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 caps
Step 3: Consolidating Tools
For this project, you will only need a few common tools. You will need:
- Hot glue gun
- Soldering Iron
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!