How to tell if a doorbell transformer is blown?

Determining whether a doorbell transformer has failed or "blown" involves testing its electrical output using a digital multimeter. First, locate your transformer (typically mounted on a junction box near the main electrical panel, basement ceiling, or utility closet). Set your multimeter to measure alternating current (AC) voltage, and touch the probes to the low-voltage screw terminals. A healthy transformer should read between 16 and 24 volts AC. If the multimeter reads zero volts or significantly lower than its rated output, the internal thermal fuse has blown or the windings have failed. You can troubleshoot low-voltage hardware safely by turning off the branch breaker before inspection. You can verify that your diagnosis is correct when replacing a dead transformer restores proper power delivery to your doorbell chime.