What is the working principle of the current transformer sampling circuit?

The current sampling unit is a sampling circuit that provides the single-chip current sampling signal when the induction cooker is working. The single-chip microcomputer detects the change of the input current at all times, and automatically adjusts the PWM signal according to the detected current sampling signal to make the induction cooker perform constant output power processing. The single-chip microcomputer will also detect the input current of the induction cooker according to the change of the detected current sampling signal, thereby automatically doing Various actions. When the single-chip microcomputer detects the appropriate number of pulses with the pot in the synchronization circuit, it will take 0.5 to 2s to detect the current change, and the difference of the current change determines whether the material of the heating pot and the size of the heating area are It meets the heating requirements. When the current sampling signal changes too much, the single-chip microcomputer makes a judgment of no pot. There are two common current sampling unit circuits in the mainstream induction cooker circuits on the market, which are the current sampling unit that uses current transformer sampling and the current sampling unit that uses resistance voltage drop sampling. The working principles of these two sampling circuits will be introduced below.

The current sampling unit is a sampling circuit that provides the single-chip current sampling signal when the induction cooker is working. The single-chip microcomputer detects the change of the input current at all times, and automatically adjusts the PWM signal according to the detected current sampling signal to make the induction cooker perform constant output power processing. The single-chip microcomputer will also detect the input current of the induction cooker according to the change of the detected current sampling signal, thereby automatically doing Various actions. When the single-chip microcomputer detects the appropriate number of pulses with the pot in the synchronization circuit, it will take 0.5 to 2s to detect the current change, and the difference of the current change determines whether the material of the heating pot and the size of the heating area are It meets the heating requirements. When the current sampling signal changes too much, the single-chip microcomputer makes a judgment of no pot. There are two common current sampling unit circuits in the mainstream induction cooker circuits on the market, which are the current sampling unit that uses current transformer sampling and the current sampling unit that uses resistance voltage drop sampling. The working principles of these two sampling circuits will be introduced below.

(1) The current sampling unit using current transformer sampling is shown in the following figure (a). The AC voltage measured twice by the current transformer CT1 is rectified by a bridge rectifier composed of D10～D13. The DC voltage smoothed by EC5 is sent to the I-A/D port of the CPU, and the CPU detects the input current of the induction cooker according to the change of this voltage signal. The current transformer CT1 has a turns ratio of 1:3000. If the turns ratio is large, the current induced by it in high current operation is linear. VR1 is an adjustable resistance of 0～10kΩ. It is mainly used to adjust the power deviation caused by structural errors. The reference of current detection can also be changed by adjusting this resistance to achieve the purpose of adjusting the output power of the induction cooker. When the resistance of VR1 increases, the corresponding current detection voltage will increase. In the case of a certain CT1 primary current, the voltage induced by the CT1 secondary will increase accordingly, and the program will perform corresponding control according to the changes in the analog signal of the A/D port. According to the constant power requirements of the software, the power will be relatively reduced.

(a)

(b)

(2) The current sampling unit using resistance sampling is shown in the following figure (b). Resistor R320 is a sampling resistor connected in series between the e pole of the IGBT tube and the negative pole of the power supply. Generally, 0.01Ω is selected so that the voltage drop reaches the technical requirement of 0.1V when the current of 10A is passed. Comparator IC4A and peripheral circuits form a forward DC amplifier with an amplification factor of 100 times. The current sampling voltage amplified by 100 times can be obtained at the VR terminal, and this voltage is sent to the IA/D port of the CPU, so that the microcontroller can act accordingly . The function of the variable resistor VR is the same as the function of VR1 in the current sampling unit sampled by the current transformer, and will not be repeated here.

If you want to know more, our website has product specifications for  current transformer sampling circuit, you can go to ALLICDATA ELECTRONICS LIMITED to get more information