Universal electrical tester. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In the manufacture, adjustment and repair of various electrical appliances, it is necessary to check the presence of mains or standard rectified voltage in circuits, the integrity of electrical connections and individual parts. Of course, you can use an avometer in these cases, but it is sometimes inconvenient, and you often have to be distracted to look at the readings of the indicator needle. It is better to use the proposed probe.

The probe allows you to determine the presence, nature (DC or AC) and polarity of the voltage, make sure there is an open circuit or not, as well as evaluate its resistance, check a capacitor with a capacity of several thousand picofarads to hundreds of microfarads for open circuit, short circuit, leakage current, check pn junctions of semiconductor devices (diodes, transistors), check the condition of the built-in battery.

The probe (Fig. 1) includes a clock generator, an input switch, two comparators, two tone (800 and 300 Hz) generators, light and sound indicators.

The clock generator is assembled on the elements DD1.2 and DD1.3. It generates rectangular oscillations in a form close to a meander (duration and pauses are equal), following with a frequency of about 4 Hz. From the outputs of the generator and the inverter connected to it on the DD1.4 element, antiphase signals are fed to the input switch and comparators.

The input switch consists of current-limiting resistors R5, R6, a rectifier bridge on diodes VD1, VD2, VD4, VD5, a zener diode VD3 and electronic switches on transistors VT1, VT3, connected according to a common collector circuit. The switch allows you to use them to power your own microcircuits when checking voltages, and to apply AC or DC voltage to them when checking connecting circuits and junctions of semiconductor devices.

Comparators work elements DD2.1, DD2.2. Cascades on elements DD3.1 and DD3.2 - matching between comparators and indicators.

Sound indication tone generators are assembled on the elements DD2.3, DD3.3 (800 Hz) and DD2.4, DD3.4 (300 Hz). They are loaded on a BQ1 piezoceramic transducer. Light indication cascades are made on transistors VT4, VT5 (they work in key mode) and LEDs HL1, HL2, respectively, of red and green glow. The brightness of the LEDs is determined by the resistance of the resistor R14.

The cascade on the transistor VT2 is used only when checking the state of the power source - the battery GB1, composed of four batteries D - 0,03. To recharge the battery, a R11VD6 circuit is installed in the probe, which limits the charging current to the required value.

Consider the modes of operation of the probe, set by the switches SA1 and SA2.

During voltage control (SA2 - in the "U" position, SA1 - "U, R"), the input signal through the X1, X3 probes, X2 connector and current-limiting resistors goes to the rectifier bridge, emitters of transistors VT1, VT3 and comparator inputs. The parametric stabilizer on the VD3 zener diode and the filter capacitor C1 are turned on - from them the voltage is supplied to the probe microcircuits and switch transistors. The clock generator starts. Transistors VT1, VT3 begin to open and close one by one.

Simultaneously with the closing of one of them, a work enable signal is sent to the corresponding comparator. If the input voltage of the comparator exceeds half of the supply voltage, the comparator fires and turns on the audio frequency generator and the "own" channel LED. For example, if there is a positive voltage on the X1 probe relative to the X2 probe, an intermittent sound signal with a frequency of about 300 Hz is heard and the HL1 LED flashes, and if it is negative, the signal frequency will be about 800 Hz and the HL2 LED will flash.

With alternating voltage in the circuit under study, both indication channels work alternately.

The frequency of the clock generator is much lower than the mains voltage frequency (50 Hz), therefore, when a rectified, but not smoothed voltage is applied to the input of the probe, the second comparator has time to work due to its ripples. As a result, the sound will be modulated, as it were, which is well perceived by ear. Due to the inertia of the eyes, the operation of the light indication cannot be noticed.

When monitoring the connecting circuit and its resistance (switch SA2 - in position "R", SA1 - "U, R"), all probe electronics are powered by battery GB1. Its voltage is alternately applied to the probes. Let's assume that in the current state of the clock generator, transistor VT1 is open, and VT3 is closed. On the X1 probe, there is a positive voltage, and on the X2 - negative. In this case, the operation of the DD2.2 comparator (and its indication channel) is prohibited and DD2.1 is allowed.

If the circuit under investigation is open or its resistance is high (more than 24 kOhm), the voltage drop across the resistor R7 is less than the response voltage of the comparator DD2.1, there is no indication.

With a decrease in the resistance of the circuit, the voltage across the resistor R7 increases. As soon as it exceeds half the supply voltage, the comparator will work, the sound indication with a frequency of 800 Hz and the HL2 LED will turn on.

With a change in the state of the clock generator, the functions of the comparators change accordingly. In this case, in the case of checking circuits with a resistance of less than 24 kOhm, both indication channels will work alternately.

In the same mode, pn junctions of semiconductor devices are checked. In the event of a break (burnout) of the transition, there is no indication; in the event of a breakdown, both indication channels work. If the transition is working, you can immediately determine the "polarity" of its connection to the probe probes. An audio signal with a frequency of 800 Hz and the green LED (HL2) lighting up means that the X1 probe is connected to the p-region (say, to the anode of the diode), the sound frequency of 300 Hz and the red LED (HL1) lighting up indicates that this probe is connected to the n-region (cathode diode).

In this case, the operation of the clock generator is terminated, since the output of the element DD1.1 is set to a low logic level (logical 0). The same level will be set on the base of the transistor VT1, and it will close. Transistor VT3 will be open, so on probe X3 there will be positive voltage.

A pre-discharged capacitor is connected to the probe probes. The charging of the capacitor begins, a positive voltage appears on the resistor R2, which leads to the operation of the comparator DD2.2. The indication turns on (the HL1 LED lights up and a signal with a frequency of 300 Hz sounds), which turns off after a while. The voltage comparator is triggered in the linear section of the capacitor charging, so you can estimate the capacitance of the capacitor by the duration of the indicator - it is directly proportional to the capacitance.

In the same mode, the leakage current of the capacitor is estimated. First, the capacitor is charged from the probe probes, then disconnected and, after waiting 10 ... 15 s, reconnected to the probes. According to the duration of the indication, it is estimated how much of the charge the capacitor managed to lose.

To check the condition of the GB1 battery, the SA1 switch is set to the "KP" position (power control), and SA2 is set to the "R" position. A stable current generator on the elements VT2, R3 and the resistor R4 form a micropower reference voltage stabilizer, to the output of which pin 12 of the DD1.1 element is connected. When the battery voltage drops below 4 V, the output of this element switches to a logic 0 state and the clock generator is blocked.

When both indication channels work in this mode when the probes are closed, you can use the probe. If a signal with a frequency of 300 Hz continuously sounds and the HL1 LED is on, the battery needs to be recharged. Then the switch SA2 is set to position "3" (charging), and an alternating voltage of 110 ... 220 V is applied to the probes. The duration of a full battery charge is 14 hours. DD3.1.

There is no separate power switch in the probe - its function is performed by the SA2 switch, which should be set to the "U" position in storage mode (the current consumed from the battery is negligible - it was not even possible to fix it). In the standby state, when the SA1 switch is set to the "R", "KP", "U, R" positions, the current consumed by the probe was 75, 130, 300 μA, respectively. With the indication switched on, the current increases to 5 mA.

Let's say the battery is completely discharged or completely absent. In this case, the probe controls the voltage using only sound indication.

All transistors, except for the field-effect, can be used with the KT315, KT3102 series with any letter index or other low-power silicon ones. When using the transistor indicated on the diagram or another field-effect transistor, a resistor R3 is selected with such a resistance at which a decrease in the battery voltage to 4 V leads to a logical 1.1 at the output of the DD0 element. Instead of the K561 series microcircuits, it is permissible to use similar microcircuits of the 564, KR1561 series. The VD3 zener diode can be with a different stabilization voltage, but not exceeding the maximum voltage of the microcircuits, transistors, capacitors used, with a maximum allowable stabilization current of at least 20 mA.

Structurally, the probe is made in a case made of insulating material (Fig. 2) with dimensions of 135x44x19 mm. Probe X1 is fixed rigidly, and X2 is connected with a stranded flexible wire insulated to socket X2 on the body. The switches are mounted on the case so that their handles can be moved with the thumb of the right hand without releasing the probe and the second probe from the hands.

The remaining parts are mounted on a printed circuit board (Fig. 3) made of double-sided foil fiberglass.

Of course, another constructive solution and installation of the probe is acceptable. The only conditions are to reliably isolate all circuits, since they are under mains voltage, and isolate resistors R5, R6, on which power up to 1,5 W can be released when the battery is charging.

When setting up a probe, first of all, as mentioned above, a resistor R3 is selected. By selecting the resistor R11, the battery charging current is set to 3 mA.

Periodically it is necessary to inspect the batteries of the battery, to clean their surface from the emerging plaque.

Author: L.Polyansky, Moscow

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