LED pn junction probe. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Before soldering a diode or transistor into the assembled device, it is advisable to make sure that they are in good condition, say, that the semiconductor junctions are intact. A relatively simple probe will help in this, the circuit of which is shown in Fig. 1. It will allow you to check not only "ordinary" diodes and transistors, for example, the KD522, KT503, KT801 series, but also many silicon bipolar high-frequency transistors and diodes, since the test takes place at a relatively low current and voltage.

Unfortunately, it is not recommended to check some germanium high-frequency and microwave low-power transistors (series GT341, GT346, 1T387, etc.) and silicon microwave transistors (series KT3115, 2T3124, etc.), requiring especially careful handling.

The device requires only one external voltage to operate, which allows it to be built into laboratory power supplies or connected to the transformer of the power supply of low-voltage 12 V soldering irons.

All possible states of the checked transition are indicated by a double-crystal LED HL1 with red and green crystals in a matte case with a diameter of 8 mm (manufactured by Kingbright).

The AC supply voltage is supplied to the rectifier from the VD1 diode and the smoothing capacitor C1, as well as to the R2VD2VD3 circuit, which limits the amplitude of the pulses supplied to the X1, X2 terminals to 3,5 ... 4,2 V.

When nothing is connected to the terminals or the junction under test is open (burned out), the positive half-waves of the voltage pass through the diodes VD5, VD4 and charge the capacitor C3 to a voltage of about 1,5 V. The field effect transistor VT1 closes, and the bipolar VT2 opens. The green LED crystal lights up.

At the same time, half-waves of negative polarity pass through the diodes VD6, VD7 and charge the capacitor C2. The transistor VT3 opens, the red crystal lights up. As a result, the simultaneous glow of both crystals leads to the fact that green and red colors, mixing, give a yellow glow color.

If a working diode is connected to the terminals in the polarity shown in the diagram, the amplitude of the negative half-waves will be limited at the level of the voltage drop at the open junction of the diode. Then the diodes VD6, VD7 will be closed, which will lead to the discharge of the capacitor C2 and the closing of the transistor VT3. The HL1 indicator glows green.

When the diode is connected to the terminals in reverse polarity, the transistor VT1 will open, and VT2 will close. The indicator will glow red.

In the event of a short circuit in the junction under test, both bipolar transistors will be closed, which will turn off the LED.

Resistor R3 contributes to a more complete opening of the transistor VT1, which allows the use of a transistor with a small initial drain current in its place.

An attempt to use one diode instead of VD4, VD5, and also instead of VD6, VD7 may lead to the fact that when checking some good diodes and transistor junctions, the result may be ambiguous.

The device can be powered with an alternating voltage of 8 ... 16 V. At a voltage of more than 14 V, for example, taken from the secondary winding of the popular TVK-110LM vertical scan output transformer of the TV, resistor R6 should be 47 kOhm.

It is permissible to use MLT, C1-4, C2-23 resistors in the device, oxide capacitors - K50-35 or their foreign analogues, C3 - K10-17, K73-17. Diode VD1 - any of the series KD105, KD209, KD243, 1N4001 -1N4007; VD4-VD7 - any of the KD102, KD103, KD521, KD522, 1N4118 series. Zener diodes VD2, VD3 can be KS126V, KS133G, KS407A. The field-effect transistor indicated in the diagram is replaced by KP103 with the letter indices E, EP, Zh, I or 2P103 with the indices A, AR, B, BR. It is necessary to select an instance with a cut-off voltage of not more than 1,3 V. KT3107I transistors are interchangeable with transistors of the same series with indices A-D, K, KT361 transistors - with indices B-E, K, M-P, as well as any of the KT668 series , KT686, KT6112, VS556, 2SA733, SS8550, SS9015. LED - with increased light output, but less bright L799SRSGW / CC, L799EGW or domestic KIPD18, KIPD37 series are also suitable, if you put up with an even lower brightness of the glow. You can also use two separate single-chip LEDs, for example, AL307NM. AL307LM.

The device can be mounted on a printed circuit board (Fig. 2) made of one-sided foil fiberglass. Parts can be placed on the side of the printed tracks of the board without drilling holes. Zener diodes are mounted vertically, the LED is placed on a printed circuit board or on the front wall of the case. It should be borne in mind that the L799 series LEDs are very sensitive to overheating, so their leads must be soldered as quickly as possible.

As X1 and X2, it is convenient to use crocodile clips with insulators connected to the board with a thin stranded mounting wire.

When checking and adjusting the device, by selecting the resistor R7, the color of the indicator glow is set, which is closest to yellow. The current through any of the LED chips must not exceed 20mA at a maximum ambient temperature of 40°C.

Author: A.Butov, village of Kurba, Yaroslavl region

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