In the process of making printed boards, some problems will be encountered after the completion of the printing corrosion process. For example, if the connection between copper and copper anchor bars is connected (bridged), there is a short circuit fault. This paper introduces a circuit of printed board short circuit detector, which can detect the above faults of printed board.

1、 Working principle

If a short circuit occurs at a place where the circuit of a certain part of the printed board should not be connected, only the signal current needs to be generated in this part, so that a magnetic field will be generated in this local area. The magnetic field can be detected by the short-circuit detector introduced here. During detection, the detector will give an alarm at the short-circuit place with sound and LED flashing to show the position of the short-circuit. The detector circuit is shown in the figure below. For example, when there is a short circuit at a in the right figure of the printed board, it is only necessary to generate a group of pulse signals at two points c and D, and then use the coil (self-made) to pick up the signal of the magnetic field. In order to improve the detection sensitivity, the signal frequency of 39khz (ultrasonic frequency) is selected here. The signal picked up by coil L1 is amplified by operational amplifiers ic1a ~ ic1c (ts9241n), shaped by Schmitt trigger circuit IC2 (74h132) and divided by IC3 (74hc4o4o).

Potentiometer P1 in the circuit is used to adjust the additional cover of operational amplifier IC1. The socket J1 in the circuit can be shorted to some two points according to the user’s needs to select the frequency division signal. The signal is then output and supplied to the alarm circuit composed of ic2c, ic2d and buzzer to make a sound. At the same time, the frequency division signal output by pin (11) of frequency divider IC3 drives LED (D2) to emit light through resistor R16 to indicate alarm. (11) The frequency division signal of the pin is 29khz / 212 = 7Hz. When the LED is illuminated, it may be flashing light or continuous light, depending on the state of (11) pin when the operation of frequency division counter IC3 stops detection.

The circuit in the figure below is powered by battery. In order to display the normal power supply of the power supply, a 2Hz multivibrator is composed of ic2b, RL3 and CIO, which is displayed by LED (D1). RL4 current limiting resistance ensures that D1 is in micro power consumption state, and the current through LED is about 1mA.

The signal source required for detection by the detection coil L1 of the detector is completed by the fourth operational amplifier ic1d of IC1. The multivibrator is composed of ic1d, R17, R18, R19, potentiometer P3 and C11, which generates the detection signal. Its frequency is adjustable by P3, and C11 and R19 can also be changed to change the frequency of the multivibrator. It can be seen from the above that since the signal frequency is a point frequency, it limits the occurrence of current and other harmonics of the detection signal source. The inductance L2 in the signal source is a low-pass filter, which is connected in series with the output.

C12 is a DC capacitor, which is connected in series with inductance L2 through resistance R2O to prevent DC current from passing through the inductance coil.

The model of operational amplifier IC1 is ts9241n, and its working voltage is 2.7 ~ + 12 (V); The output current can reach 80mA. The high output current of IC1 makes the final output of amplifier IC1 without amplification. In fact, the working current required by IC1 is only 20mA.

As mentioned above, the circuit is powered by battery, and its value is + 3V. Two AA batteries can meet the circuit requirements. As can be seen from the power supply circuit in the lower left corner of the figure below, because half of the power supply voltage (1.5V) is a virtual point, there will be two grounding points during printing plate production. When using the battery, the tap method is adopted for the two batteries to achieve the purpose of virtual grounding.

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