DIY Isolation Transformer

After I bought my test gears such oscilloscopes, I noticed that I should have an isolation transformer in my bench. Then, I forgot again with the other projects. One day, after seeing Dave and Todd videos blog about isolation transformer, I started the construction of DIY isolation transformer. For those interests, here is Dave and Todd explained videos. Thanks both.

Some designed by cascading back to back secondary winding of two transformers (220V||12V<->12V||220V). It also be worked fine but I build my own. One of my buddy’s local transformer manufacturer wound this transformer and give me as present. Thanks buddy again.

It was designed as

  • Rating : 1KVA, 220V
  • The core  cross sectional area : 60mm x 60mm
  • Primary winding & Secondary winding : 300 N, 17SWG

The two winding should be wounded together to avoid turn ratio mismatch.

DIY-Isolation-Transformer (1)

I found old charger chassis and collected some some required staffs such as sockets, fuse etc.

DIY-Isolation-Transformer (2)

Seated transformer at chassis base. Prepared face plate, cover hole with plane PCB, installed input and output fuse houses, lamp. And wiring,


I agree with Todd suggestion. This is the schematic from commercial isolation transformer. The output winding NEUTRAL is CONNECTED to main GROUND!!!


This is the edited diagram as per suggestion. Disconnected secondary winding (NEUTRAL) from main GROUND. Mine has no circuit breaker and surge protection but both input and output fuses.


Final assembly is look like that. A new tool is at my bench.

DIY-Isolation-Transformer (7)

Note :

– DUT (device under test) should be isolated with transformer rather than oscilloscope.

– BUT, Dave scenario-3… if  DUT power supply is grounded to chassis (main ground), the problem is still existed. Disconnect DUT ground ?? or use with floating power supply?! or with battery? 😛

– Test device such as oscilloscope should not be floated (must be grounded) without special reason.

– Most forum and articles talked about isolating oscilloscope not DUT. If someone know the best practice and pros, cons, let me know.

– Differential probe, usb isolator are also good but cannot still effort.

– I need GFCI outlet at my bench.


I recently found a post at Keith’s Electronics Blog. There is also original schematic from manufacturer. It isolates even ground pins of each output. Yes, system ground (Chassis ground) is purposely isolated.

– My thought is that isolating system ground has some pro and cons. Sometime we need to isolate for full safety or purposely. In most case, test equipment should be grounded for safety.

Refrubrishing My Isolation Transformer

and schematic from Keith’s Blog