This paper brings you the design of AC voltage sampling circuit.
The voltage conversion circuit is realized by Hall voltage sensor chv-50p, as shown in Figure 3-2. The output of chv-50p voltage sensor is electrically isolated from the primary circuit, which can measure DC, AC and pulsating voltage or small current. When it is used for voltage measurement, the sensor is connected in parallel with the measured circuit through the resistance ru1 in series with the primary circuit of the module, and the output current is proportional to the primary voltage.
Since the input rated current in1 of chv-50p is 10mA, the voltage detected by this circuit is 220V AC voltage, then
The power P1 consumed by resistor ru1 is
Therefore, the resistance value of ru1 is 2.2 KA high power resistor with 5W power. In addition, in order to suppress the common mode interference, two capacitors C are connected in parallel on the AC input side. Of course, in order to better eliminate these interferences, an isolation transformer can be added before the voltage conversion circuit, so the selection of resistance ru1 should be changed according to the change of voltage after the isolation transformer.
Since the input rated current in2 of chv-50p is 50mA, in order to detect the AC voltage sampling circuit, the output current must be converted into voltage, so the resistance ru2 is connected in series at the output side of the voltage sensor. If the sampling voltage range of AC voltage sampling circuit is – 5V ~ + 5V, then
Because the power consumption of resistance ru2 is relatively small, there is no special requirement for power in the selection of resistance ru2. According to the selected voltage sensor, the AC voltage sampling circuit is shown in the figure below:
AC voltage sampling circuit
It can be seen from the figure above that the sampling circuit of the output voltage of the system consists of four parts. The first part is the emitter follower composed of the operational amplifier of tl084, in which R3 and C4 are used to suppress interference, and the time constant is constant“1 ms, in line with the actual requirements; the second part is a voltage offset circuit composed of two resistors and a voltage source, because the target signal is AC signal, and the signal sampled by Hall sensor is also AC signal 0 ~ 5V, and the A / D input level of the system CPU is required to be 0 ~ 3.3V, therefore, the voltage offset is needed, the circuit principle is simple, and will not be repeated. The third part is emitter follower, and the fourth part is clamp limiting circuit, which ensures that the amplitude of sampling signal is between 0 ~ 3.3V and meets the input signal requirements of TMS320LF2407.
In the sampling circuit, voltage follower is often used. As the name suggests, the output voltage is the same as the input voltage, that is, the voltage amplification factor of the voltage follower is always less than and close to 1. The remarkable characteristic of voltage follower is that the input impedance is high and the output impedance is low. Generally speaking, it is easy to achieve the input impedance of several megaohm. The output impedance is low, usually up to several ohm or even lower. In the circuit, voltage follower is usually used as buffer stage and isolation stage. Because the output impedance of the voltage amplifier is generally high, usually in thousands of to tens of thousands of ohm, if the input impedance of the later stage is relatively small, then a considerable part of the signal will be lost in the output resistance of the former stage. At this time, the voltage follower is needed to buffer. Play a connecting role. Another advantage of using voltage follower is that the input impedance is increased. In this way, the capacity of the input capacitor can be greatly reduced, which provides a prerequisite for the application of high-quality capacitors. Another function of voltage follower is isolation.