Plastic sealing machine circuit composed of unidirectional thyristor

It is a plastic sealing machine circuit composed of one-way thyristor , which is suitable for plastic production enterprises to seal plastic parts.
    The VTH unidirectional thyristor is connected in parallel to the KA relay coil and the light-emitting diode I. Both ends of the ED1 series circuit are used to control
the status of the relay KA coil and LED1. When VTH is on, neither the KA relay coil nor LED1 will work; when VTH is off, the KA relay
coil and LED1 will be energized to work.
    The trigger signal of VTH comes from VD3 and VT1, and whether VT1 is turned on is
controlled by the delay of the RC time constant setting circuit composed of bias resistor R3, RPI potential and capacitor C2. R4 is the emitter resistor of VT1. VT1, Ra, RP1, Q, and VD2 are actually an
emitter output device controlled by RC and delayed conduction, used to control the working state of the thyristor VTH.
    The specific control sequence is: iiov AC power supply is linked to the power switch through SA1, VD1 rectifier, C] filter and Ri, step down to 24V,
relay KA is energized, and after its normally open contact KA1 is closed, the 110V AC voltage passes through the power supply The transformer reduces the voltage and outputs AC voltage from its secondary (side)
to energize the heating wire to generate heat and seal the plastic parts. After the sealing is completed, the control circuit operates, the relay KA coil is de-energized and released,
KA1 opens, and the heating wire stops heating. . Release the handle, SA1 is disconnected, and the entire machine is powered off.
    Here's how it works.
    (1) During the plastic sealing process, when SA1 is closed, the control circuit is energized. The +24V current voltage reduced by the island resistor charges
C2 through the wind and RP1. Since the voltage on G will not suddenly change, C2 is equivalent to a short circuit. state, making the base of VT1 equivalent to ground, so VT1
will not work, the emitter has no current, no voltage is generated at both ends of R4, and VTH has no trigger voltage and will not conduct. In this way, the +24V voltage reduced by the island resistor
is added to the series circuit of the KA relay coil and the light-emitting diode LED1, causing LED1 to turn on and emit light to indicate that the plastic sealing machine has entered
the sealing working state; at the same time, the current path in the KA relay coil is formed After the pull-in, its normally open contact KA1 is closed,
it provides llov AC voltage to the step-down transformer TI, which drives the electric heating wire to heat for plastic sealing after the voltage is reduced.
    (2) As the charge on the capacitor C2 gradually increases during the VTH conduction process, the terminal voltage on the capacitor C2 gradually increases. When the voltage reaches a
certain value, VT1 will be turned on. The +24V voltage will cause VD3 Zener breakdown through the ee junction where VT1 is turned on, and then a voltage
drop will occur across R4, triggering VTH to turn on. The two ends of the series circuit between the KA relay coil and LED1 are short-circuited, the KA coil and LED1 lose power, KA1
is disconnected, the heating wire loses power and stops heating, and VTH will stop conducting only when SA1 is disconnected.
    When VTH is turned on, C2 immediately discharges quickly through VD2 and VTH to prepare for the next use.
    VD2 constitutes the discharge path of C2. VD3 is used to increase the emitter potential of VT1 and correspondingly raise the base required for VT1 to turn on. voltage, eliminating
the interference of Cz residual charge, on the other hand, making the trigger signal obtained by VTH quickly reach the set value, both of which can improve the reliability of circuit operation .     Tip: Adjusting RP1 can change the RC time constant, which will change the delayed conduction time of VT1, thus changing the heating time of the electric heating wire, thereby adapting to the needs of sealing materials of different thicknesses.