20ma Current Loop Tester

Isolation Transformer CircuitsA while back I needed to test a circuit at 240vac.  Since I needed to safely test the circuit using my oscilloscope, I needed the 240vac to be fully isolated from the power line.  So, to step up the voltage, I bought a small isolation transformer from Digikey.  This device is made by Triad and has four separate windings.  As shown below this transformer can be wired up in several different configurations to either provide a 1:1 isolated voltage conversion or it can step up the 120vac input voltage to 240vac or even 360vac.

When used on 240vac power lines, it can also provide a 1:1 voltage conversion or drop the voltage down to 120vac.  The toroidal core I bought was only good for 25va but other larger transformers can also be purchased.  In addition to traditional step-up and step-down applications, the toroidal style core can be useful when you need high AC current at a low voltage.  A few turns of heavy gage wire around the core can produce 10 Amps of AC current or more.  Years ago I had to test a current transformer circuit at 100 Amps AC and used a larger toroidal core with a few very heavy gage wires around the core.