The author attempts to use commonly employed 0.5-class accuracy current and voltage Transmitter For example, start by using the following methods to distinguish between genuine and fake, and assess quality.
(1) The reference must be stable: 4 mA corresponds to the input zero-point reference. If the reference is unstable, how can we even talk about accuracy and linearity? Within three minutes of cold start-up, the zero-point drift of 4 mA shall not exceed ±0.5% of 4.000 mA (i.e., between 3.98 and 4.02 mA); the voltage drop across a 250 Ω load shall be between 0.995 and 1.005 V. Overseas IC chips often employ expensive bandgap references with a temperature coefficient of only 10 ppm per degree Celsius.
(2) The total quiescent current of the internal circuit is less than 4 mA; after adjustment, it is precisely 4.000 mA. Moreover, the active rectification, filtering, amplification, and constant-current circuit ensures that the quiescent current remains stable regardless of variations in the primary-side input. Overseas IC chips typically employ constant-current power supply.
(3) At a operating voltage of 24.000 V and a full-scale current of 20.000 mA, the reading at full scale will remain unchanged despite variations in the load resistance from 0 to 500 Ω; the deviation shall not exceed ±0.5% of the full-scale value of 20.000 mA.
(4) When the full-scale current is 20.000 mA and the load resistance is 250 Ω, the full-scale reading of 20.000 mA shall remain unchanged regardless of variations in the operating voltage between 15.000 V and 30.000 V; the deviation shall not exceed ±0.5% of 20.000 mA.
(5) When the primary side is overloaded, the output current shall not exceed 25.000 mA + 10%; otherwise, the 24 V power supply for transmitters within the PLC/DCS and the A/D input clamping circuit will be damaged due to excessive power consumption. In addition, the emitter-follower output inside the transmitter will also be damaged from excessive power consumption; transmitters without an A/D input clamping circuit will suffer even greater damage.
(6) The transmitter must be protected against damage in the event of reverse connection of the 24 V operating voltage; polarity protection is mandatory.
(7) When the induced lightning and induced surge voltage between the two wires exceeds 24 V, clamping must be implemented to prevent damage to the transmitter; typically, 1–2 TVS transient protection diodes of the 1.5KE series are connected in parallel across the two wires. These diodes can suppress the impact of positive and negative pulses with a pulse width of 20 ms occurring at intervals of 20 seconds, with a transient surge power rating of 1.5 kW to 3 kW.
(8) Is the linearity specification of 0.5% stated on the product an absolute error or a relative error? The following method can be used to determine this clearly: Only when the following criteria are met can it be considered true linearity of 0.5%.
When the primary-side input is zero, the output is 4 mA ±0.5% (3.98–4.02 mA), and the voltage drop across a 250 Ω load is 0.995–1.005 V.
When the primary-side input is 10%, the output current is 5.6 mA ±0.5% (i.e., 5.572–5.628 mA), and the voltage drop across a 250-ohm load is 1.393–1.407 V.
When the primary-side input is at 25%, the output current is 8 mA ±0.5% (7.96–8.04 mA), resulting in a voltage drop of 1.990–2.010 V across a 250 Ω load.
When the primary-side input is at 50%, the output current is 12 mA ±0.5% (11.94–12.06 mA), resulting in a voltage drop of 2.985–3.015 V across a 250 Ω load.
When the primary-side input is at 75%, the output current is 16 mA ±0.5% (15.92–16.08 mA), resulting in a voltage drop of 3.980–4.020 V across a 250 Ω load.
When the primary-side input is at 100%, the output is 20 mA ±0.5% (19.90–20.10 mA), resulting in a voltage drop of 4.975–5.025 V across a 250 Ω load.
(9) Current limiting must be implemented in the event of primary-side input overload: when the primary-side input overload exceeds 125%, the output overcurrent limit is set at 25 mA + 10% (i.e., 25.00–27.50 mA), resulting in a voltage drop across a 250 Ω load of 6.250–6.875 V.
(10) Determining whether clamping is present when the induced surge voltage exceeds 24 V: Connect an AC 50 V analog voltmeter in parallel across the two-wire output terminals, then briefly touch the two wires together using the AC 50 V source. Observe whether clamping occurs and, if so, how many volts the clamping voltage is—this will be immediately apparent.
(11) Determination of polarity protection: Use an analog multimeter set to the ×10 kΩ range to measure the two output terminals in both forward and reverse directions; if, in one of the measurements, the resistance reads infinity, polarity protection is present.
(12) Long-term short-circuit protection for no-load output current: With the primary-side input at 100% or under overload conditions exceeding 125%–200%, short-circuit the load to 250 Ω and verify that the short-circuit protection limit is within 25 mA ±10%.
The aforementioned 12 methods can also be applied to assess the authenticity and quality of other transmitters.