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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Milliohmmeter - prefix to the multimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology The prefix, together with a digital multimeter of the M-83x, DT-83x series, allows you to measure small active resistances with a resolution of 0,001 Ohm. Like the previous set-top boxes developed by the author, it is powered by the multimeter's internal ADC stabilizer. It is known that multimeters of the M-83x, DT-83x series have a small error in measuring DC voltage. Moreover, this error can always be minimized by calibrating the device by adjusting the reference voltage (100 mV). Therefore, according to the author, the development and repetition of prefixes for a multimeter that convert one or another measured value into a constant voltage at its input "VΩmA" may be of interest to a certain part of radio amateurs both from a financial point of view and from a creative point of view. With the availability of the element base and its cost, such attachments can be used to assemble a good measuring complex for a home laboratory without resorting to buying expensive measuring instruments, and often with a measurement error approaching the error of the multimeter itself. Another such prefix - a milliohmmeter - is presented below. It allows you to measure low active resistances of resistors, which is especially important when they are self-made from wire segments with high resistivity, for example, for various shunts. Main Specifications
* The measurement error of a carefully adjusted device in the above range is practically reduced to the multimeter error in the DC voltage measurement mode at the limit of 200 mV 5 ... 10 minutes after the attachment is turned on with the measuring clamps closed. There are two simple ways to measure low resistance resistors. The first is to apply a small current (mA units) through the measured resistor, followed by an increase in the voltage drop across the measured resistor. However, this will require the use in the DC amplifier of expensive and not available precision op-amps with a low zero-bias voltage and its avoidance of temperature changes. The second, easier and less expensive, is to apply more current (eg 100mA) and directly measure the voltage drop across the resistor. In the case of an appropriate source of direct current (DC), they do so. At first glance, when the milliohmmeter is powered from the ADC of the multimeter, this is not possible. But there is also a pulse method, when the current from the IT for measurement is supplied with short pulses in relation to their period. In this case, the average measurement current, as is known, decreases in proportion to the duty cycle of the pulse sequence. This method, as in some previous developments, for example [1, 2], is used to measure low resistances. The attachment diagram is shown in fig. 1. Consider the operation of the set-top box with the measured resistor R connected to the terminals XT3, XT4x.
On the logical element DD1.1 - Schmitt trigger (TSh), the elements VD1, C1, R1, R2 assembled a pulse generator. Pulse repetition period - 150...160 µs, pause - 3...4 µs. When the VD1 diode is turned on in the diagram, the generator consumes the minimum current, which is due to the peculiarity of the different current consumption of the TS during its transition from a state of logical zero to a logical unit and vice versa [3]. When the input voltage decreases from high to low (logic zero at the output), the through current through the output transistors TSh is 2...4 times greater than in the opposite case. This feature, according to the author's observations, manifests itself in all TS of buffered CMOS logic. Therefore, if the discharge time of the capacitor C1 is reduced by introducing the VD1R2 circuit, the average current consumption by the pulse generator when powered by 3 V for the 74NS series will be 0,2 mA instead of 0,5 ... 0,8 mA. Elements DD1.2 and DD1.3 are inverters, at the output of which the pulse duration is 3 ... 4 μs, and the pause is 150 ... 160 μs. They are connected in parallel to increase the load capacity. A current source is assembled on transistor VT1. Diode VD2 - thermocompensating. IT current is set to 100mA. With such a current across a 2 ohm resistor, the voltage drop is 200 mV, which corresponds to the measurement limit in the multimeter "200 mV". IT sets the current for measurement only when there is a pause at the output of the pulse generator on DD1.1, when the resistor R4 is connected to a common wire through this output for a period of 3 ... 4 μs. The "accelerating" capacitor C2 reduces the switching time of the transistor VT1 to obtain rectangular pulses on the measured resistor Rx. The inverted pulses from the outputs of the elements DD1.2, DD1.3 arrive at the gate of the field-effect transistor VT2, which is included as a synchronous detector. For the duration of the pulse, the current from the IT passes through the measured resistor, creating a voltage drop across it, which, through the open transistor VT2 of the synchronous detector, enters the "memory" capacitor C4, charging it until the voltage drops across the resistor. The voltage from the capacitor through the terminals XP2, XP3 is fed to the "VΩmA" input for measurement. At the end of the pulse, both transistors are closed for a period of 150 ... 160 μs until the next one appears. The smoothing capacitor C3 with a capacity of 220 uF eliminates the pulsed nature of the current consumption of the set-top box in the power line, maintaining it at a level of about 2,5 mA for the built-in voltage regulator +3 V of the ADC multimeter. This current is easy to determine, given that the duty cycle of the pulses at the output of inverters DD1.2, DD1.3 is 40...50 (100 mA/ (40...50)). The node on the field effect transistor VT3 and elements R8, C5 serves to limit the charging current of the capacitor C3 from the ADC voltage regulator at a level of not more than 3 mA from the moment the power is applied for 5 s. When power is applied, the voltage across capacitor C5 begins to rise due to the flow of charging current through resistor R8. When it reaches the threshold for transistor VT3, the latter begins to open smoothly, providing the charging current of capacitor C3 at a level that is safe for the ADC stabilizer. Resistor R7 and diode VD3 ensure the discharge of the capacitor C5 after the power is turned off. The prefix is assembled on a board made of fiberglass laminated on one side. A drawing of a printed circuit board and the arrangement of elements on it are shown in fig. 2. A photograph of the assembled attachment is shown in fig. 3.
Capacitors, resistors and diodes are surface mounted. Capacitors C1, C2, C4 - ceramic size 1206, C3, C5 - tantalum sizes C and B. All resistors - 1206. A little more should be said about the transistor 2SA1286 (VT1) [4]. It will replace, for example, 2SA1282, 2SA1282A with current transfer coefficient h21Э not less than 500 (additional index G) [5]. It is possible to replace it with other similar ones with a smaller h21Э (up to 300), while the resistance of the resistor R4 should be reduced to 1,8 ... 2 kOhm. The main thing is to check in the documentation or experimentally that the flat part of the output characteristic of the transistor at the collector current Iк 100 mA started with voltage Uke not more than 0,5 V. Otherwise, it is not necessary to count on the indicated measurement error - it can be significantly larger. Field effect transistor IRLML2402 (VT2) will replace, for example, FDV303N, and IRLML6302 (VT3) - BSS84. For other replacements, it should be taken into account that the threshold voltage of the transistors, the open channel resistance and the input capacitance (Ciss) must be comparable to those being replaced. Pin XP1 "NPNc" - suitable from the connector or a piece of tinned wire of suitable diameter. A hole for it in the board is drilled "in place" after installing the pins XP2, XP3. Pins XP2 "VΩmA" and XP3 "COM" - from the probes for the multimeter. Permanent connections XT 1, XT2 - tinned hollow copper rivets, soldered to the contact pads intended for them on the printed circuit board. The tinned ends of the flexible wire MGShV with a cross section of 0,5 ... 0,75 mm are inserted and soldered into the rivets2ending with XT3, XT4 crocodile clips. The length of each wire is 10 ... 12 cm. The lower inner surfaces of the "mouth" of the clamps are tinned. The ends of the wires going to them are tinned, then they are dragged into the lower "mouths" of the clamps and soldered. Solder should be applied with excess, which is then filed with a file to the level of the teeth of the "crocodile", as shown in the photograph of fig. 4.
The fixture needs tweaking. When working with it, the switch of the type of work of the multimeter is set to the position of measuring direct voltage at the limit of "200 mV". The readings, taking into account the highlighted comma, should be divided by 100. Before connecting the set-top box to the multimeter, you should check the current it consumes from another 3 V power supply with current protection so as not to disable the built-in low-power ADC supply voltage regulator in the event of a malfunction of any element or accidental short circuit of the current-carrying tracks of the board. Connect the attachment to the multimeter and close the clamps XT3, XT4, "biting" their "mouths" with soldered pads on top of each other. Let the thermal regime of the transistor VT1 be established for 5 ... 10 minutes. Despite the fact that the transistor case is cold to the touch, the crystal inside the case, even from short current pulses of 100 mA, will heat up during this time and its temperature will stabilize. To facilitate adjustment, the resistors R3 and R6 on the board are made up of two connected in parallel. On fig. 2 they are designated as R3', R3” and R6', R6”. After 5 ... 10 minutes, select the resistor R6 'so that the multimeter indicator readings are in the range of 0 + 0,5 mV, and then by selecting an additional resistor R6 ”set a "clean" zero (± 0 mV) by selecting an additional resistor R3” of greater resistance. Further, by connecting to the terminals XT4, XTXNUMX a known measured resistor Rx, for example, 1 Ohm, resistors R3' and R3 "set the appropriate readings on the multimeter indicator. To reduce the measurement error, these operations should be repeated until the desired result is obtained. On fig. 5 shows a photograph of a set-top box with a multimeter when measuring a 5 W C16-2MV wire resistor with a nominal resistance of 0,33 Ohm and a tolerance of ± 5%.
When changing the printed circuit board, the free inputs of the DD1 microcircuit elements should be connected to the positive power line or to a common wire. PCB drawing in Sprint LayOut 5.0 format can be downloaded from ftp://ftp.radio.ru/pub/2015/08/milliommetter.zip. Literature
Author: S. Glibin
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Comments on the article: Alexey Evgenevich Thank you, it was very helpful. Unfairly forgotten material.
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