After the marketization of production materials, the fierce competition is intensified, the true and false are difficult to distinguish, and the transmitter is a marginal subject. Many engineering designers are unfamiliar with this. Some manufacturers are confused with industrial grades and commercial commercial grade indicators (industrial grade). The price is 2-3 times that of commercial commercial grade. Some manufacturers can make a transmitter with a few cents of LM324 and LM431. If you don't believe it, you can open it and see if you bought it for a few hundred yuan. The LM324 and LM431, such a transmitter to send you, you dare not use it!
The author tries to use the commonly used 0.5-level precision current-voltage transmitter as an example, starting from the following methods to distinguish between good and bad.
(1) The reference should be stable, 4 mA is the corresponding input zero reference, the reference is unstable, and the accuracy linearity is determined. The zero drift of 4 mA within 4 minutes of cold start does not exceed 4.000 mA 0.5%; (ie 3.98) -4.02mA), the voltage drop on the load 250Ω is 0.995-1.005V, the foreign IC core piece uses the expensive energy gap reference, the temperature drift coefficient changes by 10ppm per degree;
(2) The total current consumption of the internal circuit is <4mA, which is equal to 4.000mA after the setting is adjusted, and the active rectification and filtering amplification constant current circuit does not change the current consumption due to the input change of the primary side, and the foreign IC core piece adopts constant current power supply;
(3) When the working voltage is 24.000V, the full-scale 20.000mA reading will not change due to the load 0-700Ω change; the change transmitter interference signal does not exceed 20.000mA0.5%;
(4) When the full scale is 20.000mA, when the load is 250Ω, the reading of 20.000mA of full scale will not change due to the change of working voltage 15.000V-30.000V; the change does not exceed 20.000mA0.5%;
(5) When the primary side is overloaded, the output current does not exceed 25.000mA+10%, otherwise the 24V working power supply and A/D input clamp circuit for the transmitter in plc/dcs are damaged due to excessive power consumption. The output of the transmitter in the transmitter is also damaged due to excessive power consumption. No A/D input clamp circuit is more smashed; (6) When the working voltage is 24V, the transmitter must not be damaged and must have polarity protection. ;
(7) When the two lines are clamped due to inductive lightning and induced surge voltage exceeding 24V, the transmitter should not be damaged; generally 1-2 TVS transient protection diodes 1.5KE in parallel between the two lines can inhibit each The impact of the positive and negative pulses of 20 milliseconds pulse width at intervals of 20 seconds, the transient withstand impact power of 1.5KW-3KW;
(8) The linearity of the product mark 0.5% is the absolute error or the relative error, which can be discerned by the following method: the linearity of 0.5% is met when the following indicators are met. Output 4mA plus or minus 0.5% (3.98-4.02mA), the voltage drop on the load 250Ω is 0.995-1.005V. When the primary input is 10%, the output voltage is 5.6mA plus or minus 0.5% (5.572-5.628mA). When inputting 25% for 1.393-1.407V primary input, output 8mA plus or minus 0.5% (7.96-8.04mA) load voltage drop at 250Ω is 1.990-2.010V
When the primary input is 50%, the output is 12mA plus or minus 0.5% (11.94-12.06mA). The voltage drop on the 250Ω load is 2.985-3.015V. When the primary input is 75%, the output is 16mA plus or minus 0.5% (15.92-16.08mA) and the load is 250Ω. The pressure drop is 3.980-4.020V
When the primary side loses 100%, the output voltage is 20mA plus or minus 0.5% (19.90-20.10mA). The voltage drop on the 250Ω load is 4.975-5.025V.
(9) The primary side input overload must be limited: when the primary input overload is greater than 125%, the output overcurrent limit is 25mA + 10% (25.00-27.50mA). The voltage drop on the 250Ω load is 6.250-6.875V;
(10) If there is no clamp when the induced surge voltage exceeds 24V: In the two-wire output port and an AC 50V pointer-type meter, connect the two wires with AC 50V to instantly touch the two-wire output port to see if there is any Clamping, how many volts can be clamped at a glance;
(11) Discrimination with or without polarity protection: use the pointer multimeter Ω multiplied by 10K file to measure the two-wire output port positively and positively. There is always an Ω resistance value infinitely large, there is polarity protection; (12) There is no pole output current for a long time Short-circuit protection: When the primary input is 100% or the overload is greater than 125%-200%, the load is short-circuited by 250Ω, and the short-circuit protection limit is measured at 25mA+10%.
(13) Identification of industrial grade and commercial commercial grade: industrial grade operating temperature range is -25 degrees to +70 degrees, temperature drift coefficient is 100ppm per degree change, that is, temperature changes by 1 degree per degree, accuracy changes to one ten thousandth The commercial commercial temperature range is 0 degrees (or -10 degrees) to +70 degrees (or +50 degrees), the temperature drift coefficient is 250ppm per degree, that is, the temperature changes by 1 degree per degree, and the accuracy changes to 10,000 points. 2.5: The temperature drift coefficient of the current and voltage transmitter can be tested and verified by the incubator or the high and low temperature box.
The above 13 methods can also be used to distinguish between the advantages and disadvantages of other transmitters.
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