and gradually increase voltage from 120VAC to 240VAC, without any load on the output.- While increasing the voltage slowly I will monitor current flow to the primary using the CAT-II/10A connector on my standard multimeter, and monitoring the output voltage of the primary using another multimeter. I will also monitor transformer temparature using my IR meter- If that's successfull I will add a 220V halogen load to the output and monitor things for a while.I realise that economically speaking I should just get a proper 220VAC rated transformer instead of going through all this. But where is the fun in that, I’ve already learned a lot…

author: erikbrenn
author_rank: Regular Contributor
timestamp: July 26, 2018, 09:58:13 pm
content: Another thing. I measured the inductance of the primary and secondary to both at around 80mH (not sure how accurate my Atlas LRC meter is though).Hence, reactance of the primary is 25 ohms at 50Hz, and 30 ohms at 60 Hz.Is there a way to actually calcuate at what voltage and frequency a transformer will saturate?

author: Zero999
author_rank: Super Contributor
timestamp: July 27, 2018, 10:13:23 am
content: Quote from: erikbrenn on July 26, 2018, 09:58:13 pmThe figure for inductance is wrong.A reactance of 30R would give a magnetising current of 4A at 120V, which is far too high for a small transformer.Are you sure it's a 60Hz mains transformer?Perhaps the LRC bridge is giving the wrong reading, because the test frequency is far too high. I've just measured the inductance of the primary of a small 230V 50Hz toroidal mains transformer with my meter and it read 4.4H. I then connected it to the mains, measured the magnetising current to be 5.72mA, which at 246V 50Hz, is an inductance of 137H. The DC resistance is 35R, which will make little difference to the measurement.Connect the primary of the transformer to a lower voltage (under 110VAC), mains frequency source, measure the voltage current and calculate the inductance, rather than using a bridge, which probably works at too higher frequency.

author: erikbrenn
author_rank: Regular Contributor
timestamp: July 29, 2018, 12:28:25 am
content: I did the test, and the transformer is not usable for 220V.I sat my Variac to 120VAC and when I switched on the transformer it was already humming quite a bit, so it's not suited for 50Hz obviously. The current was 0.4A. I raised the voltage slowly and already at 128VAC the current was 1A and the humming had increased significantly. So no point going further...I have no use of this unit, if anyone in US is interested in paying the shipping costs ($50-$60) I will happily give it away. The reseller offered to take it back for a restocking fee of 35%, so 'll rather give away. It's now a used item so customs should'n be a problem. Will prioritize those who have provided helpful answers in this thread. If more than one is interested I will make a drawThe unit has been "modded" by removing the earth reference on the output socket.(caps are in place).From experimentUnit giveaway...

author: sourcecharge
author_rank: Regular Contributor
timestamp: July 29, 2018, 01:05:01 am
content: that sucks, sorry that didn't work out for you...

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 29, 2018, 02:43:49 am
content: How much would it be to get a second one in order to do the series connection? Might be cheaper than getting a 240V transformer of the same amperage rating.

author: erikbrenn
author_rank: Regular Contributor
timestamp: July 30, 2018, 07:55:20 pm
content: Buying another one to fill up my desk is not an option.... I just swallowed my pride and ordered a proper one from a local vendor.I also just remembered that I have an 220V/110V step-down transformer without an enclosure laying around. The Trip-lite enclosure looks nice so I will replace the transformer inside with the the step-down transformer, which just fits inside it.Cheers.

author: SeanB
author_rank: Super Contributor
timestamp: July 30, 2018, 08:15:22 pm
content: Good to do that, and just leave the class X capacitors in, they are fine on 230VAC mains. the 120V transformer will probably ( because it is cheaper) have aluminium wire in it instead of copper. Easy to tell as it will be lighter than expected when removed, even if the windings look like copper through the insulation, and they do solder, the most common wires these days are copper coated aluminium, as it is a lot cheaper, and with care can be wound almost as well as pure insulated copper wire. Can be used on the 120V secondary of the autotransformer to give a 120VAC isolated output though, as you can test most modern SMPS supplies with 115VAC as well as with 230VAC, they run from 90VAC to 250VAC equally well, just different input current.