Industrial Isolation Transformer From Trash





Introduction: Industrial Isolation Transformer From Trash

If you work with electronics projects that deal with unisolated mains electricity, than you may be in danger of being lethally shocked! In addition to being dangerous to you, mains electricity can be lethal to electrical test equipment -such as oscilloscopes- if they are used on any unisolated electronic devices. Luckily, there is a device called an isolation transformer that will supply 110 volts AC completely isolated from the mains. This isolation greatly decreases the probability of lethal shock while working with electronics projects. Most people are unable to afford this great safety device because of the one hundred dollar price tag attached to the majority of isolation transformers. But fear not readers, there is a solution for this problem! In this instructable, I will show you how to build a high quality isolation transformer from broken electronic devices. The video below will accompany this instructable with a demonstration of the isolation transformer working.

Lets get started!

Step 1: How an Isolation Transformer Works

An isolation transformer works on the same principal that all other transformers use. When an alternating current flows through a coil inductively coupled to another coil, the second coil has current induced into it. The voltage in the secondary coil is proportional to the ratio of windings in both coils. An isolation transformer has a winding ratio of 1:1, so the input voltage is the same as the output voltage. In normal mains current, the live wire is referenced to ground, so if the live wire is touched ,the person touching it will get shocked because they are capacitively coupled to earth ground. The isolation transformer fixes this issue.

Step 2: Finding the Materials

Most of the materials for this project can be scavenged from trash piles.

You will need:

  • A streetlight ballast transformer(Can be found in industrial waste piles because most sodium vapor streetlights are now being replaced by LED ones.)
  • Computer power supply(For the case and AC connector)
  • Plywood or particle board(Found in a wood scrap bin)
  • AC Receptacle(Found in my spare electronics bin, can also be bought at home improvement stores.)
  • AC switch
  • Wire
  • Wire caps
  • Screws

Step 3: Consolidating Tools

For this project, you will only need a few common tools. You will need:

  • Hacksaw
  • Drill
  • Hot glue gun
  • Soldering Iron
  • Jigsaw

Step 4: Preparing the Isolation Transformer

The isolation transformer is the most important part of this project. To convert the ballast transformer to an isolation transformer, you will need to first look at the wiring schematic. All ballast transformers have one. After that, cut the white wire connecting the two coils of the ballast. Then, test the resistance of the 120 volt winding of the primary coil, and then test the resistance off all the windings of the secondary coils. The two coils with matching resistances will have matching impedances, so they will be the primary and secondary coils. Because all isolation transformers are different, it is hard to give any specific instructions on how to set it up. For this step, you mostly need to experiment with your ballast transformer. Just remember; only connect AC mains to the wires indicated on the schematic. Failure to do so could result in burning up the coils. By the end of this step, you should be able to identify two coils with matching impedances on the transformer and possibly identify any other voltages on the other windings.

Step 5: Cutting and Drilling the Base and Case

The base will house the entire isolation transformer and wiring. the base will support it. You first will need to cut out a piece of particle board that is the same size as the bottom of the computer power supply case. You can then drill holes in the side of the metal case and in the sides of the particle board so the metal case can be screwed to the base. You will also need to drill a hole on top for the switch and add screws on the bottom to hold the transformer in place.

Step 6: Placing Components

You will need to place components. You can place your components in the way most aesthetically pleasing to you, but I placed mine as shown in the pictures above. I used wood blocks to hold the electrical outlet above the wood base. I added the isolated output on the front and the input plug on the back. The switch is screwed in on the top. Most components can be attached with hot glue, though some are bolted in by wood screws.

Step 7: Wiring

To wire the isolation transformer, follow the above schematic. The mains AC goes through a switch into the isolation transformer. The outputs of the isolation transformer go directly into the outlet. To connect wires, wire nuts are used. Solder is used to connect wires to the switch. The lug nuts on the outlet are used to connect the outlet to the isolation transformer.

Step 8: Putting It Together

Finally, to put the isolation power supply together, bolt the metal case to the base and make sure all wiring is inside. It should be ready to test.

Step 9: It Works!

To test the isolation transformer, connect it to mains AC and then use the outlets how you would normally use mains power. This power supply can be used to safely power many electronic devices and projects. It puts out about 240 watts. This isolated power supply drastically reduces the chance of injury by electricity, but it does not completely get rid of the possibility. Always experiment with mains electricity with caution. I am not responsible for any injury caused by the building of this power supply.

Good luck building!



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    Interesting reuse of a Ballast Transformer. I imagine from the core size it may handle about 100 -200 Watts, but it really depends on the wire sizing of the winding's, isolation material and exact core details. So it would have been good to inform us of the maximum continuous current / wattage he managed without the core overheating, excessive secondary voltage drop. This is simple to do.

    Cannot be sure of how much isolation you actually have though. This is really important to know. The Primary & Secondary winding's are separated, that is a positive due to physical separation distance. But it also will depend on the isolation of the winding's to the core and to each other. It will probably give some isolation. But you really really need to know how much. The way to check this is performing a Hi Pot test between winding's and winding's and core. You can use a simple Megger to do a basic Hi Pot test. But please don't rely on this Transformer for serious safety use, especially for outside use as the housing is not sealed adequately which means if the it get wets, you may end up with no isolation and an even worse shock hazard due to 200V winding tap.

    The most common reason to use a Isolation Transformers is to to provide Galvanic isolation to people from AC Utility supply ground returns for safety reason.

    ie. Domestic utility AC supply Neutral is usually close to or the same as the ground potential. So defective equipment with an isolation fault to Hot may lead to human contact and a flow through the body to another part of the body connected to ground. A good isolation transformer avoids this. Obviously it still has a dangerous potential on the output but it avoids the ground loop issue.

    Modern GFI's do not isolate, they merely trip the power when a current from HOT to ground is exceeded. A shock will still happen, but due to it's momentary nature it is unlikely to cause death or serious injury to most people.

    Until Modern GFI's became available the use of isolation transformers for AC powered power tools inside was and probably for outside work is still highly recommended even mandatory for commercial professionals is some countries. And highly advisable even mandatory to use Isolation Transformers for AC powered hand tools outside, even on generator sets. In Electric Power Stations because of the various high potentials generated and the possibility to connect to them it through Ground it is still mandatory in some countries to use Isolation Transformers. I seem to remember in the UK where domestic power is 240VAC, that in UK power stations they could only use 120VAC power tools connected through a certified 240V to 120V Isolation transformers with certified isolated 120V power tools. This may have changed somewhat with the recent availability and popular use of battery powered hand tools.

    Note: I used AC power tools as an example due the obvious physical contact using them involves, but it still will apply to any AC mains powered Test Equipment, and all other electrical equipment where physical contact is involved as well. Medical instrumentation is another very specialized field where electrical isolation is very important, this has other criteria beyond scope of a simple discussion like this.

    4 replies

    I wondered why drills and such still don't use a safety ground. I assumed it is still handled by double insulation. Thanks for a great explanation. I'll have to try and understand more of it. I only really got the neutral/ground potential, ground loop and GFI parts.

    Here's something to think about, a lot of modern electric drills, sanders, vacuum cleaners ect. have an emblem on the data plate, which is a square within a square, this means that the item is double insulated, I.E. allowed to run with only two core cable, This is one of the reasons why you don't see many metal cased drills, similar to the the old "Wolf" drills, the real problem with the metal casing was that as the drill brushes wore down they scattered carbon dust through the rear ventilation slots and straight on to the operators hands, in damp conditions these created a direct link between the hand and the "Live" brush gear, I've known friends who have used these outside and couldn't let go of them, the only way their still alive, was by a co-worker kicking the tool out of their hand, (by the way I live in the U/K with 240 volts)

    A most thoughtful and erudite response, thanks for sharing your perspective.

    Nice to see young people get involved in Electronics, learning the ropes as to say in a "hands on" way. He has the basic concept. They must be encouraged, hopefully they'll become tomorrows innovative "think out the box " engineers of which we have too few nowadays.

    Good choice will be have two outputs: 110 (or 220V for europe) isolated power output, and low volatage for MCU-powered control + BT module + optorelay (fan control, timer, manual controil from cellphone,..)

    Hi Tanner, great job exploring electrical theory. I want to encourage you to keep it up. I would also like to encourage you to start expanding your research skills as you post instructables like this. It is particularly difficult to find reliable information across the internet on electrical systems, which is why you are getting some very brief comments regarding how dangerous this project can be.

    I want to ask you to make some edits in the name of safety:

    Please remove the comments about protection from lethal shock.

    An isolation transformer in no way removes the potential for lethal electric shock. A GFCI for personal protection is the only device designed for that. Electric current past .006 amps has the potential to cause the human heart to go into fibrillation, which is why GFCI receptacles open the circuit between 4-6mA during a ground fault. If that was a 300W fan you were running at 120V then the current is 416 times a potentially lethal shock.

    The potential for a human becoming part of the circuit (ungrounded or otherwise) is the hazard, and an isolation transformer has close to nothing to do with that.

    I understand this is an experiment, but if you are interested, the use of the device violates the 2014 NEC (current for a few more months) section 250.20(B)(1) -systems required to be grounded.

    The last request I have is for you work on the build a little bit. The use of combustible materials is one thing to change, the next would be to cover the exposed energized parts. Plastic trim for the receptacles are less than a dollar at a hardware store.

    Keep learning and be safe.

    2 replies

    even though i no longer have my code book since i retired, your information is in error.

    first of all it takes around 1/10 of an amp, to stop the heart. it is the frequency around 60hz that causes the heart and or lungs and diaphragm to fibrillate.

    and there was a special section, for ungrounded systems in the code book.

    and like i have already stated, non-gfi lines increase the danger of shock by adding another path for current to flow from any part of your body to ground.

    and gfi, is only required in wet areas according to us code.

    and fcc codes, supersede all nec national register codes. chassis and circuit grounds are not always earth ground for a reason.

    essencially in an isolated system, the one or the other wire is the ground. in an isolated system, your must touch both wires somehow to receive a shock.

    in a grounded system, you can receive a shock from touching neutral and hot plus ground and the hot wire. a ground wire increases the risk of shock.

    and in most audio and broadcast systems, your going to have a real problem if you connect the positive chassis ground to negative earth ground.

    Hi Jimmie, the 1/10 of an amp you mention is the threshold for muscular contraction that german scientists discovered during World War 2. You are correct that 60hz ac power causes additional problems, however it is current that kills.

    The special section of the NEC you refer to is the following article: 250.21 AC systems not required to be grounded, which this system does not meet the requirements of.

    None of these details change the fact that if you do touch either of the exposed terminals on the sides of that receptacle, or the same for anything you might have plugged into this device...

    you might die.

    I have heard of an isolation transformer before, but I really had no concept of what it was attempting to provide. Correct me if I am wrong, are you trying to provide safety to an operator from a lethal ac shock? If that is the case, why not operate electrical test equiptment from a GFI controlled outlet? I notice that your schematic does not indicate a connection to house wiring ground. The wired in computer power supply connector has a green / ground wire, but you don't show it going to anything of there a reason?

    How many amps would your set up provide?

    1 reply

    I have to ask the same question.

    We have our newly boxed isolation transformer. It has a 3pin plug top ready to plug into the mains wall socket. It has a 3pin output socket. It has an MCB on the input and another MCB on the output.

    How do we wire up the Protective earths (PE)?

    Do we wire the enclosure to the PE?

    Do we wire the output socket to the PE?

    Do we fit a GFI (RCD, RCBO) to the input wiring?

    Do we fit a GFI to the output wiring?

    Great job! Having purchased isolation transformers when a pile of scraped street lights was only 50 feet away, it was definitely a "Doh" moment for me. In our mostly low voltage world, we tend to forget how useful a mains isolation transformer can be. It is certainly a must have for working on older equipment, and still is a great way to isolate the ground on your oscilloscope when probing active circuitry.

    Also loved your VFD amp! What a cool experiment! A tremendous demonstration of the reuse, repurpose ethic. Very much out of the box, can't wait to see more of your projects!

    1 reply

    the isolated supply feeds the Device Under Test (DUT).

    The isolated supply is NOT used to supply the grounded oscilloscope.

    Never probe anything with the scope probe deliberately ungrounded. or use the specialised differential probe that costs more than most scopes.

    Lacking a ballast transformer two power transformers can be used. Connect the secondary windings together and the voltages on the primary windings are equal.

    Best to use identical transformers. Likely more isolation that way, don't know. Leave that to the experts on here.

    1 reply

    Using two identical mains transformers back to back gives an output voltage that equals the input, when there is no load current.

    As soon a load current is demanded, the output voltage drops.

    The voltage drop under full power load is roughly two times the voltage regulation of the mains transformer.

    eg, using two 230:55Vac 8% regulation 500VA mains transformers will give a 1:1 input to output voltage on any EU harmonised supply when no load current is drawn.

    When the 500W resistive load is applied, the voltage will drop to ~inputVac/(1+0.08)² ~86% input Vac. This resulting isolated voltage is well below the minimum voltage specified for an EU harmonised voltage.

    the idea is isolate your 110vac from any common ground, thus hot and neutral have no connection to ground.

    1 reply

    technically without an earth ground, there is no neutral. since the neutral, connects to earth ground as some point making the opposing polarity hot to earth ground. most equipment transformers, the secondary is isolated from earth ground. giving you the option of choosing, circuit ground, chassis ground, or earth grounding.

    I guess that I am somewhat deprived of experience. I've never seen a ballast transformer that was't potted.

    1 reply

    that is the difference between fluorescent lamp ballast and high pressure sodium or mercury vapor lamp ballasts. fluorescent lamp ballasts, are typically potted.