As we all know, the test current allowed by the test pen is generally microampere. Such a small current cannot directly drive the light-emitting tube to emit light and piezoelectric ceramics to sound, but from the perspective of energy, the ignition voltage of the neon tube is about 100V, and the ignition current is calculated as 1μA, the minimum luminous power is 0.1mw and The turn-on voltage of the light-emitting diode is 1.6V-2V, the minimum light-emitting current can be as low as 0.1mA or less, and the minimum light-emitting power is about 0.16mW, which is the same order of magnitude as the minimum light-emitting power of the neon tube. If the energy gathering circuit is added to increase the pulse power, the light-emitting diode can be driven by the electric energy of the neon tube to emit light. In addition, the current required for the piezoelectric ceramic sheet to work is extremely small, and it is no problem to rely on the test voltage of the neon tube to promote its sound.
According to the above analysis, the structure of the voltage dividing current limiting resistor R, the test terminal CS, and the touch terminal CM are similar to those of the traditional test pen. Diodes VD1~VD4 form a rectifier bridge, and the piezoelectric ceramic sheet YD not only acts as a sounding element, but also uses its own inherent capacitance to charge energy and pulse discharge. The thyristor VS and the trigger tubes VS1~VS4 form an electronic switch, which controls the charging time of the YD capacitor. The trigger voltage can be adjusted by changing the number of trigger tubes, so that the "lighting" voltage of the test pen can be adjusted. During the power test, the rectifier bridge turns the weak AC test current into DC power, and charges the capacitor of YD itself to gather energy. When the voltage at both ends rises to the trigger voltage of VS1~VS4, VS is triggered by it and quickly conducts. The YD discharges energy to the LED in a very short time through the VS, and the LED obtains a pulse current whose intensity far exceeds its minimum luminous current in an instant, and flashes with a higher brightness. With the continuous charging and discharging of the YD capacitor, the LED will keep flashing, and the YD will continue to sound intermittently, so that the electric pen has the function of sound and light dual indication. Of course, you can use a capacitor instead of YD. At this time, the test pen only has the function of light-emitting indication, but the brightness will be improved; you can also remove the light-emitting tube to form a single-function audio test pen.
