Probe for checking PN junctions. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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In the simplest version (see figure), the probe uses two transistors, on which a symmetrical multivibrator is assembled. The checked transition is connected to sockets XS1, XS2. Since the transistors of the multivibrator open in turn, in the junction circuit, for example, a diode, the current flows either in one or the other direction, depending on the polarity of the diode. If the anode of the diode is connected to the XS2 socket, the HL1 LED will light up; if the diode is switched on in reverse polarity, the current will flow through the HL2 LED.
If the pn junction is broken, both LEDs will flash, and if there is a break in the junction circuit, neither LED will flash.
Transistors can be any of the KT315 series, but with a transfer coefficient of at least 50, resistors - MLT-0,25, MLT-0,125, capacitors - KM-6.
The probe does not need adjustment. But if, when connecting a properly checked transition, the generation of the multivibrator will fail (none of the LEDs will flash) or the brightness of the LEDs will be insufficient, you need to include emitter followers on transistors VT3, VT4.
Instead of D220, it is permissible to use D219A, D220A, D220B and other silicon ones.
Author: G.Chagin
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