Circuit that converts sinusoidal AC signal into sawtooth wave signal

The circuit shown in Figure 1 is a circuit that converts a simple input sinusoidal AC signal into a sawtooth wave signal. The frequency of the sawtooth wave signal is twice the frequency of the sinusoidal AC signal. It is strictly synchronized with the frequency of the sinusoidal AC signal and is very suitable for use as a trigger pulse signal for thyristor AC voltage regulation and a control signal for a DC motor speed regulation circuit.

　　The circuit consists of a rectifier bridge D1. After full-wave rectification of the 50Hz voltage, a pulsating DC voltage Ua with a frequency of 100Hz is obtained. Its waveform is shown in Ua in Figure 2, and the peak value Um is about 1.4Ua. After the peak voltage is divided by Rl and 112, it provides the base current Ib1 for the transistor VT1. As long as the divided voltage of Ua is greater than the threshold voltage Ube1 of VTI, VT1 will be in a saturated conduction state, and the saturation voltage drop between the collector and the emitter is very small (generally about 0.3~0.4V). The transistor VT2 cannot be turned on, and its collector and emitter are in a blocked state.

　　At the same time, the DC voltage Um charges C1 through R3. The potential on C1 rises according to the RC integral characteristic curve and reaches 0.631 times of Um, thus forming the positive process of the sawtooth wave; when Ua drops below the threshold voltage Ube1 of VT1, VT1 turns from on to off, and VT2 turns from off to on, and the capacitor C1 is quickly discharged, so that the voltage on C1 drops rapidly from 0.631Um to about 0.4V (the saturation voltage drop between the collector and emitter of VT2). This process is the reverse process of the sawtooth wave voltage. The waveform is shown as Uc in Figure 2.

　　In the circuit of Figure 1, the product of resistor R3 and capacitor C1 has a great relationship with the sawtooth voltage waveform. If the value is appropriate, the linearity of the sawtooth voltage is good, otherwise, the linearity will deteriorate. In order to obtain a smaller reverse time and a positive sawtooth voltage with better linearity, the capacitor C1 is 0.47μF~2.2μF, the resistor R3 is 22kΩ4.7kΩ, and R3Cl=1/2f=0.01 seconds (f is the frequency of the AC voltage. That is, =50Hz) is the best. Because the sawtooth positive linearity and sawtooth voltage amplitude are mainly affected by the product of R3C1. If the product of R3Cl is too large, the sawtooth voltage amplitude will decrease; if the product of R3C1 is too small, the sawtooth positive linearity will deteriorate and the peak flat top area will increase. The length of the sawtooth reverse time is most affected by the output characteristics of the transistor VT2 and the capacity of the capacitor C1. At the same time, the size of R4 also has a certain influence on it. If the capacity of capacitor C1 is too large, the reverse time will increase, and the impact on VT2 will also be large. If it is too small, it will affect the discharge speed of VT2 to C1.

　　The waveforms Ua, Ub and Uc shown in Figure 2 are drawn according to the actual measurement of the circuit in Figure 1 by an oscilloscope. When the power supply is 12V AC voltage. The peak value of Ua is about 16V, and the valley value is 0V; the peak value of Ub is about 0.7V (due to the base-emitter clamp of VT2). The valley value is about 0.4V; the peak value of Ue is about 9.5V, and the valley value is about 0.4V. The forward time is about 8.5ms, the flat top width and the valley bottom width are about 0.5ms respectively, and the reverse time is about 0.5ms. The principles for selecting the parameters of each component are as follows:

　　R1=(U-0.07)/Ib1, R2=1～1.2R1, R3=1/2fc1, R4=5～8R1, the common emitter DC gain of transistors VT2 and VT2 is 100～150, among which, Ib1=1.5～2mA, f=50Hz.