Zener mystery revealed
The zener barrier is a simple circuit, but when tossed into a control loop, the unexplainable can happen. An automotive manufacturing plant in Kentucky recently experienced a ‘zener phenomenon’ on good control loops that had been in service for years.
In most cases with intrinsic safety control loop problems, the barrier is the least understood portion of the loop, and is often targeted as the cause of nuisance trips.
Technicians at the plant investigated frequent nuisance trips on a group of I/O, but could not determine the problem. Using a digital multimeter (DMM), they checked the 24-Vdc power supply, which appeared to be operating correctly.
The technicians verified that the field devices were not causing the nuisance trips. Some of the tripping barriers were spares, which were not connected to the field devices. Running bench tests on the barriers did not replicate the problem that was occurring in the plant.
Technicians at the automotive plant contacted Pepperl+Fuchs ( www.am.pepperl-fuchs.com )
The company’s service people discovered that the only way to open the fuse was to raise the supply voltage above the breakdown voltage of the zener diode. This forced the zener into its conduction state, which caused the current to rise rapidly above the fuse rating, thereby blowing the fuse. As far as the fuse was concerned, this condition appeared to be a short circuit. Regardless of whether there was a short circuit, or the zener voltage was exceeded, the fuse was blowing, and the trips were definitely nuisances.
So what was causing the trips?
It sounds straightforward, but don’t forget that the automotive plant technicians had measured the power supply voltage to be a steady 24 Vdc. Circuit grounding is another possible suspect. If the ground potential was fluctuating, the apparent zener reference could exceed the zener conduction voltage. However, the grounding network was believed to be very sound; and it would be quite time consuming to debug that aspect of the circuit.
The technicians took another look at the power supply. This time, instead of using a DMM, they used a scope to measure the output of the power supply. Wouldn’t you know it; they discovered an intermittent 30.5 Vdc spike, which was certainly enough to send the 28-V zener into conduction. The zener conducted current that was sufficient to blow the fuse, but not for a long enough period of time to show up on the DMM!
The zener mystery was solved! The power supply had a defective electrolytic filter capacitor, which tend to dry out after several years in operation. The automotive plant, armed with much more information on zener barriers, was able to get up and running again.