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    Isolation Transformer Build - Technical Questions

    After seeing so many recommendations on here, I'm going to build an isolation transformer. My original plans were to get two identical transformers out of microwaves, split them and use both the primary coils, but I have been foiled by transformers with identical part numbers having slightly different dimensions. So I'm planning on just wrapping the second coil myself. (Maybe I make two!)

    I looked up the model number on the transformer, it's a ebj60664602 and is rated for 13A on the primary coil. No mention of a duty cycle though. Considering it's coming out of a homeowner microwave, I assume it's not rated for 100%. I don't forsee any of my applications really going over 6a, but I'm still going to try to future-proof myself as much as possible, by adding extra coils to the primary and a corresponding number to the secondary.

    Here are some questions I have about this.

    1. What temperature would you suggest for a themal fuse? I don't know the temp rating of the existing primary coil insulation, but it seems that the cheapest available (retail/ebay/digikey) is rated at 155C. Would 100C fuse be ok? Does anyone have an ideal location for placement?
    2. I'm going to be doing some hand winding, so I can take muliple taps out of the secondary if needed. Would there be any use for this?
    3. Does anyone have a preferred glue for putting core laminations back together? Some of the laminations split apart when I, uh, hit it with a small sledge. Repeatedly. Turns out this transformer doesn't have both halves welded together, the laminations are glued, then the two halves are pressed together along a u-shaped cut out in the seam. My plan is to glue the laminations with varnish or shellac (I have both) and then press the halves back into place on my hydraulic press. Would anyone suggest something different?
    4. Would a fan be suggested? I have no idea how hot this will get in use.

    Thanks!

    #2
    Re: Isolation Transformer Build - Technical Questions

    Transformers should never get hot on average when being used. If it's getting hot with normal use, it's underdesigned...

    So the question is how are you planning to use it...

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      #3
      Re: Isolation Transformer Build - Technical Questions

      I plan on using it for another layer of safety when diagnosing electronics.

      I don't imagine many cases where I'd be sitting at a board for much longer than a few hours.

      Comment


        #4
        Re: Isolation Transformer Build - Technical Questions

        What about leaving it plugged in idling with no load? That's also "use"... You may need automatic cutoff if it's underdesigned and starts heating up.

        Ultimately if you're only using it for short periods of time, just need a regular overcurrent fuse and you need to make sure you turn it off. If you may accidentally leave it on, then maybe a thermal cut may be useful.

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          #5
          Re: Isolation Transformer Build - Technical Questions

          I have a 240V 2000VA isolation transformer and it does get warm just by leaving it switched on with no load and it also makes a quiet humming noise. I assumed this is normal.
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            #6
            Re: Isolation Transformer Build - Technical Questions

            eccerr0r: Yeah, I'm trying to figure out what is needed. I can build in an arduino, a relay, a fan and a thermistor. Could even do some PWM fan control, LCD display and a current transformer (to measure amps) if I wanted to go overboard with it. Pretty sure a thermal fuse will be added no matter what.

            dicky96: I'm not surprised a big transformer gets somewhat hot, there seems to be a lot of variation in transformers, and many of them barely meet their stated specs, that's why I'm considering overbuilding it with extra windings to increase the efficiency and reduce the heat. Did you build yours?

            Comment


              #7
              Re: Isolation Transformer Build - Technical Questions

              Originally posted by clearchris View Post
              eccerr0r: Yeah, I'm trying to figure out what is needed. I can build in an arduino, a relay, a fan and a thermistor. Could even do some PWM fan control,I'm considering overbuilding it with extra windings to increase the efficiency and reduce the heat. Did you build yours?
              If and when you do this project can you please tell us what you did and how you do it

              I can tell you that when you use this type of transformer it going to get real warm even if you have a transformer current controller on it
              Last edited by sam_sam_sam; 04-05-2020, 03:41 PM.
              9 PC LCD Monitor
              6 LCD Flat Screen TV
              30 Desk Top Switching Power Supply
              10 Battery Charger Switching Power Supply for Power Tool
              6 18v Lithium Battery Power Boards for Tool Battery Packs
              1 XBox 360 Switching Power Supply and M Board
              25 Servo Drives 220/460 3 Phase
              6 De-soldering Station Switching Power Supply 1 Power Supply
              1 Dell Mother Board
              15 Computer Power Supply
              1 HP Printer Supply & Control Board * lighting finished it *


              These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

              1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
              2 Hem Saw Computer Stack Board

              All of these had CAPs POOF
              All of the mosfet that are taken out by bad caps

              Comment


                #8
                Re: Isolation Transformer Build - Technical Questions

                Overbuilding = using an oversized/at least proper sized ... but especially properly designed core for the transformer!!! At most a overcurrent fuse somewhere, but that's all!

                You do NOT want or need a fan, arduino, thermal sensor, relays to "overbuild" -- these are "ricing" if I could call it as such ...

                Oh yeah - want to mention that most MOTs are undersized from the get go so you're not going to be able to overbuild the isolation transformer if you're using its core -- and thus may be required to resort to "ricing" to "fix" the problem that was introduced by the undersized core...

                Oh and I should mention, I have/sometimes use a 300VA off-the-shelf isolation transformer. Even these get a little warm... but never gets hot even if left on all day as these are properly sized.
                Last edited by eccerr0r; 04-05-2020, 05:31 PM.

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                  #9
                  Re: Isolation Transformer Build - Technical Questions

                  LOL. Well, whatever we call it, supposedly adding some more coils to both the primary and secondary should help with the cooling a bit. Also cutting the load on it, it's supposedly rated for 13a, I won't be pushing half that usually. What would you suggest for an overcurrent fuse, maybe 7.5 or 10a? I figured the thermal fuse of 100C would handle a broader number of situations and not pop on startup, etc.

                  I have read a decent amount of this, but this guy has the best boots on the ground guide specifically around wiring MOTs. Wish it wasn't all youtube, takes a while to get through it when I could read a page in a few minutes. He suggests an extra 30 wraps on the primary to combat heat issues. According to my calculations, I could comfortably fit another 40 on both.

                  https://www.instructables.com/id/Eas...-use-Tutorial/

                  It was my understanding that if your coils are properly sized (oversized?) that the core wouldn't get hot. If you overdrive anything I suppose it will.

                  He also suggests about 100 watt per pound for a transformer, fully wrapped. I didn't weigh it, but I figure it's somewhere in the realm of 5-6lbs stock.

                  As far as the other stuff goes, I figured it wouldn't be bad to monitor the transformer without having to put a hand on it, kind of defeats the purpose of being a safety device if you have to stick your hand in to check the temperature.
                  Last edited by clearchris; 04-05-2020, 09:41 PM.

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                    #10
                    Re: Isolation Transformer Build - Technical Questions

                    I would think a better starting point would be to cannibalize the transformers from UPSs. It's easy to find magnetics that are DESIGNED to run at 60Hz and 1000+ watts indefinitely.

                    [This would be particularly true of "online" technologies]

                    And, UPSs seem to find their way to the scrap pile pretty regularly.

                    Comment


                      #11
                      Re: Isolation Transformer Build - Technical Questions

                      That's actually a really excellent suggestion. When this quarantine is over, I'll make a trip to the e-waste recycler (I hope he is still there) and see what he has... Those transformers still need to be rewound, right? I assume they are running something like 120v primary and 24v secondary for the batteries, and probably seriously strong, could be a problem taking them apart.

                      Edit: For now, still continuing current plans, as I'm stuck here, and I won't let the perfect be the enemy of the good. Can always do another later, I'm sure I could find the smaller one a home. And I'm sure I'll learn something in the process that would improve a v2.
                      Edit2: I just weighed the transformer (in pieces) on my kitchen scale and it's almost 10lbs. Assuming 100watt per pound, that's 8.3a. Plus, the coils are aluminum, they are much lighter than copper, so it's probably a bit more than that. I'd be very happy if I was able to get 10a out of it sustained. 13a is probably out of reach, but this could be very workable.
                      Last edited by clearchris; 04-06-2020, 12:15 AM.

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                        #12
                        Re: Isolation Transformer Build - Technical Questions

                        Originally posted by clearchris View Post
                        eccerr0r: Yeah, I'm trying to figure out what is needed. I can build in an arduino, a relay, a fan and a thermistor. Could even do some PWM fan control, LCD display and a current transformer (to measure amps) if I wanted to go overboard with it. Pretty sure a thermal fuse will be added no matter what.

                        dicky96: I'm not surprised a big transformer gets somewhat hot, there seems to be a lot of variation in transformers, and many of them barely meet their stated specs, that's why I'm considering overbuilding it with extra windings to increase the efficiency and reduce the heat. Did you build yours?
                        No mate I bought it second hand some years ago off ebay.co.uk for about £50, along with a 2000VA variac from a different seller at the same sort of price

                        It looks like this
                        https://www.ebay.co.uk/itm/BBH-WINDI...UAAOSwT6pVrPQl

                        Though mine is 220V in 220V out

                        I've had some pretty hefty kit attached to it, I'm pretty sure its up to it's rated current. It has a simple thermal trip. I eventually brought the transformer and variac over to the Canary Islands with me

                        Rich
                        Follow me on YouTube
                        ------------------
                        Learn Electronics Repair
                        https://www.youtube.com/channel/UCFX...R8UZ2vg/videos

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                          #13
                          Re: Isolation Transformer Build - Technical Questions

                          Originally posted by clearchris View Post
                          That's actually a really excellent suggestion. When this quarantine is over, I'll make a trip to the e-waste recycler (I hope he is still there) and see what he has... Those transformers still need to be rewound, right? I assume they are running something like 120v primary and 24v secondary for the batteries, and probably seriously strong, could be a problem taking them apart.
                          If you're just looking to "hack" something together, get two of them and wire them back-to-back. That way, you don't risk damaging the laminations and increasing eddy currents (translates to heat) as a result. Maybe derate another 20%?

                          Comment


                            #14
                            Re: Isolation Transformer Build - Technical Questions

                            Now these seem to do it all

                            For cheap!!!

                            110V/220V in 110V/240V out 3000VA for the same sort of price I paid for my 2000VA some years ago around £50 - with free postage

                            https://www.ebay.co.uk/itm/220V-110V...a9ab448e2197d2

                            Or something like these for folks on 220V mains

                            220V in 110V out 3.3Kw
                            https://www.ebay.co.uk/itm/3-3-KVA-S...cAAOSwSXRc-nxJ

                            Buy two of these, connect the secondaries in series (make sure you have the secondary phase the correct way round!) and you have 220V in and 110V/220V out at 3300W for like £80

                            As many SMPS will run on 80V-240V input that also makes them a lot safer to work on if you feed them 110V isolated.
                            Last edited by dicky96; 04-06-2020, 03:47 PM.
                            Follow me on YouTube
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                              #15
                              Re: Isolation Transformer Build - Technical Questions

                              Is this isolation or just step? Autotransformers change voltage but do not provide isolation.

                              BTW a UPS powered by an isolated power supply is isolating. Another way to get an "isolation transformer" ...
                              Last edited by eccerr0r; 04-06-2020, 03:43 PM.

                              Comment


                                #16
                                Re: Isolation Transformer Build - Technical Questions

                                Originally posted by eccerr0r View Post
                                Is this isolation or just step? Autotransformers change voltage but do not provide isolation.

                                BTW a UPS powered by an isolated power supply is isolating. Another way to get an "isolation transformer" ...
                                Good question! I don't see the word isolation anywhere in the eBay add. I think that would be mentioned if it was a feature.

                                Comment


                                  #17
                                  Re: Isolation Transformer Build - Technical Questions

                                  dicky: Nice, is that in an oil bath?

                                  In regards to the ebay chinese transformer, that's far too cheap to be a real isolation transformer. You probably couldn't buy the copper (or even aluminum) wire for a 30a transformer for that price. If something like that were real, it would be a no-brainer.

                                  Comment


                                    #18
                                    Re: Isolation Transformer Build - Technical Questions

                                    There is a single toroid transformer inside but it is likely to be an autotransformer

                                    Comment


                                      #19
                                      Re: Isolation Transformer Build - Technical Questions

                                      That's really fancy having a toroid transformer inside! Or does it?
                                      Having a huge toroid transformer would cost a pretty penny due to the costs of winding such with a really heavy core...

                                      Comment


                                        #20
                                        Re: Isolation Transformer Build - Technical Questions

                                        I was referring to this one dicky96 posted https://www.ebay.co.uk/itm/220V-110V...a9ab448e2197d2
                                        This looks very similar (different name) but has an internal picture https://www.aliexpress.com/i/32858021991.html
                                        Attached Files
                                        Last edited by R_J; 04-06-2020, 08:58 PM.

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