Most of the transmitters are installed on site, and their output signals are sent to the control room, and its power supply comes from the control room. There are usually two ways of signal transmission and power supply for the transmitter:
(1) Four-wire system
The power supply and the output signal are transmitted by two wires respectively, and the wiring method is shown in Figure 2.3. Such transmitters are called four-wire transmitters. The transmitter of DDZ-Ⅱ series instrument adopts this wiring mode.Since the power and signal are transmitted separately, there are no strict requirements on the zero point of the current signal and the power consumption of the components. The power supply can be AC (220V) or DC (24V), and the output signal can be dead zero (0-10mA) or live zero (4-20mA).
Figure 2.3 Four-wire transmission
(1) Two-wire system
For the two-wire transmitter, there are only two wires connected to the transmitter, and these two wires transmit the power supply and output signal at the same time, as shown in Figure 2.4. It can be seen that the power supply, transmitter and load resistor are connected in series.The two-wire transmitter is equivalent to a variable resistor whose resistance is controlled by the measured parameter. When the measured parameter changes, the equivalent resistance of the transmitter changes accordingly, so the current flowing through the load also changes.
Figure 2.4 Two-wire transmission
Two-wire transmitters must meet the following conditions:
①The normal operating current of the transmitter must be equal to or less than the minimum value of the signal current
Since the power line and the signal line are common, the power supplied to the transmitter by the power supply is provided by the signal current. When the output current of the transmitter is at the lower limit, it should be ensured that the semiconductor devices inside it can still work normally.
Therefore, the lower limit value of the signal current cannot be too low. Because at the lower limit of the output current of the transmitter, the semiconductor device must have a normal static operating point, and the power supply for normal operation needs to be supplied by the power supply, so the signal current must have a live zero point. The international unified current signal adopts 4-20mADC, which creates conditions for the production of two-wire transmitters.
②The voltage condition for the transmitter to work normally is
In the formula:is the output voltage of the transmitter; is the minimum value of the power supply voltage; is the upper limit of the output current, usually 20mA; is the maximum load resistance value of the transmitter; is the resistance value of the connecting wire.
The two-wire transmitter must be powered by a single DC power supply. The so-called single power supply refers to the power supply with zero potential as the starting point, rather than the positive and negative power supply symmetrical to zero voltage. The output voltage U of the transmitter is equal to the difference between the power supply voltage and the voltage drop of the output current on RL and the resistance r of the transmission wire. In order to ensure the normal operation of the transmitter, the output voltage value can only change within a limited range. If the load resistance increases, the power supply voltage needs to be increased; otherwise, the power supply voltage can be decreased; if the power supply voltage decreases, the load resistance needs to be decreased; otherwise, the load resistance can be increased.
③The minimum effective power for the transmitter to work normally
Since the power supply of the two-wire transmitter is very small, and the load voltage changes greatly with the output current and load resistance, the working voltage of each part of the line changes greatly. Therefore, when making a two-wire transmitter, it is required to use a low-power integrated operational amplifier and set up a voltage-stabilizing and current-stabilizing link with good performance.
The two-wire transmitter has many advantages, which can greatly reduce the installation cost of the device, and is conducive to safety and explosion protection. Therefore, most countries in the world currently use two-wire transmitters.
Post time: Dec-16-2022