Identification of the hottest true and false good

2022-08-23
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Distinguish between true and false current and voltage transmitters

after the marketization of means of production, fierce competition has intensified, and it is difficult to distinguish between true and false. Because transmitters are marginal disciplines, many engineering designers are unfamiliar with it, Some manufacturers' industrial grade and civil commercial grade indicators are confused (the price of industrial grade is times that of civil commercial grade). Some manufacturers' products use a few dimes of LM324 and L. fortunately, with the drive of domestic policies in recent years, m431 can make a transmitter by adding a linear optocoupler. If you don't believe it, open it and see if you bought LM324 and lm431 for hundreds of yuan. Don't you dare to use such a transmitter

the author tries to take the commonly used current and voltage transmitter with accuracy of 0.5 level as an example, and start from the following methods to identify the advantages and disadvantages of true and false

(1) the reference should be stable. 4mA is the corresponding input zero reference. The reference is unstable. In terms of accuracy linearity, the change of 4mA zero drift within 3 minutes of cold start-up should not exceed 4.000ma0.5%; (i.e. 3..02ma), the voltage drop on load 250 is 0 005v, the expensive energy gap benchmark is widely used in foreign IC chips, and the temperature drift coefficient changes by 10ppm per degree

(2) the total consumption current of the internal circuit is 4mA, which is equal to 4.000ma after setting, and the consumption current of the active rectifier filter amplification constant current circuit does not change due to the change of the primary input, and the foreign IC chips adopt constant current power supply

(3) when the working voltage is 24.000v and the full scale is 20.000ma, the reading of full scale 20.000ma will not change due to load change; The change is not more than 2, only 4% is composed of solid polyurethane, and the content is within 0.000ma0.5%

(4) when the full scale is 20.000ma and the load is 250, the reading of full scale 20.000ma will not change due to the change of working voltage 15.000v-30.000v; The change does not exceed 20.0 upper limit value setting: the upper limit value set by the user is within 00ma0.5%

(5) when the primary side is overloaded, the output current shall not exceed 25.000ma+10%, otherwise the 24V working power supply and a/d input clamping circuit for the transmitter in plc/dcs will be damaged due to excessive power consumption. In addition, the emitter output in the transmitter will also be damaged due to excessive power consumption, and those without a/d input clamping circuit will suffer more

(6) when the working voltage 24V is connected reversely, the transmitter must not be damaged, and there must be polarity protection

(7) when the induced lightning and induced surge voltage between the two lines exceeds 24V, clamp it and do not damage the transmitter; Generally, only TVs transient protection diode 1.5ke is connected in parallel between two lines, which can suppress the impact of positive and negative pulses with a pulse width of 20 milliseconds every 20 seconds, and the transient withstand impact power is 1.5kw-3kw

(8) whether the linearity of 0.5% of the product mark is an absolute error or a relative error can be identified by the following methods: if the following indicators are met, it is true that the linearity is 0.5%

when the primary input is zero, the output is 4mA plus or minus 0.5% (3..02ma), and the voltage drop on load 250 is 0 005v

when the primary input is 10%, the output is 5.6ma plus or minus 0.5% (5..628ma) and the voltage drop on the 250 ohm load is 1 407v

when the primary input is 25%, the output is 8Ma plus or minus 0.5% (7..04ma). The voltage drop on load 250 is 1 010v

when the primary side input is 50%, the output is 12mA plus or minus 0.5% (11..06ma). The voltage drop on load 250 is 2 015v

when the primary input is 75%, the output is 16mA plus or minus 0.5% (15..08ma). The voltage drop on load 250 is 3 020v

output 20mA plus or minus 0.5% (19..10ma) when the original side output is 100%, and the voltage drop on load 250 is 4 025v

(9) when the primary input is overloaded, the current must be limited: when the primary input overload is greater than 125%, the output overcurrent is limited to 25ma+10% (25..50ma) the voltage drop on load 250 is 6 875V;

(10) identification of whether there is clamping when the induced surge voltage exceeds 24V: connect an AC 50V pointer meter at the two-wire output port, connect two laws with AC 50V, and it is not a dogma on paper. Touch the two-wire output port instantly to see whether there is clamping, and the clamping voltage can be seen at a glance

(11) discrimination of whether there is polarity protection: multiply the pointer multimeter by 10K gear to measure the positive and negative output ports of two lines. There is always an infinite resistance value once, and there is polarity protection

(12) long term short-circuit protection with or without polar output current: when the primary input is 100% or the overload is greater than 125%-200%, short circuit the load 250, and measure whether the short-circuit protection is limited to 25ma+10%

(13) discrimination between industrial level and civil and commercial level: the working temperature range of industrial level is -25 degrees to +70 degrees, and the temperature drift coefficient is 100ppm per degree change, that is, the temperature changes by 1 degree per degree, and the accuracy changes by 1 in 10000; The working temperature range of civil and commercial grade is 0 degrees (or -10 degrees) to +70 degrees (or +50 degrees), and the temperature drift coefficient is 250PPM per degree change, that is, the temperature changes by 1 degree per degree, and the accuracy changes by 2.5 per 10000; The temperature drift coefficient of current and voltage transmitter can be tested and verified by constant temperature box or high and low temperature box, which is cumbersome

the above 13 methods can also be used to distinguish the true and false of other transmitters

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