Isolation Transformer

Question

When a function generator is used to power a full wave rectifier an oscilloscope will not show the correct output due to a shared ground. A half wave is displayed because one of the diodes is 'shorted'. All of my research points me to the addition of an isolation transformer to separate the grounds. I am still somewhat new to transformers. We just finished covering mutual inductance and ideal/linear transformers in class so some things are still a little foggy.

I have found two different types of isolation transformers. The 'medical grade' type that have the duplex outlets and the bare 'case type-x'. The type x transformers have the 2 wires in and 3 wires out, depending on the presence of a center tap. So I guess my questions are as follows:

Which method should I use to break up my ground?

1. Should I invest in a transformer that has duplex outlets?(Tripp-Lite IS250HG or similar) I am not sure how these work, but I am guessing the generator and scope plug into the device and the device separates the two?

2. Should I buy a cheap TRIAD-N48X bare and terminate my function generator directly into the primary winding and continue my circuit from the secondary coil. Unfortunately, the TRIAD-N48x is the smallest isolation transformer I can easily find. It is rated for 115V/115V.

   • I wouldn't mind using this method if the isolation transformer will perform with 5-10 volts of potential instead of the listed 115 volts. I assume the 115 is just the max voltage.

3. I am open to other suggestions. I am on spring break so I finally get a chance to start construction on an organized in-home electronic workshop. So if there is something that I should 'build in' now would be the time! Thanks for reading.

Answer 1

It's a little unclear what exactly you are trying to accomplish. If the point is just to experiment with full wave bridges and see all the signals, then do it at a low voltage like JonnyBoats said. You can probably find a old transformer somewhere that is rated in the range 6-12V AC at 1 Amp or so. That's a good size to put a full wave bridge after, and then you also get a useful DC voltage to do lots of other things with.

As Jonny mentioned, there are also wall wart type transformers that are rated for less power but you can do mostly the same things with. The advantage is that these things are cheap nowadays. You can probably find something in the 3-5 Watt range for $5. Jameco has a broad selection of such things. That's a good place to look around.

If you really want to experiment with a full wave bridge driven by a function generator, then you should power the function generator from a isolation transformer. These are 1:1 transformers meant to take line power in and put line power out, except that the output can float. These will cost a bit more since they are usually intended for 100 W or more. Sometimes they even come in a box with a line cord for the primary of the transformer and a regular output connected to the secondary. You simply plug the cord into a wall outlet, and the thing you want isolated into the outlet on the box.

One gotcha you have to be careful of with these things is that they may not come wired up fully isolated. I bunch of years ago I bought a 500 W "isolation transformer" that was just as I described above. I used it to float a device under test so that I could hook up a grounded scope to it at various places. The first time I touched the scope ground clip to part of the power supply there was nice spark and the fuse blew. It turns out there was actually a deliberate ground wire inside the isolation transformer box connecting the ground from the line cord to the grounds on the output sockets. That's not what I consider "isolation", but someone else apparently does. Once I disconnected the two sides of the transformer and carefully verified there was no conduction path, it worked as intended.

Answer 2

If what you want to do is probe a full wave rectifier circuit for learning purposes then you are much better off with a simple transformer that puts out something like 6-12 V. 120V could kill you while low voltage should be safe, plus it will be much cheaper.

Since the output of the transformer is 2 wires, with both isolated from the AC ground, you should not have any ground loop issues. You can even go to a thrift store and get an old wall-wart that outputs AC, voltage is not critical.