Measuring Amps. Think about the changes you would have to make to a practical circuit in order to include the ammeter.
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All the current flowing in the circuit must pass through the ammeter. Meters are not supposed to alter the behavior of the circuit, or at least not significantly, and it follows that an ammeter must have a very LOW resistance. Measuring Voltage. This time, you do not need to break the circuit. The voltmeter is connected in parallel between the two points where the measurement is to be made.
Since the voltmeter provides a parallel pathway, it should take as little current as possible. In other words, a voltmeter should have a very HIGH resistance. Which measurement technique do you think will be the more useful? In fact, voltage measurements are used much more often than current measurements. The processing of electronic signals is usually thought of in voltage terms. It is an added advantage that a voltage measurement is easier to make. The original circuit does not need to be changed. Often, the meter probes are connected simply by touching them to the points of interest.
Measuring Ohms. An ohmmeter does not function with a circuit connected to a power supply. Ohmmeters work by passing a small current through the component and measuring the voltage produced. Those instructions enable the PLC to perform various useful control functions like relay logic, counting, timing, sequencing, and arithmetic computation. The PLC reads inputs, processes them through a program, and generates outputs. One reason for the popularity of PLCs is their high reliability in harsh industrial environments; occasionally, however, things do go wrong and troubleshooting becomes necessary.
Those unfamiliar with PLCs often fear troubleshooting a device that appears to be a mysterious "black box" see side bar on page 22 , but in fact today's PLCs actually are very open systems that lend themselves to relatively easy diagnosis.
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The intent of this article is to cover only the basics of PLC troubleshooting, so there are some limitations on what will be discussed. First, it's assumed that the PLC system under analysis was operating correctly at some time in the recent past, so the problems of program debugging and wiring errors that are more typical of a startup situation will not be addressed. It seems to be natural to assume that most malfunctions of PLC systems are due to processor problems, but in fact the opposite is true.
The first thing to check is the integrity of the PLC's power and ground. Visually inspect the power and ground wiring, looking for loose, corroded, or otherwise questionable connections. The integrity of the ground can be electrically checked by measuring the voltage between the PLC ground terminal and a known ground.
Using a digital meter set on the lowest scale, both the AC and DC voltages should be zero. The power supply also can be tested electrically. If the PLC processor has an AC power source, check the input voltage; it should be within the manufacturer's recommended range.
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PLC processors actually operate on DC power, so that also must be checked. Measure each of the outputs of the DC power supply and check if the voltages are within the recommended ranges. Also check the DC supplies for AC ripple. This can be done using a digital meter set on a low AC range, and the value measured should be well below the manufacturer's specifications.
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Excess ripple has drastic effects on the operation of the microprocessors and memory devices typically found in PLC processors. The final power check is to measure the voltage of any batteries in the system. Battery power is often used to prevent a PLC from losing its program during power outages, and battery voltages should be within recommended values. Other causes for erratic processor behavior are electro-magnetic interference EMI or radio frequency interference RFI.
Try to correlate the erratic behavior with an external EMI or RFI event like a large motor starting, arc welding in the area, lightning strikes, or even the use of handheld radio transmitters. Although they may seem harmless, handheld radios commonly used by maintenance personnel emit powerful RF radiation and can seriously disrupt the operation of unprotected electronic equipment. Long-term solutions to EMI and RFI problems usually involve improvements in power conditioning, grounding, and shielding.
Power, grounding, and interference problems all can cause the corruption of the PLC memory, so the next step is to verify that the program is still correct.
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All PLCs have some method for doing this, most of which involve comparing the program in the PLC with a backup copy on tape or disk. Keep the program backups up-to-date and safely away from temperature extremes, high humidity, and EMI and RFI exposure to ensure they will always be usable. Now on to the more common problem of troubleshooting inputs and outputs. This is done by using the addressing scheme for the particular PLC that you are working on, and this scheme differs from one manufacturer to another. Most digital input modules detect changes in voltage levels, and they are available with various AC, DC, or universal ratings, with universal modules typically accepting a fairly wide range of either AC or DC signals.
A typical AC input channel is shown in Fig. Note that the figure shows indicator lamps on both the power and logic sides of the circuit; many modules, however, have only one or the other of these. If only one indicator is present, it's important for troubleshooting purposes to determine where it is connected. If the threshold unit on an active input has failed, for example, a power-side indicator would be ON while a logic side indicator would be OFF. The power to drive PLC inputs usually is not supplied by the input module, so it's important to find out where that power comes from.
There are two types of inputs: Isolated and nonisolated. Troubleshooting differs depending on which type you are dealing with.
Each channel on an isolated input module is electrically separated from the others and may have a different source of power. On the other hand, one side of each input channel on a non-isolated module is connected to a common reference. Ship to: Finland.
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