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- When troubleshooting AC outputs controlled by TRIACs within a PLC system, confusion in meter readings can arise due to the solid-state nature of series circuits.
- Unlike the straightforward measurements in digital and relay outputs, AC outputs driven by TRIACs can yield perplexing results.
Multimeter as a Key Tool
- A multimeter equipped with a voltage measuring function proves invaluable for electrical troubleshooting technicians.
- Swift readings enable the identification of open and closed circuits, pinpointing potential issues with just a couple of measurements. In industrial control circuits, voltage readings typically register either 0 volts or the complete source voltage (often 24 VDC or 120 VAC for control circuits).
Challenges with AC Outputs Driven by TRIACs
- While partial drops in voltage are infrequent, they can occur when diagnosing AC output devices such as contactor coils or solenoids.
- When these 120 VAC outputs link to a relay output on a PLC module, a closed relay contact should convey 120 volts, and an open contact should transmit 0 volts.
- Consequently, the technician should observe the same voltages across the output device, offering insights into whether the issue lies in the device, wiring, or the PLC itself.
- However, when these coil devices connect to a TRIAC AC output (sometimes referred to as an ‘AC Output’), readings across a malfunctioning coil may display intermediate values ranging from 100 to less than 120 volts — an illogical range. While instinct may suggest a faulty PLC output delivering insufficient voltage, caution is warranted. This discrepancy can be particularly confusing for technicians.
- As we will explore, this reading may signify an open wire within a malfunctioning output device, with the TRIAC itself remaining operational.
What is TRIAC output in PLC?
- The term refers to a solid-state component known as a Triode for AC, or TRIAC, which is a silicon-based switch activated by a minimal control voltage from the PLC.
- Analogous to a transistor, such as a MOSFET, specifically designed for AC loads, it exhibits a slight voltage drop during activation, resulting in current limitations compared to relay circuits.
- Nevertheless, owing to its absence of moving parts, TRIACs boast an almost limitless switching life, provided they operate within the specified current limits.
- In line with all solid-state devices, the TRIAC never achieves complete ‘on’ or ‘off’ states. Instead, its resistance alternates between extremely low, facilitating current supply to the output device, and extremely high, regulating the current flow to the output device.
- When the terminal receives power, this exceedingly low but measurable resistance induces a minute voltage drop, leading to power dissipation and heat.
How do you troubleshoot a TRIAC output?
Testing a TRIAC Output
- To conduct a straightforward multimeter examination of a TRIAC AC output, position the AC voltmeter leads between the output terminal and the neutral (N) of the AC line.
- Activate the output terminal to the ON state. The meter should register either 24 or 120 VAC, depending on the system specifications.
- Subsequently, switch the output terminal to the OFF state. The voltage reading at this point should surpass 0 volts, settling at a level slightly above half of the power supply. It’s challenging to pinpoint the exact anticipated value due to variations in TRIACs and meter resistances.
- The intermediate voltage reading arises from leakage current in solid-state devices.
Leakage Current Problem
- For widely used TRIACs, ‘leakage current’ denotes a manufacturer-specified limit on the current range anticipated when the TRIAC is in the off state.
- This value is exceedingly minimal. For instance, the datasheet for the BT136, a TRIAC rated at 4A, outlines an off-state current quantity, which is synonymous with leakage current.
Interpreting Intermediate Voltage Readings
- In this practical scenario, the specified range for off-state current in the TRIAC application is between 0.1 mA and 0.5 mA with an applied voltage of 600 volts. Naturally, lower applied voltages result in smaller leakage currents.
- Calculating the equivalent off-state resistance for the provided current values yields a range of 6 Megaohms to 1.2 Megaohms.
- Considering this information, a technician troubleshooting a device with an open coil measures a voltage of 112 volts across the coil, expecting to observe 0 volts. Despite verifying that the PLC terminal is in the OFF state, the 112-volt reading raises concerns about the integrity of the output.
- An intermediate voltage reading across an output device connected to a TRIAC output may indicate an open wire within the device rather than a faulty PLC output.
TRIAC Series Circuit Representation
- Conceptualizing the TRIAC as a series circuit, the multimeter completes the circuit upon connection due to the open coil.
- The small leakage current passes through the meter, registering a voltage. The critical question, however, is whether the 112 volts constitutes a reasonable measurement.
- A typical Digital Multimeter (DMM) possesses internal resistance ranging between 10M and 20M Ohms, with high-quality tools typically closer to the 20 Megaohm level.
- With a measured voltage of 112 and an internal resistance of 20 Megaohms, the resulting current is approximately 5.6 microamps.
- Returning to the PLC, the TRIAC accounts for the remaining 8 volts. With a leakage current of 5.6 microamps, the equivalent resistance falls around 1.5 Megaohms, aligning with the TRIAC specifications.
- Generally, the voltage value for a 120 VAC supply would likely range between 80-110 volts, and for a 24 VAC supply, it would be approximately 20-23 volts. The smaller supply voltage corresponds to reduced leakage current and less voltage dropped by the meter.
Factors Affecting Voltage Drop
Role of Multimeter Resistance
- The multimeter’s internal resistance influences the measured voltage. A lower multimeter resistance reduces the voltage reading, following the principles of a series circuit voltage divider. Use a multimeter with a high input impedance.
Terminal State Consideration
- Ensure the output terminal is in the ‘OFF’ position during troubleshooting. An ‘ON’ terminal will display the full supply voltage, potentially masking underlying issues with faulty coils.
Functional Coil vs. Faulty Coil
- When the coil is operational, its resistance typically remains considerably lower than that of the meter, often not exceeding a few hundred ohms. Even under the worst-case scenario of maximum specified leakage current, which should be less than 0.5 mA as per the datasheet, the resulting voltage drop is anticipated to be below 0.25 volts for a 500-ohm coil. When the coil is in good condition, the troubleshooting process closely aligns with expectations for a relay output.
- If the measured value raises significant concerns, introducing a resistor in parallel with the coil is a viable solution. Ideally, the resistor’s value should be approximately 10 times higher than that of the coil (around 5k to 10k ohms). While this won’t significantly impact the circuit’s current consumption, the resistance will remain notably lower than that of the TRIAC itself. This configuration ensures that the measured voltage for an open coil will be closer to 0 when the terminal is in the OFF state.
- If troubleshooting gets difficult or the problem persists, involve with competent experts who are knowledgeable about industrial control systems and PLC programming.