How can I make DIY Isolation Transformer? - Page 1
Forum section: electronics / beginners

author: bshi02
author_rank: Regular Contributor
timestamp: April 06, 2024, 11:27:31 pm
content: Hello.I want to have Isolation Transformer because I sometimes tried to repair some of electronics and I heard that "If I accidentally touched on the voltage, I will still feel the pain but very slight pain but I will not get electrocuted, Assuming I have used the isolation transformer."But in my country,There is no where to purchase any Isolation Transformer,So I want to make DIY Isolation Transformer on my own.I can purchase below 1phase transformer[WY22-100AF pri(V):0-200-210,sec(V):0-210-220-230 (E)] from electronic stores and the outlet voltage is 220-230v in my country,https://e-punglim.co.kr/product/%EC%9A%B4%EC%98%81-%EB%8B%A8%EC%83%81%EB%B3%B5%EA%B6%8C-%ED%8A%B8%EB%9E%9C%EC%8A%A4-wy22-100af/4139/But What is other part or tools need to make DIY Isolation Transformer?and Is there any schematics for making DIY Isolation Transformer?

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 07, 2024, 12:22:16 am
content: If the transformer is the same asthe one listed here, then it should work OK.You just need to put it into a chassis/box with the appropriate plugs/sockets/fuse for your country.Maybe add an on/off switch and indicator light if you want.

author: bshi02
author_rank: Regular Contributor
timestamp: April 07, 2024, 01:03:51 am
content: Thank you very much for your reply.It seems that this 1phase transformer[WY22-100AF] is the one listed.But I don't have any experience to make isolation Transformer from appropriate parts(plugs/sockets/fuse/1phase transformer).Is there any schematics which demonstrate the place where I should solder appropriate part and each soldering point of 1phase transformer?

author: MathWizard
author_rank: Super Contributor
timestamp: April 07, 2024, 01:22:56 am
content: Well this also adds a variac (auto-transformer) , for changing the overall AC voltage, and series light bulbs, for limiting the max current, but Mr.C has the isolation transformer as well, and switches, maybe extra fuses too.Most of the time I don't use an auto-transformer, but I usually use the IT any time I work on anything AC powered, and if it hardly draws any current, I'll add a 100W series light bulb. Some circuit's that pull a lot of current, would need a much bigger bulb, other wise they will drop the voltage to the DUT, when there isn't really an over-current.For my setup, I have an AC wall outlet mounted on a 2"x4" (wooden post), and an AC wall light switch, and a fuse, and a lightbulb stand/socket. So I can use that with the IT, and/or the AT. And I can put a short connector in the lightbulb socket too, and still have AC power to my DUT on a real 125VAC rated switch.

author: calzap
author_rank: Frequent Contributor
timestamp: April 07, 2024, 05:01:54 am
content: From the nature of the OP’s posts, it sounds like they need more education and experience in electronics and mains wiring before attempting to construct an isolation transformer. Mistakes can lead to fires, damaged equipment or shocks.Find it hard to believe that isolation transformers can’t be bought in the ROK.Mike

author: bshi02
author_rank: Regular Contributor
timestamp: April 07, 2024, 12:28:55 pm
content: Frankly speaking,It is possible to purchase isolation transformer in south korea, BUT it cost at least $400 for a Korean-made isolation transformer.Because almost all of korean use Isolation Transformer for the purpose of eliminating audio noise,So It is usually purchased by audio enthusiast, and They tend to seek for somewhat high-class Isolation Transformer that make hard to purchase any Isolation Transformer at a reasonable price in south korea.Although I have fiddled with soldering iron more than 2 years but I don't have any experience to deal with Autotransformer or isolation transformer.So I do want any tutorial of making Isolation Transformer for complete novice.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 07, 2024, 12:31:13 pm
content: On the DIY side?For example, get two 220->24/48/whatever transformers from anything, then connect as 220/24->24/220.

author: tooki
author_rank: Super Contributor
timestamp: April 07, 2024, 02:50:23 pm
content: Quote from: bshi02 on April 07, 2024, 12:28:55 pmThen why do you think you need one? If you’re a true complete novice, you shouldn’t be working on mains stuff if you don’t understand it. Bear in mind that an isolation transformer reduces some risks and introduces others (because it will cause your RCD/GFCI to not trip!!), so it’s not a good idea to use one just because you heard somewhere that it’s a good idea.

author: David Hess
author_rank: Super Contributor
timestamp: April 07, 2024, 04:30:44 pm
content: Wiring two common power line transformers back-to-back works well enough.

author: calzap
author_rank: Frequent Contributor
timestamp: April 07, 2024, 05:58:40 pm
content: To OP:You should be aware that “isolation” does not have a consistent definition. Some “isolation” transformers provide noise isolation or voltage regulation, but do not provide galvanic isolation. For example, some Tripp-Lite isolation transformers only provide noise isolation.There are a number of cautions to be aware of when powering a device with an isolation transformer. For example, use a battery-powered meter to check that the transformer output is really isolated from earth ground before each use. Be aware that mains powered test equipment, power supplies and tools can defeat the purpose of an isolation transformer in some circumstances. And there is more.Mike

author: bshi02
author_rank: Regular Contributor
timestamp: April 08, 2024, 12:47:59 am
content: In fact,I have repaired a little of retro game console which require in the majority of cases, soldering tiny jumper wire to trace damaged mainboard or replace capacitor or rectifier diode,voltage regulator and It only need to measure voltage of each PSU's pinout(12v,5v,3.3v).But when I tried to fix CRT TV, I realized that it is nearly impossible to fix CRT TV without any guidelines.So I have asked question on the various forum about How to fix CRT TV and began to read book of electronic repair guide such as Testing electronic component,CRT TV Repair Course,My Best Collection of electronic repair articles.https://jestineyong.com/at-last-my-brand-new-e-book-is-ready/testing-electronic-components/https://jestineyong.com/fifth-ebook-launched/in the result,I concluded that I absolutely need isolation transformer in order to safely examine suspicious component with switching on CRT TV.By the way,I plan to make DIY Isolation transformer by following below link.But There is no guideline to where I should connect ground wirehttps://okelectronic.wordpress.com/2012/12/04/diy-isolation-transformer/And It seems that CRT TV which I attempt to fix has type C plug like below link(2 pins,not grounded)http://itempage3.auction.co.kr/DetailView.aspx?itemno=D301209143So I doubt that whether I don't have to connect ground wire to chassis body of Isolation transformer or not..

author: calzap
author_rank: Frequent Contributor
timestamp: April 08, 2024, 06:50:01 pm
content: Your schematic for the isolation transformer looks good. The chassis/box of the isolation transformer should be connected to mains ground if it’s conductive. The output ground connection should not be connected to anything and be so labeled. I'd add an on/off switch on the primary side. Once you get it put together, use a meter to check that the outputs are indeed isolated from earth ground and mains ground, neutral and hot. Although the two live connectors on the output can be labeled hot and neutral based on connector geometry, those terms will have no relevance to mains wiring.And remember, it’s the device under test that is to be connected to the isolation transformer and not your test instruments. And you can get a shock by touching two parts of your body directly or indirectly to the DUT. First line of defense is thinking things through and second is being careful not to touch anything that can be live (even if supposedly isolated).Mike

author: bshi02
author_rank: Regular Contributor
timestamp: April 09, 2024, 04:22:39 pm
content: Thank you very much for your reply!After wade through below youtube video, Now I understand that ground connection of ac input must be connected to any conductive metal like aluminium chassis of the isolation transformer,and In contrast, ground connection of output receptacle should not be connected to anything.But If I purchase 1phase transformer of 300VA capacity to examine broken CRT TV of 220v voltage then what Amp fuse should I mount on both input and output connection in order to prevent from burn out or extreme overheat of isolation transformer?

author: calzap
author_rank: Frequent Contributor
timestamp: April 10, 2024, 05:21:06 am
content: The fuse should be on the hot line of the input and first thing the hot line from the cord connects to. For a 300 VA transformer and 220 V, maximum A should be 300/220 = 1.36 A. 1.5 A fuse should be OK. Is that enough for the device(s) you want to test? Use a reputable brand fuse; no-name fuses may blow at a current much higher than their printed rating.A fuse on the output would be OK, but not really needed for most applications. An overload on the output should blow the fuse on the input. In some circumstances, a fast blow fuse on the output could protect the transformer from current coming back from the DUT.Noise suppression, voltage regulation, etc. should be on the input side.I haven’t watched the video. But the toggle switch on the front of the device appears to be capable of switching off isolation. I would never have that. Too easy to forget or accidentally nudge the switch the wrong way. Isolation transformers should have no easy way for their isolation to be nullified.Mike

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2024, 05:46:38 am
content: I had this thought. a GFCI is literary supposed to prevent you from getting killed, when wet, by a outlet in the bathroom.I think that is a pretty trust worthy technology compared to a isolation transformer.Its a globally used well developed technology. It is super mass produced and cheap.Its even recommended for out door power equipment like pressure washers. And its lighter.Don't bother with the transformer?

author: dietert1
author_rank: Super Contributor
timestamp: April 10, 2024, 05:47:26 am
content: Be aware that a large transformer of 300 or 500 W produces considerable heat, like 5 or 10 % of its nominal power, so for continuous operation it may need a fan. In this respect toroids are better.When i made an isolation transformer many years ago, i added a second smaller transformer to produce -5% and +5% output voltages - a simple adjustable transformer. I also added one of those cheap consumer power meters. It produces voltage, current and power readings.Regards, Dieter

author: jonpaul
author_rank: Super Contributor
timestamp: April 10, 2024, 11:03:55 am
content: find junked microwve oven , remove transformer.Disassemble and rewindj

author: Gyro
author_rank: Super Contributor
timestamp: April 10, 2024, 11:21:19 am
content: Quote from: jonpaul on April 10, 2024, 11:03:55 amProbably the least safe and most inappropriate method I can think of! CCA primary, core running near saturation, secondary winding to be cut out (or angle grinder to the welded core laminations, magnetic shunt to be removed. Suitably insulated bobbin to be constructed. Primary - Secondary winding high voltage isolation to be tested. All by a beginner?For somebody so experienced, who keeps jumping in with one liner unsubstantiated safety warnings, it's a ludicrous suggestion.

author: themadhippy
author_rank: Super Contributor
timestamp: April 10, 2024, 01:19:22 pm
content: Quotewhilst (hopefully) stopping you getting blown up an rcd might be a bit slow in stopping your expensive test gear from going pop

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 10, 2024, 03:10:07 pm
content: For oscilloscope use a GFCI/RCD and a high voltage differential probe is the more modern method, a lot more straightforward to stay safe. To say a isolation transformer provides touch safety is a dangerous half truth at best. The key thing to understand is the isolation transformer removes the earth reference from the mains power, allowing you to connect earth (e.g. oscilloscope probe or signal generator ground lead) to the device under test without creating a live-earth short, this reintroduces the shock hazard as now parts of the circuit are hundreds of volts above earth. Also note a RCD or GFCI cannot "see" through a isolation transformer so one can actually be LESS protected using one, safety is not the purpose.Keep in mind a CRT TV is a big box of shock hazards no matter what you do, the primary safety device here is your own mind. You work on them live only when needed, otherwise it is unplugged with the plug placed somewhere visible. Understand sections can hold serious energy for days or weeks after power off, and try to understand what you're poking at ahead of time. I've personally not yet needed to make oscilloscope measurements or other mains earth referenced test gear connections (what the isolation transformer really enables) on a CRT TV and I own about 5 at the moment and have fixed a few. I think cracked solder joints killed more of them than anything else. The CRT TVs worth fixing mostly aren't of the "hot chassis" (entire set mains referenced) variety anyway, as those generally only have a RF input, models with video inputs generally have an isolated power supply with the hot section outlined and labeled.Have a good size mirror set up at all times so you can stand behind the running TV and observe the picture from a safe working position. NEVER reach behind an opened TV. Always one hand in the pocket, rubber shoes, nonmetallic workbench, etc. To do it safely takes more than one piece of gear and a fair amount of thought.I'd say if one can safely and effectively use an isolation transformer things like wiring up and fusing one should be self evident. I agree with other posts that you need to study more before dealing with mains or other high energy circuits, a certain level of understanding really is required to safely work with such things.

author: bshi02
author_rank: Regular Contributor
timestamp: April 11, 2024, 01:19:18 am
content: Thanks very much for all of reply.I want to add Noise suppression or voltage regulation to input side.But I have no information what type of Noise suppression or voltage regulation should be added to input side of isolation transformer.As I am novice at repairing electronics,I prefer to examine suspicious component without power on.But It seems that it is impossible to examine suspicious component in PCB of CRT TV without power on.By the way Is it really dangerous reach hands to PCB of CRT TV which is behind an opened TV and which is already powered on?If I power on CRT TV and try to examine PCB of CRT TV without connecting its power cord to Isolation transformer,Then Should I use only one hand to measure voltage of suspicious components in order to avoid electrocute?

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2024, 01:35:57 am
content: *NEVER* reach behind a CRT TV or monitor with its back off. Fumbling around a board carrying live mains, high energy DC circuits up to 360V, and tens of kV EHT capable of several mA continuous current, without being able to see *exactly* where your fingers are is hazardous in the extreme, both for you and the unit. Instead, use a wall mirror to see the screen or if you need a better view of the screen, place the unit sideways on the bench, so you can move slightly to view the screen or see the board as needed.

author: bshi02
author_rank: Regular Contributor
timestamp: April 11, 2024, 05:41:18 am
content: I discharged the crt tube by following below link and the biggest capacitor(which is written 400kv and 220uf on its surface and still hold 280 voltage before discharging) with 10watts 2k ohm ceramic resistor and successfully detached PCB of CRT TV from crt tube.https://www.wikihow.com/Discharge-a-CRT-MonitorBy the way,Is it still hazardous to touch hands on any soldering point of PCB of CRT TV even if I successfully detached PCB of CRT TV?

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2024, 07:35:37 am
content: Most TVs are designed so that under normal conditions larger electrolytic capacitors in the circuit will discharge within seconds or minutes of power disconnection, however under fault conditions, some may hold charge for hours or even days. Never assume the board is safe to touch until you have confirmed all large capacitors are fully discharged.Another trap for the unwary is that discharging the CRT tends to induce a static charge on its face plate, and if that induced charge is discharged by someone touching the faceplate, or otherwise leaks away, if the final anode cap connector isn't kept grounded to the grounding spring across the aquadag coating, it can recharge the CRT to a significant percentage of its normal EHT voltage - usually not enough to electrocute you, but certainly capable of giving a big enough shock to cause you to drop the tube - I've seen it happen, it wasn't pretty and the tech was lucky to have been wearing heavy enough trousers that protected them from the flying glass from the resulting implosion.

author: bshi02
author_rank: Regular Contributor
timestamp: April 11, 2024, 09:13:57 am
content: Thank you very much for reply!i usually don't pay much account when I discharge the CRT because I think it is alot safe if I wear two layer of gloves,But I understand that I have to use strong alligator clipsin order to firmly attach it to grounding spring.I have tried to fix broken CRT TV which have no display problem,So I examined this CRT's PCB and found that After I powered on and off this CRT TV a week ago, the biggest capacitor(400kv,220uf) won't fully discharged at all, It still held 270 voltage since I powered off.I desoldered all of resistor in HOT area and diode ,and test it with LCR T4 But there was no faulty.below screenshot is MC059C diagram which seems to be the same with CRT TV excepting for part of ic501 and ic661,ic751 and ic701.the biggest capacitor(400kv,220uf) is C803 in HOT area which is next to T803 transformer.As I examined all of resistor in HOT area,So I want to test STR-W6754 switching generator(IC801) But It seems that It require to measure its voltage with power on CRT TV,But I am afraid to get electrocute if I make a slip measuring.But I don't know how to test STR-W6754 with power off.

author: calzap
author_rank: Frequent Contributor
timestamp: April 15, 2024, 07:03:29 pm
content: This thread prompted me to start a thread on misuse of isolation transformers. It'shttps://www.eevblog.com/forum/beginners/how-to-kill-yourself-with-an-isolation-transformer/. Be sure to read the follow-up comments.Mike

author: wizard69
author_rank: Super Contributor
timestamp: April 16, 2024, 01:05:41 am
content: Quote from: coppercone2 on April 10, 2024, 05:46:38 amI actually believe that GFCI can provide better protection than an isolation transformer. This isn't to say that an isolation transformer is always a bad idea, but you can make a mistake wit electronics operating on an isolation transformer that will kill you just a well. The death comes from current flowing through the heart and any goof that induces that flow will kill.QuoteOn the flip side there are good reasons to use an isolation transformer especially for sensitive equipment. Also people need to realize that dim bulb protection circuits are more about protecting old hardware if there are hidden shorts or failures at voltage. They really have nothing to do with protecting the tech, but are rather low tech current limiting.

author: dietert1
author_rank: Super Contributor
timestamp: April 16, 2024, 04:57:01 am
content: Our isolation transformer has been used for scope work on switched power supplies - to separate circuits normally connected to mains. Yes, this work is dangerous and only for experienced techs aware of the risks involved.A DR is for a different purpose and working on a mains connected device with difference probes is as dangerous. The light bulb is a auto-reset fuse. It doesn't prevent accidents, like touching metal parts with a loose probe Gnd clip. Better use a separate ground wire and pull the clips from the probes. And avoid probes with a 1:1 - 10;1 switch when working on a mains circuit and don't do this type of work at night when you are getting tired.An isolation transformer can also help to break ground loops in analog audio. This applies to semi-professional equipment without ground lift. Once more a situation that requires knowledge and experience to maintain safety.Regards, Dieter

author: jonovid
author_rank: Super Contributor
timestamp: April 16, 2024, 07:29:04 am
content: image 1an old 700 watt microwave oven transformerwith its primary 240 volt windings. the secondary winding gaps are for another Do it yourself project.the original high voltage 1kv secondary winding was cut with a hacksaw and pushed out, without disassembling the core.leaving the low voltage side of the secondary intact. red wires.image 2here is an un-cut 700 watt primary 240 volt winding in its varnish collar from another old microwave oven transformer were the core was disassembled.leaving the wire collar intact.this is a lot of wire to unwind then poke through a new core to make a mains voltage isolation transformer.

author: Gyro
author_rank: Super Contributor
timestamp: April 16, 2024, 08:55:50 am
content: Quote from: jonovid on April 16, 2024, 07:29:04 amI referenced this in reply #17. A re-wound MOT is absolutely the worst way for somebody to make a safety isolation transformer...- The windings are probably CCA.-Neverre-use old wire in a transformers - especially a mains winding, and you're uncoiling a varnished coil- The chances damaging the wire insulation while threading the core are high.- The secondary-core insulation is nowhere near good enough for a safety isolating transformer- The average person doesn't have the necessary Hipot test equipment to ensure that the primary-secondary and secondary-core insulation and voltage withstand meet spec.Don't do it, it's not safe for this application -just don't.

author: Ian.M
author_rank: Super Contributor
timestamp: April 16, 2024, 09:38:24 am
content: + it would be running on the ragged edge of saturation and overheating if you run it for long, as Microwave oven transformers are designed as cheaply as possible and rely on their limited duty cycle + fan cooling to survive their normal use. To avoid saturation, you need 10%-20% extra primary turns or the same reduction in input voltage or increase in mains frequency (e.g. a 50Hz transformer in a 60Hz country), if using them to build a normal transformer, and the few who are competent to add to the primary safely, know its not worth the effort as they can buy transformer laminations to assemble a core round a bobbin for less than the labour cost of faffing around with a recycled MOT.