Analysis of the circuit of DL-01 low-frequency electrotherapy instrument

Low-frequency current has a strong ability to penetrate the human body, so low-frequency electrotherapy instruments have a certain effect on cervical spondylosis, frozen shoulder, lumbar disc herniation, rheumatism, arthritis, cerebral insufficiency, and non-viral inflammation in the human body. The principle circuit of the DL-01 low-frequency household electrotherapy instrument drawn based on the actual object is shown in the attached figure. The core of the circuit is a complementary output voltage-controlled pulse width adjustment integrated circuit KW9712, which can accurately adjust the oscillation frequency and duty cycle over a wide range, with an operating voltage of +5V to +15V. The static current is less than 10mA. The functions of each foot of KW9712 are listed in the attached table.

The oscillation frequency is determined by the values ​​of R2, RP2, and C2. Adjusting RP2 can make the oscillation frequency between 1Hz and 500Hz. Pin ③ is the pulse width adjustment voltage input terminal. When the input voltage rises from 1/3VCC to 2/3VCC, the output pulse width of pin ④ rises from 0 to 100%, and the output pulse width of pin ⑤ drops from 100% to 0, otherwise it is the opposite. The sum of the pulse width duty ratios of pins ④ and ⑤ is 1, and the rated output current is 10mA. When adjusting RP3, the output pulse width of pin ④ is 10% to 40%, and that of pin ⑤ is 90% to 60%. The oscillation signal with controllable pulse width output by pin ④ drives transformer T1 to work through VT2, generating a signal with the same amplitude and opposite polarity, controlling the push-pull output part of VT3 and VT4. The power supply of the push-pull output is supplied by the adjustment tube VT1. Adjusting RP1 can change the conduction degree of VT1, thereby changing the working voltage of the output stage, and achieving the purpose of changing the strength of the low-frequency current. The oscillation signal output from the ⑤ foot drives the piezoelectric buzzer HTD (FT27) and the two-color light-emitting diode LED (2EF203) through VT5. SA-1 and SA-2 are 2x2 double-pole double-throw linkage switches. When set to "1", VD3 is short-circuited. The secondary of the push-pull transformer T2 outputs bidirectional pulses with the same amplitude and opposite directions, which are used for conventional treatment. At this time, the red LED (R) tube is on; when set to "2", a unidirectional pulse is output for drug ion introduction treatment. At this time, the green LED (G) tube is on. KW9712 ⑦ foot is the shutdown control terminal. When the external voltage of ⑦ foot is lower than 1/20Vcc, the chip is allowed to work; when the voltage of ⑦ foot is higher than 1/20VCC, the chip is prohibited from working, the circuit stops oscillating, and there is no output from ④ and ⑤ feet.

Appendix KW9712 pin functions

┏━━━┳━━━━┳━━━━━━━━━┓

┃Pin ┃ Symbol ┃ Function ┃

┣━━━╋━━━━╋━━━━━━━━━┫

┃① ┃VCC ┃ Positive power supply ┃

┣━━━╋━━━━╋━━━━━━━━━┫

┃② ┃RT／CT ┃Oscillation timing resistor and capacitor ┃

┣━━━╋━━━━╋━━━━━━━━━┫

┃③ ┃ ┃ Voltage-controlled pulse width adjustment ┃

┣━━━╋━━━━╋━━━━━━━━━┫

┃④ ┃TWA ┃ Complementary pulse width output A ┃

┣━━━╋━━━━╋━━━━━━━━━┫

┃⑤ ┃TWB ┃Complementary pulse width output B ┃

┣━━━╋━━━━╋━━━━━━━━━┫

┃⑥ ┃GND ┃ Ground ┃

┣━━━╋━━━━╋━━━━━━━━━┫

┃⑦ ┃OFF/ON ┃ Off/on control ┃

┣━━━╋━━━━╋━━━━━━━━━┫

┃⑧ ┃RT ┃Oscillation timing resistor ┃

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