Step down transformers are used to take a high voltage and transform it to a lower workable power. Typically, step down transformers are used in control circuitry. The most common home application may be the doorbell transformer. In this case, 120 volts alternating current (VAC) is stepped down to 24 VAC for use at the outdoor button. The lower voltage aids in retarding electrical shock potential. Regardless of the end usage, all step down transformers operate in the same way. The basic construction also is the same, as two separate continuous coils are used for the low voltage transformer. The high side, high voltage, is typically identified as H1 and H2 while the low side, low voltage, is X1 and X2.
Disconnect all electrical power from the transformer. This can be done by either switching off the circuit breaker or removing the fuse that supplies power to the circuit. Industrial applications may have a separate knife switch to control the power feed.
Remove all wires from the transformer terminals using the screwdriver. Identify the wires if they are not already identified. Use a clear tape and pen. Write the terminal that the wires are attached to and place the identified tape on the wire's end. The transformer must not be connected electrically to any outside circuit as this may give a false reading to the next steps.
Turn the volt ohmmeter to the "Ohms" position and place the red lead into the connector identified as "Ohms." The black lead should be connected into the common plug. Touch the black lead to the metal frame of the transformer.
Touch the red lead to the transformer's terminals in the following order: H1, H2, X1 and then X2. The meter should read infinite ohms or wide open. Infinite ohms on a digital meter will be identified as a blank screen or a wide open will have the word "Open" displayed. Different digital meters may have various screens. An analog meter, needle display type, will show no movement on the display. If the meter registers any form of resistance, there is an internal problem with the windings. The copper coils may be shorted to the metal frame of the transformer. The transformer will have to be replaced.
Check the continuity of each separate coil using the ohmmeter. Touch the black lead to H1 and the red lead to H2. The meter should give a resistance reading. Generally, it should read in the range of 3 to 100 ohms, depending on the style and type of transformer. Perform the same test to the X1 and X2 terminals. You should receive the same results. If the meter reads infinite ohms or a wide open when checking between the terminals of the same coil, the wires are broken. Replace the transformer.
Use the ohmmeter to conduct the transformers isolation circuit. Touch the red lead to H1 and the black lead to X1. The meter should read infinite ohms or a wide-open circuit. Perform the same test, but to H2 and X2 respectively. If any resistance at all is read on the meter other than a wide-open circuit, the isolation of the transformer has been compromised and must be replaced.