The – terminal of the first item on the loop should be run to the + terminal of the second item on the loop, and so on until the wiring returns to the – terminal of the power supply. The power supply + terminal should be run to the + terminal of the first item in the loop. If the current is less than 22mA and the supply output voltage is low, then the power supply could be faulty. Ensure that the transmitter is installed with the proper polarity. Possible causes for excessive loop current are miswiring, a ground loop, and a problem with the transmitter. Insert a milliamp meter between the supply and the disconnected wire to measure the loop current.Ī loop current much larger than 26mA would indicate an excess load on the loop. If the voltage output level returns to the specification a high loop load might be the cause. If the voltage output remains at the faulty level, the supply might be bad. Disconnect the loop and measure the voltage output. Variation of the output voltage with load is normal for an unregulated supply.ģ) If the supply is regulated, and the output is low, it may be caused by a high loop load. Measure the loop power supply voltage, and ensure that it is at the proper level.ġ) If the supply output is zero, determine if the supply is being powered, if a fuse is blown, or if the supply is damaged.Ģ) If the supply voltage is a little low, check to see if the supply is unregulated. The first step in troubleshooting any circuit is to check the power supplies. Otherwise, start with the loop device section. Please refer to the display manual or the setting up document in this series. If the only malfunction is that the display is reading the wrong numbers, then you should first investigate setting up the display. If it appears that the loop is functioning in some manner then the nature of the malfunction becomes important. If the loop does not appear to function in any manner then one should first check the power and wiring. Problems can be caused by power, wiring, or loop device issues. The first thing to do is determine where the problem is. The loop wire must be disconnected, and the DVM attached into the circuit to cause all the current to run through the DVM.Ī malfunctioning 4/20mA loop can be caused by many things. Current is measured by attaching the DVM in series with the circuit. Voltage is measured by attaching the DVM leads across the device, essentially attaching the DVM in parallel with the device. It is possible to damage the loop, the DVM, or yourself by using it improperly. Besides the usual sources, a suitable DVM can be found in the hardware stores, auto parts stores, and department stores. The DVM should be able to read Volts and milliAmps DC. To troubleshoot a 4/20mA loop electrically, one must use a Digital Volt Meter.
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