home made isolation transformer - Page 1
Forum section: electronics / beginners

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 11, 2015, 07:50:05 pm
content: Somewhere way back in my memory I recall that I can make a (low-watts) isolation transformer by taking two normal transformers, and wiring the secondaries back to back. One trip to the RS clearance yielded a pair of 12.6V 3A (CT) transformers. So I'm thinking I might try this, including fusing both the line side and the output side at 1A. Is this a reasonably safe approach ?The CT would be ignored, and sealed with heat shrink.

author: cjk2
author_rank: Contributor
timestamp: February 11, 2015, 08:23:57 pm
content: I don't see why this would not work provided both transformers are rated for the isolation you require. I suggest using a variac if you have one to do your initial power test and check for any heating of the transformers.I once made an isolation transformer by taking two microwave oven transformers apart and sticking both of their "primaries" on one core. The core was from one of the MOTs and was clamped back together after the windings were in place.

author: electr_peter
author_rank: Unknown
timestamp: February 11, 2015, 08:45:43 pm
content: 40W is very low power, what are you planning to do?It can work, but be aware of safety issues. Add 3 fuses on all lines: AC input, AC between transformers, AC output. Use DPDT switch on the output. Thermal cut-out may be useful, but it is an overkill at low power levels. Some AC filtering on input could reduce potential spikes from damaging insulation. Ground all metal parts as well (except intermediate and output lines).Apart from overloading/overheating, there is an issue of transformer isolation. If mains->12.6V transformer fails, you will have full mains on the input of 12.6V-> isolated mains transformer. Stepping up mains with such high ratio will almost surely 1) damage anything on the output and 2) break isolation on second transformer. Now you have non-isolated mains or much higher voltage on the "isolated" output - very dangerous. This is unlikely, though.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: February 12, 2015, 12:10:30 am
content: The voltage regulation is going to suck with all that extra copper.Since you have two transformers, you can take them apart, and make a single transformer with the parts of both. Find a way to install two "primary" windings on one transformer.

author: BradC
author_rank: Super Contributor
timestamp: February 12, 2015, 01:52:43 am
content: Quote from: cosmicray on February 11, 2015, 07:50:05 pmYeah absolutely. I did it for years with a pair of 240v-24v transformers. Not the most efficient, nor the best power rating or regulation but for quick and dirty mains isolation it worked a treat until I could score a real isolation transformer.

author: Whales
author_rank: Super Contributor
timestamp: February 12, 2015, 02:44:46 am
content: Quote from: electr_peter on February 11, 2015, 08:45:43 pmWhat sort of situation would this occur in?

author: electr_peter
author_rank: Unknown
timestamp: February 12, 2015, 09:00:51 am
content: Quote from: Whales on February 12, 2015, 02:44:46 amLighting, supply transformer failures, wiring/neutral breaks, ... I am talking more about primary winding insulation - in case of many high voltage events primary side can short out.

author: Whales
author_rank: Super Contributor
timestamp: February 12, 2015, 10:14:18 am
content: Ah, I was interpreting that as having the two transformers back to back could provide some different sources of failures, but I wasn't about to figure out how. Eg some interesting inductive collapse scenarios.

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 12, 2015, 02:25:32 pm
content: Thanks to everyone for the warnings, provisos and other as-sundry gotchas.

author: Fred27
author_rank: Unknown
timestamp: February 12, 2015, 03:41:31 pm
content: In the UK we have shaver sockets (for plugging in electric shavers in the bathroom). These have a 1:1 isolation transformer in them. Normally rated about 15-20W I think. Anything similar where you are?

author: richard.cs
author_rank: Super Contributor
timestamp: February 12, 2015, 07:28:02 pm
content: For those worried about what happens if you have an insulation failure simply earth the low voltage winding between the two transformers. Any insulation failure on the first transfomer will simply blow the input fuse. A failure in the second teansformer compromises isolation but doesn't result in voltages > input.If you choose split bobbin transformers it can't happen anyway.

author: BradC
author_rank: Super Contributor
timestamp: February 13, 2015, 01:01:45 am
content: Quote from: richard.cs on February 12, 2015, 07:28:02 pmWhat a brilliantly simple and elegant solution. I've really just replied to make sure it's seen twice.

author: aneng
author_rank: Regular Contributor
timestamp: August 27, 2018, 07:59:45 am
content: As a noob, could someone please explain how you can earth the low-voltage connection between the transformers and have it still work ? I'm assuming you don't earth both wires ! ...and whatisearth in this context - mains earth ?I know my question shows a fundamental lack of understanding so bear with me!I'm halfway through building my own from two big UPS transformers.

author: Brumby
author_rank: Unknown
timestamp: August 27, 2018, 08:27:36 am
content: Quote from: aneng on August 27, 2018, 07:59:45 amThe secondary has no connection to the primary. It is totally floating. You can earth either side of the secondary and all that will do is set a reference point - operation will be completely unaffected.QuoteCorrect - otherwise you would be shorting the secondary.QuoteYes.The recommendation to earth one side of the secondary gives safety protection when the primary is fused in the active (line) connection. It will not work if only fused in the neutral. Fuses in both will cover active/neutral swap situations.If there is a short between primary and secondary, then a fault current can flow from the primary through that short to the secondary and to earth. NOTE: If the short is near the active side of the primary, the current will be significant and the fuse should blow reliably. However, if the short is near the neutral side of the primary, the voltage through the short will be low (where neutral lines are earth referenced) and the fuse may not blow. Even though it's not ideal, this actually presents a lower risk. If, then, the mains plug is reversed (swapping active and neutral) the fault condition will turn into the previously described and the fuse will blow.

author: helius
author_rank: Super Contributor
timestamp: August 27, 2018, 08:41:25 am
content: Quote from: aneng on August 27, 2018, 07:59:45 amThe topic is a method of building an "isolation transformer" from two identical, common power supply type transformers. They arenotthe type of transformer actually made and sold as an isolation transformer. All transformers (besides autotransformers/autoformers) have galvanic isolation: the difference is their intended purpose: to supply power to a low-voltage device, or to substitute for a line outlet.The first of the two power supply transformers inputs line voltage and outputs a low voltage (24V or 12V, perhaps), and the second is turned around so it inputs the same low voltage, and outputs line voltage. A real isolation transformer would have a primary and secondary at the same voltage.The two-transformer approach has no advantage other than cost. Since isolation transformers are only used for specialized purposes, they are quite expensive and not carried by stores. But the two-transformer approach can work with any type of power supply transformer, as long as you have two the same. Some power supply type transformers are very cheap, and they are easy to find in the trash. By converting twice, they have higher magnetic losses and waste more energy than a single isolation transformer, and are bigger and heavier for the same current capacity.Since there are two transformers, and every transformer provides isolation, there are actually three separate isolated domains. The line input side (with its dangerous to touch live wire), the middle with low voltage only, and the output side with isolated mains voltage. The suggestion is to tie the middle domain to earth—it can have no effect on the other two domains, because they are isolated from it. Earth in this context always means the Protective Earth conductor of the electric outlet.Isolation transformers are the subject of a peculiarly large number of threads on this site. I say peculiar, because their usefulness is limited and they are rarely encountered in engineering labs or equipment installations. The primary purpose of isolation transformers appears to be a way to instill unjustified confidence in inexperienced technicians, to weed them out of the gene pool. They do not prevent deadly electric shock, and they actually stop the GFCI from saving your life. The best that can be said is that they make some measurements easier on line voltage equipment; think carefully whether it's really worth it.

author: Ian.M
author_rank: Super Contributor
timestamp: August 27, 2018, 09:57:54 am
content: For those of us who grew up with tube era equipment, there's nothing particularly scary about working on equipment with a 340V DC bus with the chassis grounded, which is what you get if you use an isolating transformer in front of a typical SMPSU and ground the negative side of the reservoir capacitor(s) for ease of making measurements. Yes it could kill you if you got shocked across your chest, so it was essential to use working practices that minimise the risk of that, but with appropriate PPE and work environment the risk of serious injury was minimal. When one wasn't working on the PSU, and no ground was connected to it, the use of a isolation transformer made accidental contact with any single point in the whole PSU primary side minimally hazardous.Isolating transformers were therefore more or less essential equipment in the CRT TV service trade, as at the time, the alternative of CAT II rated isolated differential probes with enough bandwidth to be useful just weren't affordable.With the drop in the cost of suitable isolated differential probes, the increasing lack of exposure of junior techs to safe working practices for high voltage work, and their (unjustified) faith that a GFCI (RCD) will always save them from serious harm from accidental contact with a mains live circuit, its becoming less and less advisable to recommend their use.They are still a good option if you have been properly trained and you need to do development work on a MCU with a non-isolated capacitive dropper PSU or other line referenced supply, and it is impractical to isolate a PC hosted logic analyser or in-circuit debugger due to USB bandwidth constraints.The O.P's proposed 300mA one is a bit light for serious bench use. Maybe if they are developing IoT hardware it will be useful. There's also the issue of voltage drop - the turns ratio of the transformers will be non-ideal so the second one operating backwards will deliver less than the mains supply voltage. That's not really a problem on the output of a Variac, but without one, it would probably be helpful to have a third transformer that could be connected to boost (or very rarely buck) the output voltage, possibly allowing the 'tap' to be adjusted to compensate for the voltage drop on load.Personally I have one that's a legacy of my TV field service tech days, with a 250VA torodial transformer built into an old teak instrument case so its readily portable. It has a fused primary and a 1A circuit breaker on the secondary. It still sees occasional use when I'm working on equipment that has an exposed mains PSU.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2018, 11:34:03 am
content: Back to back power transformers operating as a line voltage isolation transformer work fine and even provide better isolation because now there are two electrostatic shields available plus the floating middle winding which can be grounded.Pmbrunelle identified the issue with increased series resistance leading to poorer regulation but this may actually be seen as an advantage since it lowers short circuit and surge currents. If this is a problem, then use bigger transformers.Copper and magnetic losses are doubled but so what? They were already small. For 120 volt operation, magnetic losses may be lowered by using transformers configured for 240 volts at the expense of doubling the copper losses (and series resistance) but this can be made up for by using larger transformers. The same can be done for 240 volt operation if the number of transformers is doubled but this gets out of hand quickly.Isolation transformers have no effect on GFCI operation but see below. GFCIs protect against currents to *ground* which isolation transformers also protect against. Neither protects against currents between hot and neutral or hot and hot. If the secondary (or floating with back to back transformers) winding was grounded and the primary to secondary isolation failed, then the GFCI would trip. If the secondary winding was grounded in two places, the GFCI would not see it but the GFCI would not see a hot to neutral or hot to hot short either.The only disadvantages are greater size and weight, increased losses, and higher resistance leading to poorer regulation. The advantages are good availability and low or even no cost.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 27, 2018, 01:41:25 pm
content: I agree with those posters who say this will work and are puzzled by some of the responses that raise issues that aren't actually there.First, the insulation on most transformers I've seen is typically 2500 volts so I'm really baffled at all the primary to secondary breakdown hysteria that would never be raised if the transformers were just used as 120:12 transformers in a power supply.Second, I'm puzzled at those who would fuse every lead in the circuit. Again, they would never consider this necessary if the transformer was used as a step-down in a power supply. One properly rated and placed fuse is sufficient.Third, the original post states that they realize this will be low wattage so reminding the poster of something they already know isn't that helpful. This could have simply been a 'what if' type of question as the poster pondered the concept.I have used this method in the past and it can allow you use a couple of identical transformers with useless voltages and put them to good use. At one time I had a number of 120 to weird voltage high wattage transformers and connecting them back to back worked fine and beat scraping the transformers.Using two 120 to higher voltage transformers back to back lessens the problem of copper losses you would get from using low voltage secondary transformers. If you have transformers with voltage adjustment taps on the primaries you can correct for any losses. Regulation won't be quite as good as using one isolation transformer but generally this hasn't been a consideration for me.Yes, having a good variable isolation supply is great but not everyone can find or afford them starting out. I now have a B&K Precision variable/metered/isolated/fused A.C. supply that didn't cost that much on Ebay.

author: exe
author_rank: Unknown
timestamp: August 27, 2018, 02:02:01 pm
content: Quote from: ArthurDent on August 27, 2018, 01:41:25 pmI'm not sure about that. All step-down transformers I saw have the biggest loss on the primary side. That's because it has a lot of turns of a thin wire. So, while the current in primary winding is much smaller, it seems manufacturers "compensate" that with unproportionally higher primary resistance.

author: aneng
author_rank: Regular Contributor
timestamp: August 27, 2018, 02:05:15 pm
content: Thanks for the replies everyone. What are the opinions on this design.... ?http://www.leomeyer.de/leomeyer/electronics/isotrans.htm

author: Ian.M
author_rank: Super Contributor
timestamp: August 27, 2018, 02:35:20 pm
content: Leo Meyer is an idiot. Downstream of an isolation transformer, *BOTH* conductors present an equal shock risk unless one of them is intentionally grounded. Also WTF is the 1M8 resistor for? Presumably its to bleed away static electricity to protect the transformer insulation, but in that case, why isn't it to ground, why is it 2W (even if it ends up with 340V DC across it from a SMPSU DC bus, that's well under 0.1W dissipation), and also it should be either a special HV resistor or two or three in series as typically the voltage rating of a through hole resistor is around 200V peak.Ground the center tap of the left hand transformer secondary. Fuse between the transformers with a T (timelag) characteristic fuse of no more than the max rated secondary current. As your location is Wales, get rid of that crap for indicating if the mains supply is miswired.You may want to include a 250V RMS working voltage rated varistor across the primary on the left and 500V rated gas discharge surge suppressors to ground to protect the insulation.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 27, 2018, 03:08:37 pm
content: "What are the opinions on this design.... ?"I don't think it is that bad but I would make at least 4 changes. I do not like switching neutrals although I realize there are some areas that may not have a real ground (earth) carried through the input cord, the plug can be inserted either way. If there is a ground and the neutral is at zero potential at the breaker box, jumper the circled switch section. Also I always fuse the input-period. The output fuse value can be changed depending on what you are testing to protect the DUT but I always have an input fuse on the hot line before the switch.I have found that the newer LEDs are much brighter and 27K may make the LED too bright so I'd try a much higher value.If there is an actual neutral at zero potential then the drain resistor makes sense but I would use three equal value resistors in the 500K range to increase the voltage rating of the series string. The purpose of the original 2W rated resistor was to use a physical larger resistor to make the breakdown path greater but I'd still go with 3 resistors for greater safety. If the neutral is always truly grounded then connecting the left end of the resistor to neutral works but connecting it to ground instead is a better idea.

author: Ian.M
author_rank: Super Contributor
timestamp: August 27, 2018, 04:11:30 pm
content: In 230V land, it makes more sense to use a neon for a reversed supply phasing indicator, and the only reason to include it is the prevalence of Schuko plugs that are reversible in most types of European socket.In North America, there is no reason to suspect that a three pin socket will have reversed L and N as they should all have been installed in compliance with code, and the circuit isn't critically dependent on phasing anyway (once you've removed that stupid 1M8 resistor), so its best to delete the phasing indicator.Fusing the low voltage connection between the transformers is essential if the transformers are going to be used anywhere near their max VA rating, and the input should also be fused.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 27, 2018, 05:05:12 pm
content: Saying that the grounding indication LED isn't needed in North America is not correct. What should be and what is in the real world are two different things plus everything can fail in unpredictable ways. This is why high end power strips have these LED indicators and all electricians have some way of checking a duplex outlet for proper wiring. The photo below shows examples.Fusing the secondaries between the two transformers isn't necessary because if you have a 120VAC to 12V/10A transformer it won't make any significant difference whether you fuse the primary for 1A or the secondary for 10A (ignoring inefficiencies ) but you want to certainly have a fuse as close to the line as possible so having a second redundant fuse on the secondaries between the two transformers isn't needed.

author: aneng
author_rank: Regular Contributor
timestamp: August 27, 2018, 05:09:21 pm
content: Thanks for that Ian. My transformers are Chuan Shun 430-7142A models.... from APC UPSs. They are big, hefty buggers:-I'm not certain what the primary windings are, there are four wires on the primary side, Yellow, White, Black and Blue. I suspect they are 2 x 120V but I can confirm that with my meter.

author: Ian.M
author_rank: Super Contributor
timestamp: August 27, 2018, 05:38:58 pm
content: Quote from: ArthurDent on August 27, 2018, 05:05:12 pmThe Sperry socket tester is a useful bit of kit. Everyone with an IQ above room temperature should have one or an equivalent, and use it to check that sockets are properly wired when moving in to a property. However, sockets don't rewire themselves magically overnight, so unless you've got a serious electrical fault like a bad neutral, the indicator in Leo Meyer's circuit is of little use. If you've got a bad neutral, in an American split phase 115V-0-115V instalation, you shouldn't be relying on an indicator on an occasionally used bench isolating transformer to spot it, as it can do a massive amount of damage very quickly. If you need that sort of protection, then you need a permanently installed device to monitor the two phases' voltage balance and also detect the EMI from arcing connections.The power strip with fault indicators is somewhat useful if you travel a lot. Otherwise its fairly pointless (except that its indicators may be able to tell you if its surge protection as been compromised). However, if your equipment is critically sensitive to proper phasing and grounding you should always check *BEFORE* you connect it, e.g with the Sperry tester.QuoteFor large transformers you are correct. For smaller transformers, it will be difficult to fuse the primary of the first transformer so its protected against overload as the fuse will have to handle the startup surge of both transformers. Its easier to protect against overload between the transformers where the current is higher and only the second transformer is contributing to the startup surge. You still need a primary fuse on the first transformer in case it develops a shorted turn or some other fault, but it doesn't have to be sized for close tolerance overload protection.

author: glarsson
author_rank: Frequent Contributor
timestamp: August 27, 2018, 05:44:21 pm
content: I don't know about that particular transformer, but APC transformers tend to be for a specific voltage. The extra wires are then for the buck/boost that a APC line interactive UPS can do when not running on batteries.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 27, 2018, 06:48:58 pm
content: "For large transformers you are correct. For smaller transformers, it will be difficult to fuse...."The discussion is about making an isolation transformer to power some piece of equipment and posters have mentioned that 40 watts is probably too low so I really doubt that there would ever be a case making a DIY isolation transformer where the primary fuse would need to be so low as to cause a concern.As to proper grounding indications, I wired my own house with a 200A main and my 'lab' has full 20 amp capacity with proper double duplex outlets rated for 20 amps, not 15 amps like a lot of houses. The outlet strips I bought that plug directly into those outlets have built-in cartridge fuses and have digital LED load indicators and fault protection indicators as well so I'm pretty sure I'm safe. Each has 24 outlets and were designed for IT so they had to be reliable. I have, however, seen many of these cheap outlet strips that rely on poor metal to metal spring contact to the frame and a number of these do fail. The also loosen up their grip on the plug pins and can overheat (see photo). Most work benches will not have stuff plugged directly into a wall outlet but will probably cascade a few outlet strips of unknown parentage and this can present problems. To say that you should eliminate a few cents worth of parts that give a visual indication of a possible failure is not good engineering practices and false economy. Leave the parts in for safety. It is when you get to the point that you feel you know that there can be no problem that you find out that there was at least one possibility you hadn't considered.I also have a separate circuit for the lighting so in the unlikely case that I were to trip the 20 amp breaker downstairs I could still see to get out of the room without killing myself in the dark. Oh, and I also have battery powered safety lights (and an LED pocket light) if the house power goes out. Some may view this as overkill but I'm hoping that it will prevent that from happening.

author: Bratster
author_rank: Frequent Contributor
timestamp: August 27, 2018, 11:42:24 pm
content: Quote from: glarsson on August 27, 2018, 05:44:21 pmQuote from: aneng on August 27, 2018, 05:09:21 pmGlarsson is correct, those APC Transformers are set up for 120 volts input(I'm in the US so that's all I have seen), with an additional buck and boost tap so it can regulate the line voltage to some extent without switching the battery.Sent from my Fi Moto x4 using Tapatalk

author: BradC
author_rank: Super Contributor
timestamp: August 28, 2018, 01:59:12 am
content: Quote from: Bratster on August 27, 2018, 11:42:24 pmAll the transformers from most of the larger (>1500VA) SU/SUA series SmartUPS are 120V primaries.The 24V units (>=1500VA except XL models) have two different transformer configurations (either 120 or 230V model).The 48V units (>1500VA and all XL models) all use 120V/24V transformers with primaries in parallel for 120V and series for 230V. Secondaries in series. All variants have buck/boost taps in the primaries, although I don't recall now how they are wired on a 48V model for buck/boost and I don't have one handy to pop the lid off.If you were in a 110V country, an old SU2200 would be a pretty neat source for a pair of well matched transformers that'd work happily back to back, remembering that these transformers are specifically designed to be bi-directional. There is a *lot* of iron and copper in those transformers.

author: exe
author_rank: Unknown
timestamp: August 28, 2018, 07:19:14 am
content: Quote from: BradC on August 28, 2018, 01:59:12 amAren't all transformers bi-directional? Please correct me if I'm wrong, I thought more iron and copper help efficiency and regulation, but nothing to do with directionness.

author: BradC
author_rank: Super Contributor
timestamp: August 28, 2018, 07:52:31 am
content: Quote from: exe on August 28, 2018, 07:19:14 amNo, you are right. A transformer is theoretically symmetrical. In practice I've found cheap transformers lossy when being used in reverse. They have just enough iron and copper to do what they need to do and hit the target they are designed for in the direction they were designed for. You seem to lose more going the other way, so by the time you connect a pair back to back you generate less light and more heat (so to speak).

author: exe
author_rank: Unknown
timestamp: August 28, 2018, 08:01:00 am
content: Quote from: BradC on August 28, 2018, 07:52:31 amI don't doubt this, but what's the mechanism behind?

author: aneng
author_rank: Regular Contributor
timestamp: August 28, 2018, 09:20:27 am
content: I've not been able to find a datasheet for my transformers or a schematic for a UPS that uses them so, I'm guessing my best option is to use the highest-resistance pair for my 220V primary and just use the same pair on the other transformer?

author: BradC
author_rank: Super Contributor
timestamp: August 28, 2018, 10:14:34 am
content: Quote from: aneng on August 28, 2018, 09:20:27 amThat'd be the way I approach it. Did those transformers come from a UPS with a single 220v primary or did it use 2 in series? If it's a 120v transformer you may well saturate it shoving 220v into it and bad things might happen.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2018, 10:21:25 am
content: First cold check resistances to determine which wires go to which separate windings. Next you need to take a good look at it to see if its construction actually has reinforced insulation between primary and secondary, as a UPS with batteries entirely within the case doesn't actually *need* isolation - the batteries can be live, and tied to the incoming neutral. If it looks good so far you need to do 1KV HiPot insulation tests between primary and secondary and from primary to core. If it fails that, forget about using it for anything without rewinding itIf its got a tapped primary, power one up using what you *think* is the full winding connections, with a 100W mains incandescent bulb in series with the Line to limit the current if you got it wrong or its out of a 120V UPS, then, assuming the bulb doesn't light, check the voltages at the other taps with respect to Neutral. Alternatively, there may be a seperate buck/boost winding on the primary connector.You'll probably want to tap down on the first one a bit and use the full winding of the second one (on the load side), to boost the voltage a bit to compensate for losses. It depends on the input:output voltage ratio under load, how close your local supply is to a nominal 230V RMS, and the taps available.A bit of googling indicated your transformers are probably used in 1000VA or 1500VA models. However UPSes typically overload their transformer considerably as the runtime is limited by the battery capacity so the actual continuos rating will be significantly less. Post dimensions of the core (including aperture dimensions) and the weight, and hopefully one of our transformer experts will be able to estimate a continuous VA rating for you.

author: aneng
author_rank: Regular Contributor
timestamp: August 28, 2018, 10:40:23 am
content: I don't own a Megger, so the insulation test is out. What would your definition of 'reinforced insulation' be, and how can I tell without stripping the things apart ?@BradC - I don't know for certain but I think they were one transformer for one UPS, and I'm not convinced of this buck/boost winding theory. I can email the manufacturer and check how they are wound (if they reply). I'm 70% sure they are just 2 x 120V primaries. I vaguely recall measuring the windings when I first acquired them and I'm sure they were 2 x equal resistance pairs. I'll measure them tonight and also email the factory.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 28, 2018, 11:02:09 am
content: Quote from: BradC on August 28, 2018, 10:14:34 amUnless the transformers came from big (or industrial) UPSs, I'll stay away of transformers scavenged from home UPSs.Most of home use UPSs have class B transformers, or even higher temperature rating.https://www.electricalengineeringtoolbox.com/2014/08/nema-insulation-classes-for-transformers.htmlFor small and low cost transformers like the ones that can be found in a home UPSs, this means that such a transformer runs VERY hot, even when no load is attached to the secondary. Home UPSs are also designed for an intermittent regime, with maximum 10-20 minutes run at nominal power, followed by a much longer cooling period (while the UPS battery recharges). At nominal load, a home UPS transformer will go so hot that it can easily go over the water boiling point in a couple of minutes.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2018, 11:58:29 am
content: You are looking for visual signs of extra insulation between primary and secondary - either a split bobbin or multiple layers of polyester tape over the full width of the winding (you'll only be able to see the edges - *IF* you are lucky).Photos of the sides of the transformer would help.You are intending to trust your life to those transformers' isolation, but it comes from an application that doesn't require any isolation. It would be foolish in the extreme *not* to do a HiPot test. If you cant get access to a Megger or an old enough PAT tester to have a 1KV DC insulation test that can be run separately, you'll need to scratch-build a tester.You'll need a current limited near 1KV DC source that can supply 1mA. Slightly under 1KV is preferable because you can then use an ordinary 10Meg input impedance DMM on its voltage range to measure the test current. V/10 gives I in uA. Measure the output voltage of the supply. Then take the transformer, ground its secondary and core and measure voltage on the DMM between the HV supply and the primary to get the leakage current.How you get 1KV DC, current limited is another matter. I'd probably build a Royer oscillator using a small mains transformer with a CT secondary backwards, and drive a Cockroft Walton voltage multiplier to boost its output. Add stages till you get near 1KV, then reduce the input voltage enough to be certain its under 1KV with one more stage and cautiously trim up to just under 1KV. Don't forget a bleed resistor on the output - 5x 1Meg in series should do nicely. If you want to get fancy you can add voltage feedback with a 10K resistor in series with the ground side of the bleed resistor, which will give you 1V for every 500V of output voltage, and use an adjustable regulator with ground refernced feedback the feed the center tap of the Royer circuit, but stabilising the control loop will be difficult due to the multiple large time constants. For the current limiting, use 6x 180K resistors in series with the output, which gives you 1080K with enough margin on its voltage rating to be safe.N.B: charged caps at that sort of voltage can kill you stone dead - work safely, and treat the output of the voltage mutiplier before the current limiting resistor with *EXTREME* respect. Switch off and short the output to circuit ground before touching or moving any of the connections. A grounding lead with a heavily insulated croc clip on one end and a ring terminal or fork terminal at the other to bolt it to the circuit ground point, is useful for that. You can hold an axial 22K 5W wire wound resistor in the clip by one lead as a surge limiter for discharging caps etc to its other lead, then drop it and finally clip the ground lead onto the terminal for the safety short to ground.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 28, 2018, 12:04:08 pm
content: Quote from: aneng on August 27, 2018, 05:09:21 pmI would say your assumption is correct.You will need however, to know the proper phasing of the pair of individual primary windings.Should they be connected incorrectly, a big BANG! will follow upon application of mains voltage.

author: BradC
author_rank: Super Contributor
timestamp: August 28, 2018, 12:12:14 pm
content: Quote from: aneng on August 28, 2018, 10:40:23 amThe part number prefix leans towards an APC SmartUPS, and the 2006 date would make it a late SUA model. That would probably mean that the pinout on the transformer is most likely :- L- Trim (APCs name for Buck)- Boost- NIn normal mode the incoming mains is put across the L & N. In boost mode the incoming active gets fed to the Boost winding (pin 1) and in trim mode the output gets fed from the Trim winding (active still connected to L). Remembering the boost/buck only affects the "line-interactive" mode. So those extra windings just make up part of an auto-transformer it uses to boost or buck the "on-line" output voltage.Have you got a small transformer somewhere that will spit out 9 or 12V AC? If so, connect it up to the secondary of the transformer in question and measure the AC voltage coming from the primary. The two outside pins on the plug are most likely the main primary taps based on the boards I have here and schematics I've seen. That'll give you a rough idea of the primary/secondary ratio at half (or less) than the actual line voltage. What you'll see is one neutral line, and 3 actives with roughly "volts, volts + 15%, volts + 30%" (or as its used volts - 15%, volts, volts + 15%).I've just had a squiz at some board pics I took of an SU2200 and the primaries are both wired in parallel. Boost & Buck windings included, so my comment on them having 120V primaries was a faulty memory on my part.The inverter / battery on the APC is connected to chassis, so it doesn't float and must comply with all the normal isolation & safety regulations. The XL units use the same transformers and they have external battery connections. The only APC units that I'm not across are the newer RT units, and they use a switched mode converter, not one of the iron boat-anchors you have.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2018, 12:24:39 pm
content: Quote from: schmitt trigger on August 28, 2018, 12:04:08 pmWhich is why I advocated a 100W 'dim bulb' test for initial powerup.Testing the primary with a low-ohms meter could help eliminate some possibilities and help figure out whether its got 2x 120V windings or one 230V winding + a buck/boost winding for the AVR, or whether its got three taps, e.g 250V, 230V and 210V for the AVR.If you don't have a four wire meter that can measure milliohms, take a LM317 regulator on a heatsink and rig it as a current source with a 15R resistor and a 100R preset in parallel so you can trim it to exactly 100mA, with an anti-parallel diode between Vin and Adj so it doesn't get killed by back-MF, apply 100mA to each winding in turn and measure the voltage. V in mV *10 gives you the resistance in milliohms. Use four wire techniques - apply the test current to the very ends of the leads and probe for voltage on the wire right up against the end of the insulation without touching the clips for the test current.If you need greater sensitivity, its possible to build a battery powered x10 OPAMP buffer to get direct readout in mV for milliohms, but some care must be taken to clamp the inputs to protect it against damage from back-EMF.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 28, 2018, 12:42:29 pm
content: On the last line of the white stamped markings in the pictured transformer, partially covered by the rubber foam, is written "CLASS H(180*C)" ~350 Fahrenheit. Expect it to run very hot at any load.I wouldn't like an isolation transformer that can melt solder.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 28, 2018, 12:45:44 pm
content: exe - "Aren't all transformers bi-directional?"Ratio wise that is correct for no load on an ideal transformer. Here's what I think without any equations to back it up but it makes sense to me. Basically what BradC is saying about "you generate less light and more heat (so to speak)," is correct. A transformer isn't 100% efficient because the energized primary has to use some additional wasted power to create a magnetic field in the core. next the collapsing magnetic field in the core has to transfer most of this energy to the secondary winding. Each of these steps will use some of the energy that would all go to the secondary in an ideal transformer and this wasted power (called watts lost in some industries) is the heat generated by the transformer.If you need 12 volts at 10 amps out of a transformer there are tables to calculate the secondary wire size needed and years ago when I couldn't find a transformer I needed I would remove the secondaries on a scraped transformer and wind my own using some of this information-and it worked. For 12/10 output at full load you would probably need to calculate wire size for the primary for something like 120 volt at 1.1 amps because of the loses I described. This means that the primary wire size would have to be slightly larger than what you would need for 120 volts at 1.0 amps in an ideal transformer. This would also mean that if you used the transformer backwards with 12/10 in, which would be correct for the wire size used on the 12 volt winding, that to create the same heat rise you could expect to get something like 120 volts at 0.9 amps out. If the transformer windings were conservatively rated or you use the transformers under their ratings the heating effects will probably remain the same.The core size shouldn't matter which way you connect the transformer because you will still be magnetizing it to the same level but the wire size (or resistance) of the secondary used as a primary will generate more heat if you expect to get the output you expect without derating.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2018, 12:56:23 pm
content: Quote from: RoGeorge on August 28, 2018, 12:42:29 pmThat's the max insulation temperature, deep in the winding. The surface temperature will be significantly lower.However the UPS its out of must have been rated at a minimum of 1000VA, and if derated to lets say 500VA the transformer is going to run a *LOT* cooler. When Aneng gets back to us with dimensions and winding resistances etc. we'll be able to estimate how much to derate it for continuous use, and what sort of surge rating it should have.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 28, 2018, 01:27:36 pm
content: Try that:- Plug just the primary of a UPS transformer to the power line, leave the secondary disconnected.- Let it run for about an hour in open air (without any load).- Measure the external temperature of the transformer.When I did that for the first time, I was very intrigued. Was expected it to be at room temperature, but instead, it almost burnt my fingers. The transformer was OK, with no internal short circuits. Indeed, that transformer was scavenged from a cheap Mustek PowerMust 600 USB, an APC might run a little colder, but still, UPS transformer are not designed for continuous load, so expect them to go very hot very fast.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2018, 01:47:58 pm
content: That depends on the UPS. Line interactive UPSes need transformers that can run continuously at no load. Standby UPSes only have to energise the transformer for battery charging and when powering the load, so can get away with much smaller lossier transformers, by interrupting the charging phase once the transformer has reached max temperature, to allow the transformer to cool back down.Also the safe to touch surface temperature limit is generally accepted to be 60 deg C (140 deg F), and there is a *LOT* of margin above that for a transformer with 180 deg C insulation. e.g. if suitably guarded against contact, running it at 100 deg C surface temperature wouldn't be an issue.

author: aneng
author_rank: Regular Contributor
timestamp: August 28, 2018, 03:30:55 pm
content: OK, so here are some photos of my actual transformers (the previous image was from a similar one I found in a google search)....As far as the winding resistances are concerned, there isn't much to go on I'm afraid - everything is under 3 Ohms !What I have got though is here:-White to Black = 2.3 OhmsWhite to Yellow = 2.5 OhmsWhite to Blue = 2.6 OhmsBlack to Yellow = 0.9 OhmsBlack to Blue = 1.0 OhmsYellow to Blue = 0.9 OhmsAs you can see, I had already joined the secondaries together before posting here !

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2018, 04:35:38 am
content: You are going to need to disconnect the second transformer while testing, and you should be grounding one end of the first transformer's secondary anyway for safety so insulation breakdown will trip the breaker or RCD, so you'd have to redo that wiring anyway.The lead and contact resistance or an ordinary DMM that uses two terminals for resistance prevents you getting meaningful readings for anything much below one ohm, hence my suggestion to cobble together a four wire measurement low ohm meter that uses a 100mA current source. However your results seem to indicate a tapped winding with white as the common, black as the lowest voltage tap, yellow as the nominal voltage tap and blue as the highest voltage tap. Connect white to Neutral and yellow to Live with a 100W incandescent bulb in series then you can confirm the tap voltages with your DMM.To get an estimate of its VA rating we really need good numbers for the core dimensions, including the dimensions of the apertures and thickness of the center and side limbs. As they are obscured by the bobbin, some feeling around with a very thin stiff wire with a tiny rightangle bend right at the tip may be needed to locate the aperture corners, so you can mark in line with the aperture edges on the face of the core and take measurements.

author: JS
author_rank: Frequent Contributor
timestamp: August 29, 2018, 04:57:36 am
content: Quote from: exe on August 28, 2018, 08:01:00 amCould be coupling, the way you do the winding changes transformer properties, some are done primary first, secondary on top. Others are two coils side by side. The coupling of the first is usually a bit better, specially if the secondary doesn't fill the whole gap. The two coils side by side are usually in a special plastic reel, insulation is much better, coils are far away from each other, as coupling could be a bit worse (so core could be bigger than the counterpart) but more symmetrical and insulation better I would choose this for a safety isolation transformer.I've used a pair of those to build a tube preamp, using the 12V in the middle to power the heaters. Then a smaller one to go back to the HV. As 12V transformer wouldn't give me enough voltage at the output I took my guts from the drawer and used a 9V transformer (don't do that) and/but it did worked, the core got a bit buzzy but nothing some dipping wouldn't stop as I found out, and the transformer was still pretty oversized so it took the job getting just a bit warm.JS

author: aneng
author_rank: Regular Contributor
timestamp: August 29, 2018, 06:44:19 am
content: Something's just occurred to me.... those transformers are rectangular, not square. Looking from the top, it looks as if there could be two separate transformers in the core. I've never seen one like this before. What's going on inside then ? - Is it two, or one big bugger that has rectangular windings ?

author: Ian.M
author_rank: Super Contributor
timestamp: August 30, 2018, 10:09:37 am
content: I'd have to see one in-situ to be certain but I suspect its width is *either* simply constrained by the UPS chassis form factor, *or* they sub-optimally change the VA rating by implementing the required change of core area by adding/removing fixed size laminations rather than having different sizes of lamination that increase/decrease the 'face' area to keep the center limb squarish. Of course that increases your copper costs and copper losses as the windings are longer per turn . . . . .

author: aneng
author_rank: Regular Contributor
timestamp: June 16, 2020, 04:17:33 pm
content: Bit of a necro-bump but.... I finally got around to wiring it up and testing it. I have connected the mains earth to the transformer laminations and one side of the low-voltage link between the trannies.I only lost 2 Volts across the whole thing, though I have yet to test it with a load.I have a couple of questions though.... I am reading significant voltage between the output of the second transformer (TX2) live and mains earth - so I disconnected the earth from both transformer chassis and the low voltage link, leaving just two transformers, back to back with only live and neutral.With it like this, I'm reading 148V from the live output of TX2 to the chassis of the transformer and 7.8V from the neutral to the chassis. If I measure the TX2 transformer chassis to mains earth, I get 77V.Obviously, I get the full 238V across the TX2 output.The chassis of TX1 is at 106V relative to mains earth.When I say 'chassis' - I mean the laminations. I've noticed that the bolts holding the laminations together have insulating washers at either end and I'm curious why this would be so. If I measure from the insulated bolts to any of the input or output lines, or mains earth, on either transformer, I get roughly 20V.At first I thought perhaps the transformers were junk and there were internal shorts, but measuring with my meter that does 100M ohms, I'm getting infinite DC resistance between any lead and either the laminations or the bolts.What's going on here ?

author: bdunham7
author_rank: Super Contributor
timestamp: June 16, 2020, 04:46:55 pm
content: You're measuring what is probably perfectly normalcapacitiveleakage, but without a well defined impedance (your voltmeter). You need to measure leakagecurrentby connecting resistor across each thing you want to measure and then measure the voltage across that. A 1K resistor will give you 1mv/uA, but I'd recommend you try a 100K first, then if the voltage is low, go to 1K. Obviously not across the mains....

author: ArthurDent
author_rank: Super Contributor
timestamp: June 16, 2020, 06:31:11 pm
content: "I've noticed that the bolts holding the laminations together have insulating washers at either end and I'm curious why this would be so. "There only needs to be an insulating washer at one end of each screw and generally a paper tube like a drinking straw the length of the the screws through the laminations. If the screws aren't insulated they will in effect act like a shorted turn, waste power, and the transformer will heat up.

author: Alti
author_rank: Frequent Contributor
timestamp: June 16, 2020, 06:45:24 pm
content: The problem with wiring two back2back is that output voltage == input voltage at 0A load.Typical 230V/24V 40VA transformer is going to have 28V under no load and 24V under nominal load. Now, if you feed 24V into secondary of the other one, you will get under 200V under nominal load.You need a third 24V transformer to compensate for that.I think a more practical idea would have been a set of 10 small 230V/24V transformers wired with secondaries in series.If you want identical pieces, either 3*40VA transformers or 10*4VA transformers.Alternatively, you could use 2*400VA transformers which would be idling at 40VA with minimal voltage drop, to get near 230V.

author: aneng
author_rank: Regular Contributor
timestamp: June 16, 2020, 07:58:39 pm
content: Quote from: bdunham7 on June 16, 2020, 04:46:55 pmRight.... measured with a 980 ohm resistor connected to mains earth at one end, and the other end connected to a) - gives b) mV across the resistor.a) Laminations of T1 b) 27.3mV ACa) Laminations of T2 b) 19.1mV ACa) Hot output of T2 b) 32.5mV ACa) Neutral output of T2 b) 17mV ACa) one side of LV link between T1 & T2 b) 30.8mV ACSo, that means the largest leakage current in the whole thing is 32 uA - which I'm guessing is absolutely fine ?As an aside, I'm a member of another forum (not electronics related) where I've mentioned that I've built this isolation transformer and one of the guys on there, who I've previously regarded as knowledgeable about electronics, has gone crazy saying I'm going to kill myself if I use this, that it defeats the RCD (I know that !) and that if I were to touch either of the isolated output leads, I would get a potentially fatal shock due to the "capacitance of the human body". I can't see the logic in his claim, as unless I complete the circuit in some way (impossible if the output isn't earth-referenced and I'm not stupid enough to grab both output leads!), I can't see how any current could flow through me, let alone give me a potentially fatal shock.He's stated that "the circuit is actually completed by your capacitance to earth (all your surroundings, in fact) and the capacitance of the transformer secondary to earth in series - which you have also made far worse (approximately doubled, I think) by earthing the low-voltage link between the two transformers".Is he talking out of his *** ?

author: bdunham7
author_rank: Super Contributor
timestamp: June 16, 2020, 08:10:40 pm
content: Quote from: aneng on June 16, 2020, 07:58:39 pmYes, those numbers look OK.QuoteWell, the numbers matter. Yes, the circuit is completed by your impedance (of whatever type) to earth and the transformer's capacitance to earth (which you just measured) in series. Assuming you are standing barefoot in a puddle, if you touch the various areas that you just measured, the corresponding current will to through you. Below 500uA you'll be OK, although you might feel it. As for earthing the center section (and T2, for that matter) I don't think it is helpful. I don't know why there is all the worry about the transformer breaking down and so on. I suspect you will have the lowest leakage with either both the center section and the case of T2 isolated completely from ground, or the center section grounded and the case of T2 isolated. It's worth an experiment.

author: The Electrician
author_rank: Frequent Contributor
timestamp: June 16, 2020, 08:23:33 pm
content: With a proper shoulder washer at each end of the bolts, no paper tube is needed:https://www.tecni.uk/Nylon-Shoulder-Washer-7.9mm-x-6.5mm-172.150.068

author: tkamiya
author_rank: Super Contributor
timestamp: June 16, 2020, 08:53:12 pm
content: What are you going to use this for?As proposed, 6.3V 3A transformer won't give you much of capacity at 120V. I doubt it would actually be useful. The purpose of isolation transformer is for safety. If you are unsure of what and why certain things are important, I would strongly suggest purchasing a small one already made for the purpose.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 17, 2020, 02:07:21 am
content: I have a pair of 500VA transformers wired back to back that I have been using for years. Works just fine.