Sound probe-ohmmeter (4 options). Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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For "ringing" radio components and wiring circuits, an avometer in the resistance measurement mode or a separate ohmmeter with a dial indicator is often used. When working with him every now and then you have to look at the arrow. If special measurement accuracy is not required, a simpler probe with an indicator light on an incandescent lamp or LED is used. But you still have to look at such a device often. Therefore, it is more convenient to use a probe with an audible alarm, which we propose to assemble according to one of the above schemes (Fig. 1-3).

The sound indicator is a miniature head telephone built into the probe body or connected separately via the microphone jack. The use of silicon transistors will provide high reliability and efficiency of devices. With the probes open, the current consumption from a voltage source of 1,5 V (element 316 or 332) is practically absent, and in the indication mode its value does not exceed 3 mA.

All devices are assembled on the basis of an unusual blocking generator, made according to the "three-point" scheme. At the first probe (Fig. 1), sections Ia and Ib of the primary winding of the transformer T1 are directly connected, respectively, to the base and collector circuits of the transistor VT1, and the phone BF1 is the load of the secondary winding T1. In the initial state (probes XP1 and XP2 are open), the power supply G1 is disconnected from the generator, and there is no sound in the phone. If the probes are closed to each other, the supply voltage through the limiting resistor R1 is supplied to the device. A positive bias occurs through the Ia section of the transistor-based transformer, and due to the strong positive feedback (POF) between the I winding sections, the generator is excited. A low tone sound will be heard from the phone (its frequency is determined by the parameters of all elements included in the generator).

If there is resistance in the circuit under test, it will naturally be in series with resistor R1. As a result, the collector and base currents will decrease, thereby reducing the depth of the PIC acting between the collector-base circuits of the transistor, which, in turn, will lead to a change in the nature of the sound in the phone - the tone will increase, and the volume will decrease.

Based on these features, one can approximately determine the resistance value within the measurement interval, which is about 1 kOhm for a given probe. When only rustles are heard when the probes touch the section of the measured circuit in the phone, this indicates that the resistance of this section exceeds 1 kOhm. The complete absence of sound means an open circuit, or indirectly suggests that the resistance of the circuit under test is too high.

Fig. 1,2

But if you need a probe that responds with a sound signal to a higher circuit resistance, say up to 100 kOhm, use the circuit shown in Figure 2. Its difference from the previous version is that here the operation of the blocking generator is controlled by a measuring circuit connected by means of probes between the extreme output of section 1a of the winding of the transformer T1 and the output of the base of the transistor VT1. If the section under test is not violated, through it, firstly, the bias voltage is supplied to the VT1 base and, secondly, the POS circuit will close: the transistor will open and the sound generator will start working. When the connection between the probes is broken, the common bias supply circuit and the PIC will be broken, the transistor VT1 is closed, the generator will not work. The current consumed by the device in this mode - no more than 0,1 μA - is so meager that it practically does not affect the resource of the element. Therefore, the switch was not needed.

The adjustment of both probes is reduced to the selection of the resistance of the resistor R1, the loudest low-pitched sound is achieved with the probes closed.

The third probe is more perfect than its counterparts. The presence of a push-button switch SB1 (Fig. 3) and associated resistors R2 and R3 made it possible to introduce two indication limits: 0-20 Ohm and 0-200 kOhm. The expansion of the measurement limits was achieved through the use of two transistors (VT1 and VT2), connected according to the so-called composite transistor circuit. Moreover, the internal resistance of the "collector - emitter" section VT1 depends on the resulting positive bias at its base, created by a voltage divider, composed of the resistances of the circuit under test and resistor R2 (or R3). This transistor controls the operation of the blocking oscillator on VT2, thus affecting the frequency and amplitude of its oscillations reproduced by the BF1 capsule.

If the probes XP1 and XP2 are open or the circuit under study is open, there will be no sound, since the transistor VT1 will be in the closed state, breaking the common power supply and PIC circuit from the transformer winding Ia to the base of the transistor VT2, which, due to this reason, also turns out to be closed. In this mode, the current consumed does not exceed 0,1-0,2 μA, which is much less than the self-discharge current of the G1 element. In the considered design, there is no need for an additional resistor that limits the base current VT1, since in any case this current does not exceed the maximum allowable values ​​for this type of transistor. This is explained by the fact that VT1 operates in the microcurrent mode - the current through its “collector-emitter” section is limited by the active resistance of the winding of section Ia of transformer T1, resistor R1 and the “base-emitter” junction VT2 and is no more than 0,4-0,6 mA; the base current VT1 is always much less than this value.

Fig. 3

It is more convenient to set up an ohmmeter probe beforehand by assembling it on a temporary breadboard, excluding the elements SB1, R2, R3. The probes are shorted and, by selecting the resistance of the resistor R1, they achieve the loudest sound of a low tone. Then, by connecting a variable resistor of 680 kOhm or 1 MΩ to the input of the device and slowly increasing its resistance, the full range of the probe indication is determined, noting the position of the slider at the moment the sound disappears in the background. The resistor is turned off and the resulting resistance is measured with an avometer, which is usually 350-500 kOhm.

Within these limits, any two measurement limits can be formed. Say, to set the limit "20 Ohm", a constant resistor of the same value (standard 22 Ohm resistor) is connected to the probe input and, having temporarily turned on the resistor R2 between the VT2 emitter and the VT1 base, select its resistance according to the minimum volume in the phone - get the upper limit this limit. Then, in the same way, a 200 kΩ resistor is connected to the probe input and, choosing the value of the resistor R3, set the limit to "200 k". After that, the parts from the temporary setup board are transferred to the permanent one.

If only one measurement limit is sufficient, the probe circuit can be simplified. Eliminating the elements SB1, R2, R3, we obtain the measurement limit corresponding to the operating range of the device. In the case when a lower indication limit is needed, a shunt resistor is installed between the VT2 emitter and the VT1 base, the resistance of which is selected in accordance with the above recommendations.

Fig. 4

In practice, however, more often there is a need for a probe with several measurement limits, which allows you to more accurately determine the resistance of the circuits under study. The diagram of such a device is shown in Figure 4. The probe has five indication limits, and four of them are formed at the moment the corresponding button SB1-SB4 is closed, and the most high-resistance, fifth limit, equal to the full range of the device, is created when all buttons are released (this position is displayed in figure 4).

The following items apply to the probe. Transistors - any series of KT201, KT312, KT315, KT342, KT373 structures of npn, with a base current transfer coefficient of more than 30. And by changing the polarity of the power supply G1 to the reverse, you can use transistors KT104, KT203, KT350 - KT352, KT361 with any letter structure index pnp.

Resistors MLT-0,125 - MLT-0,5. T1 - output transformer from any small-sized transistor radio. Switches of limits of indication - push-button small-sized types KM-1, KMD-1. Home-made ones made on the basis of the MP1-1, MP3-1, MP5, MP7, MP9, MP10, MP11 microswitch, or the MT1-1 toggle switch (Fig. 3) are also suitable. BF1-electromagnetic capsule DEMSh-1, microtelephone TM-2A or another with a coil resistance to direct current 180-300 Ohm. It is possible to use telephone capsules with a lower coil resistance, however, in the latter case, the upper limit of the measuring range will be lower.

The described probes are suitable for "ringing" the installation of various designs, checking fuses, switches, incandescent lamps, heating elements, inductors, transformer windings, electric motors and electromagnetic relays, resistors and other parts. Semiconductor devices - diodes and transistors - are checked by comparing the direct and reverse resistance of their pn junctions. In the event of a breakdown, the sound will be at any position of the probes; when disconnected, there is no sound.

In addition, you can check the quality of capacitors and roughly estimate their capacitance. The higher the measurement limit of the probe, the lower the capacitance it is able to respond with an audible signal.

Author: E. Savitsky, Korosten, Zhytomyr region; Publication: cxem.net

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