Buying an Isolation Transformer - Page 1
Forum section: electronics / repair

author: watchmaker
author_rank: Unknown
timestamp: December 23, 2023, 02:21:28 pm
content: I am pretty sure I understand their use and importance. They protect my scope and might protect me. The isolation receptacles should not return to earth. Check by testing socket ground to earth ground, or visual. If earthed, disconnect internally.Now the question is size and manufacturer. I am not averse to used and looking at new under $150 the stuff like Jameco and Alltronic look suspect and the BK looks underpowered.I do not expect to do more than consumer type things (guitar amps, etc).In terms of used, what else should I look at other than medical transformers like Torroid/Trip Lite (2 to 5 VA) or Hammond? Other suggestions? Am I wrong about Alltronics and Jameco?Regards,Dewey

author: Jwillis
author_rank: Super Contributor
timestamp: December 24, 2023, 06:14:02 am
content: The manufacturer is not really important, after all it's just a transformer. With that being said, you need to keep in mind that toroids have better efficiency, regulation, less losses, and lower audible harmonics that the conventional E core transformer.Hammond ,Triad, Antek are all good transformers. Some may be more pricey than others depending on where they are made and how well they are built. I'm not familiar with Anatek/Altronics or CES transformers, so I have no opinion on those. Just to clarify, Antek and Anatek are not the same company.The Real power of Watts is equal to the Apparent Power(VA) X Power Factor. So lets assume that the power factor is 1 for the sake of simplicity. A 300VA 120V transformer will give you around 2.5 Amps. A 500VA is around 4A. How many AC amps does a typical guitar amp or other consumer products need to operate properly? If it's going to be more than 4amps at 120V then you may need to get a 1000VA Isolation Transformer. You'll typically need about 10 to 25% over head on output amps to cover any losses.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 24, 2023, 09:25:51 am
content: I have an off-the-shelf isolation transformer that I bought second-hand. I also made another one by myself out of a pair of identical power transformers salvaged from AV gear, back to back, without modifying anything - simply from 230V to 80V then back to 230V, which gives me two stages of insulation (though each is not necessarily up to the standard for isolation transformer). Only a bit of loss of power efficiency but it's not a big deal.

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 24, 2023, 10:42:00 am
content: Quote from: Jwillis on December 24, 2023, 06:14:02 amIf you aren't that cash strapped at least get one with shield between the windings.I do agree that almost any transformer (except auto duh) works just fine for most "casual" isolation use.

author: Psi
author_rank: Super Contributor
timestamp: December 24, 2023, 11:39:25 am
content: Transformers out of some types of UPS's have two separate mains voltage windings and so can be repurposed to make an isolating transformer.From memory. One is just a basic 230V winding, and the other winding has tap at 215-230-245 for doing some basic mains stabilization.I can't recall if it had this feature to cope with the wide battery voltage range you get when you stack 3 SLA's in series.Or if maybe it was just to stabilize the incoming mains voltage a bit when its a bit low/high

author: watchmaker
author_rank: Unknown
timestamp: December 24, 2023, 07:46:45 pm
content: Thanks all!I just ordered a Toroid ISB 60 for $90 shipped and return privilege.I will make sure the outlet ground pins are disconnected. Man there is a fair bit of BS about what is fundamentally a simple application.Checking what I thought I knew got me thinking about portable generators. I am going to have the ground pin on mine. Does it go to the frame? Never thought about it.Read the discussions on Class 1 and Class 2 devices and how Iso xfomrers for repair are an exception. I had no idea what prosmustunderstand, and what civiliansshouldunderstand. Very much like chainsaws!

author: Jwillis
author_rank: Super Contributor
timestamp: December 24, 2023, 08:56:50 pm
content: Medical grade isolation transformers are are not truly isolated. They are designed to filter undesirable noise from entering sensitive medical equipment like EEG's and are still tied to earth ground through the Neutral.Here's really good video on isolation transformer types and experimentation.

author: bdunham7
author_rank: Super Contributor
timestamp: December 24, 2023, 09:03:22 pm
content: Quote from: watchmaker on December 23, 2023, 02:21:28 pmI wouldn't be so sure if I were you. What exactly are you trying to do? I would not disconnect the ground wiring internally, but rather use a 'cheater' plug if the situation warranted it. The passing through of the earth ground is unrelated to the primary function of an isolation transformer which is to remove the ground reference from the LINE and NEUTRAL wires.

author: DisasterNow
author_rank: Regular Contributor
timestamp: December 24, 2023, 10:24:40 pm
content: Practically downstream of the isolation transformer we will have a floating AC line, with no reference to the earth.I have an old transformer in the workshop, quite huge, removed from a UPS (230AC-12AC). If I were to find another one like it I could create an isolation transformer with two transformers.

author: IanJ
author_rank: Unknown
timestamp: December 25, 2023, 12:32:01 am
content: I managed to pick up an isolation transformer from the medical industry via Ebay.I modified the wiring to give me the option of isolating or linking the input earth and the output earth……something you have to understand before using either way.Worth noting that a good isolation transformer will have better isolation between the primary and secondary windings……..so depending on what you want general transformers may or may not suffice.Ian.

author: watchmaker
author_rank: Unknown
timestamp: December 25, 2023, 01:49:07 pm
content: Quote from: bdunham7 on December 24, 2023, 09:03:22 pmRead the post above yours. Yes, you disconnect the output ground from the case ground Otherwise with a 3 prong plug you have defeat4ed the entire purpose.

author: watchmaker
author_rank: Unknown
timestamp: December 25, 2023, 01:51:46 pm
content: Quote from: IanJ on December 25, 2023, 12:32:01 amIan, I thought about this but do not think I will do it. I have no idea who will have it next. I do intend to mark the case as grounded and the outlets as floating only.

author: bdunham7
author_rank: Super Contributor
timestamp: December 25, 2023, 06:13:50 pm
content: Quote from: watchmaker on December 25, 2023, 01:49:07 pmNo, not the case. The video in the post you mentioned has a reasonably clear explanation at about 15 minutes in that matches what I said if you understand it. If you don't believe me, I'd suggest you review the design and operation of actual commercial isolation transformer units intended for electronics servicing. BK Precision and Sencore have or had a good selection of them.The issue raised there and byJwillisis not the pass-through of the earth ground, it is the bonding of the output neutral of the transformer to ground. An isolation transformer by default will have an output that is almost entirely isolated--except for a bit of capacitive coupling--from the input and from ground. Thus if you connect a meter or your arm between one of the outputs and ground, almost no current will flow and you won't be zapped. The presence or absence of a separate earth ground wire is irrelevant to this issue and indeed some isolation transformers had two-prong plugs and sockets--no ground at all. This is what I mean when I state that the primary function of an isolation transformer is to remove the ground reference from the LINE and NEUTRAL wires.There are some additional features as well, as the isolation transformer will also reduce available fault current and may reduce noise and transients that are passed through, especially if it is a of a shielded or double-bobbin design. Some devices may have site wiring detection that won't work on an isolated suppy or will set off alarms. These devices, just like one of those little three-light socket testers, are expecting to see the neutral wire at about the same potential as ground and a definite voltage between line and both neutral and ground. A specific setup to accomodate these devices is to bond the neutral of the output to ground so it functions just like a local service entrance. You still get the other benefits, but there is no protection against a shock hazard or other issue due to the LINE wire still having full potential relative to ground. This is the exact point the guy in the video was making when he demonstrated that the bulb did not light when he connected one end to his grounded green alligator clip.Most isolation transformers will not have the output neutral bonded to ground. If you are buying one originally intended for medical or other very specific usage other than electronic servicing, you would want to check that. But this is an entirely different issue from disconnecting the actual green ground wire internally, something I strongly recommend you not do. There are times when you do want to interrupt that ground, but as I said earlier I'd use an external cheater plug so that you can do it either way and never make a mistake about whether your ground is interrupted or not.

author: watchmaker
author_rank: Unknown
timestamp: December 25, 2023, 09:04:41 pm
content: Quote from: bdunham7 on December 25, 2023, 06:13:50 pmRead what I wrote; not what you want me to have written. This is all ado about nothing. Thanks for your concern, but it is misplaced.Merry Christmas.

author: bdunham7
author_rank: Super Contributor
timestamp: December 26, 2023, 12:01:52 am
content: Quote from: watchmaker on December 25, 2023, 09:04:41 pmHere's what you wrote:Quote from: watchmaker on December 25, 2023, 01:49:07 pmPerhaps you intend your words to mean something different than I understand them as? If not, either your second statement is false or companies like BK Precision, Sencore, etc have been manufacturing and selling entirely useless products for decades.

author: fmashockie
author_rank: Frequent Contributor
timestamp: December 26, 2023, 04:04:14 am
content: Seems like you already purchased one, but I am privy to the Sencore PR-57 and BK 1653. I like that they are both isolation and variac in one.

author: EHT
author_rank: Frequent Contributor
timestamp: December 27, 2023, 09:52:48 pm
content: As was noted above, it is worth testing your actual device especially if 2nd hand. I bought one on eBay. Someone had cut the Earth (PE) wire inside and not put any sticker on the case. My understanding of the most sensible way to use these is the neutral floating but PE connected.Another problem which has not been mentioned is the inrush current. The one I bought to start with was 1kVA and had no inrush protection. FWIW it's an EU brand (Bronson) but made in China. The inrush on this huge transformer was enough to trip the whole domestic circuit. So I put some NTCs in series and measured the voltage across these with a scope. The peak dies away after about 300ms. I then added a time-delay relay to short the NTCs out after 300ms. All good apart from the time-delay relay's internal circuit seems to emit a frequency whistle (in the air, not on the line) which is a bit annoying...Some of these transformers like the Toroid one in the video have NTCs in place but not the time-delay shorting. They've calibrated the value of the NTCs such that the inrush is about 10-15A, which is still not great, then also have the problem that the NTCs will then dissipate some heat.BTW, I don't get the thing about the purpose of the medical isolation transformer being noise-reduction. Surely HF noise is passing through; why not just use a good filter, with MOVs etc?

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 28, 2023, 09:16:37 am
content: Quote from: EHT on December 27, 2023, 09:52:48 pmUnable to get your point here. Do you mean PE of the input side connected to the case (but if so what does "neutral floating" mean, if again it's the input side)? Or anything else?

author: MathWizard
author_rank: Super Contributor
timestamp: December 28, 2023, 09:38:49 am
content: I got a 500 or 1000VA 1:1 transformer, I believe it was the Hammond brand. It did have an Earth GND, on the input and output 3-prong plugs. I un-hooked it internally from a grounding screw.That was a few years ago, I better open it up and see if it still makes sense to have unhooked. If for any reason the metal core/chasis become live, I'd still want the panel breaker to trip.

author: RFDx
author_rank: Regular Contributor
timestamp: December 28, 2023, 03:51:48 pm
content: Quote from: max.wwwang on December 28, 2023, 09:16:37 amThe PE wires on input and output are connected to each other and to the (metal) case. Do not cut the output PE wire as suggested by the OP multiple times.The two main wires on the output plug are galvanically isolated through the transformer from L/N on the input.Although your body is (normally) grounded, the isolation barrier keeps you now safe in case you are touchingoneof the output wires by accident (or on purpose).

author: Jwillis
author_rank: Super Contributor
timestamp: December 28, 2023, 09:44:24 pm
content: Quote from: EHT on December 27, 2023, 09:52:48 pmThe noise that is referred to is the transient voltage spikes that can occur over mains power. The secondary winding has significantly reduced voltage spikes present on the primary side. In the case with medical transformers, theirs also an added Faraday shielding between primary and secondary that shunts these transients further along with HF noise to ground. So no further circuitry or components are required.

author: watchmaker
author_rank: Unknown
timestamp: December 28, 2023, 11:03:50 pm
content: Quote from: max.wwwang on December 28, 2023, 09:16:37 amAgree. If something is earthed, it is not floating by definition. FWIW, contrary to what one person claimed, OP NEVER said to disconnect the primary side from earth. What was SAID was he disconnected the OUTPUTS from earth. Otherwise, the outputs are NOT floating.Most electrical circuits have a ground which is electrically connected to the Earth, hence the name "ground". The ground is said to be floating when this connection does not exist."Conductors are also described as having a floating voltage if they are not connected electrically to another non-floating (grounded) conductor. Without such a connection, voltages and current flows are induced by electromagnetic fields or charge accumulation within the conductor rather than being due to the usual external potential difference of a power source."Most electrical circuits have a ground which is electrically connected to the Earth, hence the name "ground". The ground is said to be floating when this connection does not exist."https://en.wikipedia.org/wiki/Floating_ground

author: vk6zgo
author_rank: Super Contributor
timestamp: December 29, 2023, 02:00:32 am
content: Quote from: watchmaker on December 25, 2023, 01:49:07 pmThe only time "you have defeated the whole purpose" is if you connect one side of the secondary to ground.If neither side of the secondary is connected to the external metalwork of the transformer, it is immaterial whether the metalwork is connected to the house ground or not.The 115v/230v available at the secondary of a true isolating transformer is afloatingsupply.There may or may not be reasons to extend the house ground through to the output socket, but the usual reason for connecting the house ground to the transformer metalwork applies (blowing a fuse or tripping a RCD in the event of a breakdown between the incoming active ("hot") & the case/metalwork), unless your isolation transformer is double insulated.

author: watchmaker
author_rank: Unknown
timestamp: December 29, 2023, 03:20:01 am
content: Quote from: vk6zgo on December 29, 2023, 02:00:32 amI NEVER proposed disconnecting the case from Earth. I SAID I was disconnecting the outlet sockets fed from the secondary from the case Earth so they would be floating. To leave them connected to Earth indeed would entirely defeat my purpose for using an isolation transformer.To be cyrstal clear, here is what I acutally wrote in post one:"I am pretty sure I understand their use and importance. They protect my scope and might protect me. The isolation receptacles should not return to earth. Check by testing socket ground to earth ground, or visual. If earthed, disconnect internally."The ONLY way it can be suggested I talked about removing the case Earth is if you take the last sentence out of context. Which one respondent chose to do while others understood how to read the whole parargraph and addressed my request for manufacturer recommendations. I wrote this off as a language difference.Your statement is in precise agreement with what I have said Thank you.Sadly, some have made sport of putting words into my mouth Not only is this rude, it is intellectually dishonest. If something is ambiguous to you, ASK for clarification. But do NOT put words into another's mouth.FWIW, I have been struck by the imprecision of language in this thread. A ground is an arbitrary voltage reference point. Earth means ZERO potential energy by definition.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 29, 2023, 04:34:18 am
content: Quote from: watchmaker on December 29, 2023, 03:20:01 amDifferent terms in different countries.Earth is normally the term used in UK & Australia, with ground being mainly a USA thing.

author: Dan123456
author_rank: Regular Contributor
timestamp: December 29, 2023, 05:20:12 am
content: Quote from: vk6zgo on December 29, 2023, 04:34:18 amOff topic but, Oooh that makes a lot of stuff make more sense now!I had always kinda assumed they were interchangeable terms but it was causing me some confusion on why they were called different things and a little concern that I could be doing something wrongThanks for that

author: soldar
author_rank: Super Contributor
timestamp: December 29, 2023, 05:08:00 pm
content: Quote from: max.wwwang on December 24, 2023, 09:25:51 amWhen you do this you should be aware of two things.One is that there will be a voltage drop when you load so the output voltage will be lower than the input voltage, substantially so. My isolation transformer has taps to compensate for this.Also, the output impedance of the mains is close to zero for all practical purposes. The output impedance of an isolation transformer is not only resistive but also inductive and this can have unwanted effects on some circuits. You should be aware of this.

author: soldar
author_rank: Super Contributor
timestamp: December 29, 2023, 05:10:32 pm
content: Quote from: vk6zgo on December 29, 2023, 04:34:18 amIn the deep south they also use "soil" and "dirt".

author: Synthtech
author_rank: Regular Contributor
timestamp: December 30, 2023, 11:16:59 am
content: Given that toroidal transformers have very poor physical isolation between primary and secondary windings compared to traditionally wound iron core transformers I can’t see that a toroid would be a good choice for an isolation transformer.

author: EHT
author_rank: Frequent Contributor
timestamp: December 30, 2023, 10:38:59 pm
content: Quote from: Jwillis on December 28, 2023, 09:44:24 pmThanks, yes, makes sense. I suppose it must give a lot more HF attenuation than filter + MOV since it is far more expensive and bulky.

author: EHT
author_rank: Frequent Contributor
timestamp: December 30, 2023, 10:44:04 pm
content: Quote from: RFDx on December 28, 2023, 03:51:48 pmYes, that's what I meant. The Bronson one I got from eBay had the PE wire to the output snipped, presumably by last owner, so I recommend people check second-hand units.Actually in my post, I mainly meant to focus on the inrush current problem with these. The Bronson 1kVA had none. The Toriod Corp ones have NTCs with no bypass relay. I saw a take-down of some other model which had NTC & bypass but can't recall what that was.

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2023, 11:09:02 pm
content: corroded missing PE and cut away mains folded under the connector that is basically shorting to chassischeck your stuff! with a light at all angles because you can get downright sabotage

author: GnomeZA
author_rank: Regular Contributor
timestamp: December 31, 2023, 01:38:52 pm
content: Quote from: Psi on December 24, 2023, 11:39:25 amI've salvaged a lot of transformers from UPS and I have yet to find one that is isolated. They have taps yes, but it is an auto-transformer. Even APC professional true sine wave series had the same arrangement. APC also sells a voltage stabiliser and no dice, it is simply an auto-transformer.I know this is a bit of a side-track to the topic, but I wouldn't want a rookie caught in this trap, thinking they've isolated themselves

author: bdunham7
author_rank: Super Contributor
timestamp: December 31, 2023, 05:00:38 pm
content: Quote from: GnomeZA on December 31, 2023, 01:38:52 pmIn those transformers the primary has taps and is used as an autotransformer for buck/boost voltage stabilization, but thesecondaryis certainly isolated.

author: GnomeZA
author_rank: Regular Contributor
timestamp: December 31, 2023, 05:37:27 pm
content: Quote from: bdunham7 on December 31, 2023, 05:00:38 pmYou are arguing about technicalities here because the next thing I'll say is, "what do you think secondary voltage is?"You'll say "Oh it is 12v/24v". Or maybe you naively think they put a 1:1 transformer in a UPS? (spoiler alert, they don't)Then I'll say ok, so we are back to it being useless as an isolation transformer.Then you say, ok so use two. Now you need two transformers instead of 1, from equipment that has a bunch of other expensive electronics in it, and we are back to discussing the problem above with a huge voltage drop and all kinds of undesirable properties due to using two transformers back to back.Oh and every single UPS, even APC (maybe especially APC) use woefully undersized transformers. The APC Pro series range has a no load draw of 50 watts on the 1kw unit for example. They use way too high voltage per turn to save on copper and core size. (ie. magnetization current is crazy compared to a true isolation transformer which is nearly twice the physical size for the same rating as APC. And APC transformers are nearly twice the size of the cheap Chinese units I've salvaged from)Thoughts?

author: bdunham7
author_rank: Super Contributor
timestamp: December 31, 2023, 06:02:46 pm
content: Quote from: GnomeZA on December 31, 2023, 05:37:27 pmI suppose that's all true, but when people discuss making their own isolation transformers from salvaged equipment, they almost always mean the back-to-back variety since as you say no manufacturer is likely to stick an expensive 1:1 (or tapped) isolated transformer in the front of their design for no good reason. Dead APC UPS units aren't all that rare and the advantage of the tapped primary is that you could compensate for the sag from the back-to-back arrangement by selecting the right taps. IDK about all models, but the old SUA and SMT models that I look for use enough iron in the core to stay relatively cool and efficient. They need to because they are line energized 100% of the time. Newer models may be worse.

author: GnomeZA
author_rank: Regular Contributor
timestamp: December 31, 2023, 06:20:53 pm
content: Quote from: bdunham7 on December 31, 2023, 06:02:46 pmMy country has serious power problems so we buy and sell these UPS/inverters/etc. like crazy over here (2nd hand market).My company also bought the server grade APCs a few years back (moved on now, overpriced for what it is, and with our power situation they don't last more than a few months).Anyway these new generation units, the server grade APC have an auto-transformer but it has no secondary and they are even smaller than before.Instead they use the high frequency or "transformerless" design used in 99% of modern pure sine inverters now (if you are familiar with these).Pretty much a switch mode power supply that can operate in both directions.So in one direction AC -> rectifier -> buck using high frequency transformer -> rectifier -> battery charger (so a SMPS)Other direction DC/Battery -> boost through high frequency transformer -> rectifier -> AC wave form using PWM with some capacitors and inductors to improve THD.I was speaking more to the older generation APC units that still had a secondary through which it would boost the low voltage AC during inverter operation(ie. referred to in inverter world as "low frequency" design)In the UK they need to use "site safety transformers". That has been my source of "cheap" isolation transformers (anything but cheap tbh but still much cheaper than the medical grade or transformers actually marketed for electronics lab type use). There are a few grades of those transformers, 230 -> 110v (split phase, ~ 55v per phase) but 1:1 230v -> 230v also exist.They also provide a 100% duty cycle power rating and 10 minute duty cycle rating from what I've been able to gleam.Almost all of them also had a earth strap between primary and secondary too (so bonus less common mode noise and more safety).

author: max.wwwang
author_rank: Frequent Contributor
timestamp: December 31, 2023, 10:32:21 pm
content: Quote from: RFDx on December 28, 2023, 03:51:48 pmThanks for your clarification. So you meant the output(secondary) side. Note though for an isolation transformer (perhaps except medical ones, which I don’t know much), there is no such thing as secondary L/N since they are symmetrical, identical, and both floating.If I understand you correctly, you seem to strongly oppose cutting the connection of the secondary PE to the primary PE (then both will be connected to the core of the transformer and its case. But you did not explain the problem of leaving the secondary PE also floating. Can you please expand a bit more? Thanks.Please correct me if I'm wrong. I cannot think of any benefit of having PE of the secondary connected to PE of the primary (and both connected to the transformer core and the metal case). But I can think of at least one problem with doing this. We know the purpose of an isolation transformer is for safety, of either person or device (and this includes not only the device connected to the transformer, but also - very often - any testing equipment such as an oscilloscope used to test the device). In such a configuration, if your device-under-test (DUT) is a class I appliance (i.e. has a PE wire connected to its metal case), which is supplied by the isolation transformer, when you clip the probe of your oscilloscope to any point of the DUT that has a potential different from its case (and PE), this will cause a short circuit across such potential difference (from the clip, through mains PE, to secondary PE, which is connected to the case of the DUT).There does not seem to be an increased safety risk to a person, because the secondary winding is floating anyway (both ends). (No increased safety either as far as I can see.)

author: max.wwwang
author_rank: Frequent Contributor
timestamp: January 01, 2024, 09:29:11 am
content: Quote from: Jwillis on December 28, 2023, 09:44:24 pmThat makes sense, because a 1:1 transformer (actually it does have to be 1:1), isolated or not, does have the effect of suppressing harmful spikes, because of the characteristics of inductance.However, if this is the sole purpose of a medical 'isolation' transformer (which is not isolated), itis a misuseof this term. A 1:1 transformer does not have to, and shouldn't, be called, an "isolation transformer", unless, obviously, its primary and secondaryare isolated.

author: Psi
author_rank: Super Contributor
timestamp: January 02, 2024, 12:29:01 pm
content: Quote from: GnomeZA on December 31, 2023, 01:38:52 pmStrange, my DIY isolation transformer is definitely from a UPS I tore down. Unless I'm remembering wrong but 99% sure i'm not.It also has a high current 12VAC winding that I have disconnect since i'm just using it for isolating.It wasn't anything special, like a 500-1000VA PC UPS with single 12V SLA.It's definitely isolated. I guess I just assumed it was a more common UPS design than it really is.If it's that uncommon I'll stop recommending it to people has a source for isolation transforms.

author: GnomeZA
author_rank: Regular Contributor
timestamp: January 02, 2024, 12:42:30 pm
content: Quote from: Psi on January 02, 2024, 12:29:01 pmSo to clarify, the UPS transformer you have has a mains voltage primary (with multiple taps?) and an isolated secondary that is 220v (1:1 with the primary)?Or rather what taps does it have exactly?Could you link me to the UPS you had?Hey I'm not attacking you or saying don't recommend it. I'm saying I've never seen a UPS transformer with a secondary tap that is give or take 1:1 with the primary. I mean it *could* exist (probably does?), I've just never seen anything like that.

author: Psi
author_rank: Super Contributor
timestamp: January 02, 2024, 12:49:09 pm
content: Quote from: GnomeZA on January 02, 2024, 12:42:30 pmThe UPS model is long forgotten sorry.I would have to take my isolation transformer box apart to check the transformer to be sure what it has.But it definitely has 3 separate isolated windings. Two mains voltage windings and one 12V winding.The two mains voltage windings may not be exactly 1:1, but close enough if not 1:1.

author: GnomeZA
author_rank: Regular Contributor
timestamp: January 02, 2024, 01:00:32 pm
content: Quote from: Psi on January 02, 2024, 12:49:09 pmOk fair enough, guess they do exist. Best to check for anyone thinking of using them with multimeter tho!

author: Haenk
author_rank: Super Contributor
timestamp: January 02, 2024, 01:34:04 pm
content: Quote from: GnomeZA on January 02, 2024, 12:42:30 pmI just took the time to take this picture. This hefty combo (likely 30kg or more) is from our former 2kW online-UPS, made by Invertomatic. This comes with a 1:1 transformer.Obviously too nice to just throw away or sell for scrap...

author: Psi
author_rank: Super Contributor
timestamp: January 02, 2024, 01:38:03 pm
content: Is that one isolated though? It seems to not have any wires coming out the side facing us, which hits that its an auto transformer.Or do you mean the red one?

author: Haenk
author_rank: Super Contributor
timestamp: January 02, 2024, 01:59:44 pm
content: In hindsight, that picture is to be rated "not helpful"Yes, the left transformer has 2 in (on the right side) and 2 out (on the left side, not visible, and wired as the right side); the bridging is probably for easy adaption to 110V.

author: GnomeZA
author_rank: Regular Contributor
timestamp: January 02, 2024, 03:38:35 pm
content: Quote from: Haenk on January 02, 2024, 01:34:04 pmAh this is a relevant point.I've only been salvaging from line interactive or offline types.Online-UPS are typically pretty expensive and I fully expect them to be much more robust in this regard.Not sure what you paid for that but to compare, APC line interactive UPS sell for ~$500 right now.Their online versions would almost certainly be $5000 or something crazy like that.