Analysis of the production principle of high-power constant light control circuit

The circuit is shown in the figure below. UC3842 is a single-ended switching power supply control chip. Pin ① is the internal error amplifier output terminal, and a negative feedback network is connected between pin ② to determine the gain of the error amplifier. Pin ② is the sampling voltage input terminal. The sampling voltage is compared with the internal 2.5V reference voltage to generate a control voltage to control the output pulse width. Pin ③ is the overload protection control terminal. When the voltage exceeds 1V, pin ⑥ stops outputting. Here, it is used as the light on and off control terminal. Pin ④ is connected to an external timing element to determine the oscillation frequency. Pin ⑥ is a pulse output with a driving capacity of 1A. Pin ⑦ is the power input terminal, between 16 and 36V. Pin ⑧ outputs an accurate +5V reference voltage. P1 is a power adjustment tube, model 2SK727. V1 is a fast recovery diode, D1 is a photosensitive diode, and W1 is a precision multi-turn adjustable resistor. The circuit works at 100kHz, with a normal output of 180W and a maximum of 230W.

When the light bulb L2's luminous efficiency decreases due to aging or other reasons, the internal resistance of the photosensitive diode D1 increases, the voltage at pin 2 of UC3842 decreases, the pulse width at pin 6 increases, and the voltage output by the circuit increases, which increases the light intensity of the bulb. And vice versa. This achieves the control of the brightness of the lighting source.

When designing and applying a mechanical vision system to accurately measure dynamic workpieces, this light source is used to replace the imported original lighting source costing more than 5,000 yuan, achieving the same technical requirements (this circuit has been used in the lighting control of Siemens VS710 vision sensor).