DIY Isolation Transformer

One piece of test equipment that I had been missing for a while was a bench isolation transformer. This would have allowed me to perform tests and repairs that would protect me and the equipment being tested.

After looking around, I found it very difficult to find a device that met my requirements. Everything I found was too expensive or did not have what I wanted – a device with both voltage and current meters, with fuses on both the primary and secondary and an auto current trip on the input.

In the end I decided to build my own unit.

The core of the system was two identical transformers, wired back to back, to provide the isolation. These were sourced from some faulty UPSs, sourced on eBay. They were missing their batteries, but were working. I scrapped the UPSs and removed the parts that were worth keeping.

I recovered the current trip switch from an old Intermec 3400 Barcode Printer with a damaged chassis.

I was able to get a combination voltage/current/power meter on eBay, with a 6 week delivery from China.

The last few parts were a case and some IEC power connectors for both the input and the output, as well as fuses and fuse holders that I had in stock for many years.

The secondaries of the transformers were wired back to back – this provided dual isolation via the windings on both transformers. So when I tested it and connected the mains to the primary of the first transformer, I measured 230v AC on the primary of the second transformer. This proved that my theory worked, yay!

The meter gave me an indication when the device under test drew too much current, as well as showing the power draw. You may be thinking why did it show the current and voltage when the device was connected to the mains?

Well, I planned to connect the primary side of the isolation transformer into the output stage of a variac. A variac is a transformer with a variable output.

This allowed me to slowly increase the voltage to the device under test. I could see if the current draw was excessive and identify faulty equipment without having to connect the device to the mains and possibly blowing components, causing even  more damage.

Note:Variacs are very handy devices that can be used to recondition old power supplies that may not have been used for a long period of time.

These two units coupled together provided me with a good environment to test devices for repair, while protecting myself and the customers device being repaired.

A female IEC connecter is used for the power output. This allowed me to have a selection of different leads that could plug in here.

Here is a photo of the Isolation Transformer connected to a Frequency Inverter. You can see that the meter is showing the output voltage, current that was being drawn as well as the power consumed.

The back of the unit just has a power input connection and an input fuse for protection.

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