1. The fifth foot of UC3842 is ground.
2, first test the fifth and eighth pin, pin 8 output reference voltage 5V, test this foot will soon be able to judge the UC3842 good or bad
3, if the 8th pin voltage is normal, there is no problem with UC3842 under normal circumstances, and the others do not need to be measured.
4, if the 8th pin voltage is not, test the 7th pin to see whether the power supply voltage is correct. However, the 7-pin voltage value depends on the specific circuit design. ① The foot is the output end of the error amplifier.② The foot is the input end of the feedback voltage. The voltage of this foot is compared with the 2.5V reference voltage at the phase end of the error amplifier to generate the error voltage, so as to control the pulse width; ③ The foot is the input end of current detection. When the detection voltage exceeds 1V, the pulse width is reduced to make the power supply in intermittent working state; ④ The foot is the timing end, and the operating frequency of the internal oscillator is determined by the external resistance and capacitance time constant, f=1.8\/(RT×CT); ⑤ The foot is the public ground end; ⑥ The foot is the push-pull output end, the interior is the totem pole type, the rise and fall time is only 50ns, the driving capacity is ±1A; The pin is the DC power supply side, with undervoltage and overvoltage locking function, the chip power consumption is 15mW; The pin is a 5V reference voltage output end, with a 50mA load capacity.