Month: May 2024

Remote pressure gauges are often used in variable frequency water supply control systems, with resistance values ranging from 100 ohms to 500 ohms, corresponding to continuously changing pressure range values, such as 0-10mpa. This continuously changing resistor can be directly applied by serializing a larger resistor and applying a 0-10V signal to obtain the continuously changing voltage for use by the frequency converter, PLC, and other controllers. In practical engineering applications, 4-20mA has stronger anti-interference ability than 0-10VD, so it is often necessary to use 4-20mA signals for transmission control. At this time, resistance signals or voltage signals need to be converted into 4-20mA signals. Some frequency converter manufacturers have developed such water supply signal acquisition boards, and there are also related signal conversion boards on the market. These boards can be applied in various situations, not just constant pressure water supply control systems. The following are three conversion circuits for learning and reference.

A certain frequency converter manufacturer has developed a water supply substrate R/I conversion circuit board for this purpose, which converts the resistance change of the remote pressure gauge into a 4-20mA current signal and inputs it to the control terminal. This signal is compared with the given pressure signal (parameter setting value), automatically adjusting the output frequency of the frequency converter to maintain a constant pressure in the water supply network. A 4-20mA signal source circuit is actually a constant current source circuit with a large internal resistance, and the output current depends on the internal resistance, regardless of the external load resistance. In a closed circuit, even if the load resistance value is 0, the original output current value can still be maintained unchanged. T2 and T3 form two constant current source circuits, the former being a “constant” constant current source circuit and the latter being a “variable” constant current source electrical appliance. The 12V DC voltage from the CPU motherboard is transmitted through D1 C4 isolation and filtering become the power supply for the Vcc1 water supply substrate. R1 and TL431 further process Vcc1 into a reference voltage of 2.5V, which is then processed by TL431, operational amplifier circuit R2、Z2、 Internal resistance of remote pressure gauge T2 circuit, forming 2.5V/510 Ω=4 A constant current circuit of 9 mA. Due to Z1 The Z2 and T2 circuits have a constant current circuit of 4.9 mA, and the resistance change of the remote transmission pressure gauge is converted to the voltage change on the Z2 resistor. This pipeline pressure signal is input to pins 5 and 6 of the second stage operational amplifier circuit through R3. Second stage operational amplifier circuit T3 forms a “variable” constant current source circuit, where the change in resistance inside the remote pressure gauge is converted into signal voltage input to the two input terminals of the operational amplifier. This stage amplifier forms a constant current source circuit with deep negative feedback (amplification factor of 1), and the output current depends on the size of the resistance inside the remote pressure gauge. Z1 and Z3 are voltage embedded protection diodes for signal input and output terminals. The internal resistance of the current input terminals of the frequency converter is generally 250 Ω.
The bottom left image shows a dedicated signal conversion chip for 0-10V/4-20mA, with the chip model of AD694. The control terminal of the frequency converter itself has a 24V power supply, and the chip only needs to connect a current limiting resistor and a transistor to accurately convert the signal. The anti-interference performance of the chip is also relatively good; The bottom right image is a 0-10V/4-20mA signal conversion circuit composed of an operational amplifier circuit and discrete components. The circuit requires two power supplies and adjustment of the initial output current, which is relatively cumbersome and not widely used.