Four-level economical probe. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The economical probe described in the article, using four LED indicators, shows one of the intervals of the DC resistance value of the monitored circuit. In addition to monitoring the conductivity of linear circuits, such a probe can check capacitors for an open or short circuit of the plates, as well as the serviceability of p-n junctions of semiconductor devices.

The proposed probe is designed to test electrical circuits in industrial equipment - windings of transformers, relays, filaments of signal lamps, pn junctions of semiconductor devices, contact resistance. Often it is not necessary to know exactly the resistance value of a circuit section: it is enough that it fits into a certain range. In such cases, a probe with an indication of several resistance intervals in the range of 0 ... 10 kOhm is suitable for testing.

The proposed probe was developed on the basis of the device from the article "Peak power indicators" ("Radio", 1982, No. 9, p. 61). The original circuit was redesigned so that the indicator readings corresponded to a series of electrical circuit resistance intervals.

On fig. 1 shows a diagram of the probe. It is based on a threshold device based on CMOS logic elements with a voltage divider that forms four different switching levels of four LED indicators.

When checking, the measured circuit is connected in parallel with the resistor R9, through which the total current of the voltage divider flows. To bring different currents in the external circuit to the thresholds of operation of the elements DD1, the resistances in voltage dividers from resistors R1 - R9 were experimentally selected. When the current through the external circuit decreases (i.e., with an increase in the resistance of this circuit), the elements DD1.1 - DD1.4 are switched in series. The elements of the DD2 chip act as a decoder, including one of the transistors VT1 - VT4 and, accordingly, the indicators HL1 - HL4.

The table shows the correspondence between the intervals of the controlled resistance of the external circuit and the glow of the probe indicators (if necessary, the intervals can be changed).

If the resistance of the measured circuit is within the range of the measured parameter, one of the LEDs HL1-HL4 turns on. When the probe probes are not connected anywhere or the resistance of the measured circuit is more than 10 kOhm, none of the LEDs is lit. In this mode, the current drawn by the probe is only 70 µA. The operability of the device and its battery is checked by shorting its probes.

In addition to checking resistance within the specified limits, the probe allows you to check capacitors in the same way as it is customary to do with any avometer in the resistance measurement mode - by alternately changing the polarity of connecting the device to the capacitor being tested. At the same time, on the LEDs of the probe, as the capacitor recharges, a single effect of "running lights" is observed in the direction from HL1 to HL4. Moreover, the larger the capacitance of the capacitor, the lower the switching speed of the LEDs. This allows you to roughly judge the value of the capacitance of the capacitors. It is really possible to check their capacitance from 1 uF or more. The probe also confidently "rings" the pn junctions of semiconductor devices: in the direct connection of the pn junction, the HL1 LED lights up.

If an AC voltage of 220 V is accidentally applied to the input, which happens in the practice of a repairman, the probe does not fail. In this case, only the HL1 indicator is lit.

It is also possible to use the device with a "ringing" of a four-core cable. To do this, three resistors should be connected to one end of the cable, connected according to the circuit shown in fig. 2, and on the other hand, having connected the common wire of the cable and the device, check with a probe that the number of the luminous indicator indicated in fig. 2 wire number. If any two cores are broken or shorted to each other, the indication will show the corresponding deviation.

The body of the probe is made of sheet textolite and has external dimensions of 115x52x22 mm. One probe is made from a guide pin taken from a rectangular connector; it is installed from the end of the case. On the inner side of the removable side wall, a second probe is fixed with screws - a crocodile clip with a wire. Inside the housing, next to the removable wall, there is a microswitch type MP7, which opens the power supply circuit of the probe when the side wall is put back in place. Microcircuits are glued to the inner surface of the body of the probe, and the rest of the parts are soldered above them by volume mounting. Resistors - any small-sized. Transistors - series KT315 or similar.

When setting up a probe for selecting the resistance of resistors R1 - R8, a block of four sliding variable resistors SPZ-23v with a linear control characteristic was used. After completing the settings, instead of the block of resistors, selected fixed resistors are installed.

The probe is powered by a Krona battery or similar with a voltage of 7,5 ... 9 V.

Author: S. Stashkov, Perm

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