What you'll learn
Welcome to fault-finding. From here on every lesson hands you a pre-wired panel with a problem buried in it; your job is to find the fault using the multimeter on the toolbar.
Lab time: ~8 minutes.
Lesson briefing
Photoeye Polarity Reversal
Welcome to fault-finding. From here on every lesson hands you a pre-wired panel with a problem buried in it; your job is to find the fault using the multimeter on the toolbar.
The multimeter has two probes — red and black. Pick a mode:
- Voltage (DC): reads the DC voltage at the red probe relative to the black probe. A correctly-powered 24 VDC bus reads +24 V at a red-on-(+V) / black-on-(0V) probe; reverse the probes and you read -24 V.
- Continuity: beeps when the two probes sit on terminals that are electrically joined. Useful for tracing wires and checking for breaks.
Symptom: the line operator reports the photoeye on the conveyor infeed has stopped detecting boxes. You walk over to the panel: the sensor LED is dark, the PLC input X0 LED is dark, and the rest of the panel looks healthy. The PSU's own indicator LED is on.
Probe the panel, decide what's wrong, then submit a diagnosis.
Hints
Start at the photoeye. Probe across its V+ and 0V terminals with the multimeter in voltage mode — what reading would you expect on a healthy install?
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This lesson uses 6 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.
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