Summary
Highlights
The video starts by introducing a non-working output on a MicroLogix 1400 PLC, specifically a light. The PLC's output signal is confirmed to be active digitally, but the light is not turning on. The first troubleshooting step is always to check the bulb, which is assumed to be good for the purpose of the demonstration.
The next step is to measure the voltage at the output terminal connected to the light. The presenter uses a multimeter set to AC volts, noting that the red wire color indicates an AC circuit, but advises verifying voltage type if unsure. Upon measurement, zero volts are detected at the light, indicating a lack of power reaching the device.
Since no voltage is reaching the light, the troubleshooting moves to the control panel to determine if the issue is wiring or a faulty PLC output. The output is forced on through the PLC, and then voltage measurements are taken inside the panel. The neutral connection is established as a reliable reference point for voltage readings.
Measurements are taken at the terminal block where the light's wire connects, and then at the PLC's output terminal with the output still forced on. No voltage is detected at either point, even at the PLC's output. This indicates that the PLC's internal relay contact for that specific output is not closing, suggesting a bad PLC output.
To confirm the diagnosis, another PLC output (output zero) is forced on, which is known to be working. Voltage measurements at output zero's terminal block and the PLC's output show 120 volts, confirming that the PLC is correctly sending power when the output is functional. This comparison solidifies the conclusion that the initial output is indeed faulty.