In detail: DIY repair of a switch voltmeter from a real master for the site my.housecope.com.
To begin with, if there is a malfunction, the voltmeter must be opened. To do this, you need to take a knife and clean its sides from glue or other adhesive materials. Next, you need to determine its malfunction. The device can be faulty only for the following reasons: lack of balance, measurement error, overwriting, non-return of the arrow to zero. To adjust the balance, you need to take a soldering iron and evenly apply solder to the antennae of the arrow so that the arrow is at zero in any position. This can be quite problematic, especially when the Voltmeter has a high sensitivity.
To eliminate the measurement error, you need to choose a resistor, at which the readings of the device are exactly in the accuracy class. This can be done using a special resistance store. Overwrite is a condition in which the needle gets stuck while moving along the scale. Here you need to clean the ring and the magnet of the device so that not a single speck of dust remains anywhere around it.
And when eliminating the non-return of the arrow to zero, you need to align the frame or replace the thrust bearing. It happens that you need to do both at the same time. Here, in general, the whole rather simple repair. There are practically no other malfunctions in it, except, of course, that there may be an open circuit somewhere, but such a malfunction is eliminated in the same way as with all other electronic devices.
Previously, I saw this device only in color photos on the Internet, but now I saw it on the market; the glass is broken, some ancient batteries are tied to the body and all this is covered with a layer, to put it mildly, of dust. And I remember the ammeter-voltmeter - a tester of TL-4M transistors in that, unlike many others, in addition to the gain, other characteristics of transistors can be checked:
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Video (click to play). |
- reverse current of junctions collector - base (Ik.o.) and emitter - base (Ie.o.)
- initial collector current (Ic.p) from 0 to 100 μA;
At home, I disassembled the case - the measuring head burst in half, five wire resistors burned out almost to the state of coals, the balls fixing the position of the dial switch are far from round, only lumps stick out from the block for connecting the tested transistors. I didn’t take pictures - but now I’m sorry. The comparison would also give a clear confirmation of the fairly widespread opinion that the devices of that time were practically not killed.
Of all the restoration work, the longest and most painstaking was the general cleaning of the device. I didn't wind up the resistors, but put the usual OMLT (it is clearly visible - the left row, all "sawn"), with a fine finishing to the required value with a "velvet" file. The rest of the electronic components were intact.
Finding a new original block for connecting the tested transistors, as well as restoring the old one, was not realistic, so I picked up something more or less suitable and cut something off, glued something, and as a result, in a functional sense, the replacement was a success. I didn’t like to turn the dial switch every time after the end of measurements (turn off the power) - I put a slide switch on the power compartment. Fortunately, the place was found. The measuring head turned out to be in good order, only glued the body. I put plastic balls of the switch ("bullets" from a children's pistol).
To connect transistors with short "legs" I made extension cords with crocodile clips, and for convenience in work, two pairs of connecting wires (with probes and with "crocodiles"). And that's all. After turning on the power, the device started working in full. If there are any measurement errors, they are clearly insignificant. Comparisons in measuring current, voltage and resistance with a Chinese multimeter did not reveal significant differences.
I strongly disagreed to search for regular batteries for the power compartment every time. Therefore, I invented the following: I removed all the contact plates, so that two "finger" batteries would enter the compartment along the width, I made a cut of 9 x 60 mm in the side wall from the side of the device compartment, and removed the excess free space along the length thanks to the made inserts with contact springs.
If someone happens to "repeat", then using this sketch, it will not be difficult to do it.
It even turned out to be somehow cozy. There is no more question about power supply, there is no shortage of AA batteries. I will not deny myself the pleasure of bringing to your attention a circuit of an ampere-voltmeter - a transistor tester. With such simplicity and so much the device can.
This is a diagram of the installation of lamellas (contacts) in the switch of the device. Without it, there is a risk of not assembling the device at all. Here is a complete instruction manual. The renovation was done by Babay.
Such repair is understood as the implementation of adjustments, mainly in the electrical circuits of the measuring device, as a result of which its readings are within the specified accuracy class.
If necessary, the adjustment is carried out in one or more ways:
change in active resistance in series and parallel electrical circuits of the measuring device;
changing the working magnetic flux through the frame by rearranging the magnetic shunt or magnetizing (demagnetizing) a permanent magnet;
a change in the opposing moment.
In the general case, first, the pointer is set to a position corresponding to the upper measurement limit at the nominal value of the measured value. When such compliance is achieved, calibrate the measuring device at the numerical marks and record the measurement error at these marks.
If the error exceeds the permissible, then it is determined whether it is possible, by means of adjustment, to deliberately introduce the permissible error at the end mark of the measurement range, so that the errors at other numerical marks “fit” into the permissible limits.
In those cases when such an operation does not give the desired results, the instrument is re-calibrated with a retraction of the scale. This usually occurs after a major overhaul of the meter.
The adjustment of magnetoelectric devices is carried out with a direct current supply, and the nature of the adjustments is set depending on the design and purpose of the device.
By purpose and design, magnetoelectric devices are divided into the following main groups:
- voltmeters with nominal internal resistance indicated on the dial,
- voltmeters whose internal resistance is not indicated on the dial;
- single-limit ammeters with an internal shunt;
- multi-range ammeters with a universal shunt;
- millivoltmeters without temperature compensation;
- millivoltmeters with temperature compensation device.
Adjustment of voltmeters with nominal internal resistance indicated on the dial
The voltmeter is included in a series circuit according to the milliammeter switching circuit and is adjusted so as to obtain, at the rated current, the deviation of the pointer to the final numerical mark of the measurement range. The rated current is calculated as the quotient of the rated voltage divided by the rated internal resistance.
In this case, the deflection of the pointer to the final numerical mark is adjusted either by changing the position of the magnetic shunt, or by replacing the coil springs, or by changing the resistance of the shunt parallel to the frame, if any.
In the general case, the magnetic shunt removes through itself up to 10% of the magnetic flux flowing through the interglandular space, and the movement of this shunt towards the overlap of the pole pieces leads to a decrease in the magnetic flux in the interglandular space and, accordingly, to a decrease in the angle of deflection of the pointer.
Spiral springs (stretch marks) in electrical measuring instruments serve, firstly, to supply and withdraw current from the frame and, secondly, to create a moment that counteracts the rotation of the frame. When the frame is rotated, one of the springs is twisted, and the second spins, in connection with which a total opposing moment of the springs is created.
If it is necessary to reduce the angle of deflection of the pointer, then the spiral springs (stretching) in the device should be changed to "stronger" ones, that is, to install the springs with an increased counter torque.
This type of adjustment is often considered undesirable, since it is associated with painstaking work to replace the springs. However, repairers who have extensive experience in soldering coil springs (stretch marks) prefer this method. The fact is that when adjusting by changing the position of the magnetic shunt plate, in any case, as a result, it turns out to be displaced to the edge and the possibility of further moving the magnetic shunt to correct the readings of the device, disturbed by the aging of the magnet, disappears.
Changing the resistance of the resistor shunting the frame circuit with an additional resistance can only be allowed as an extreme measure, since such a branching of the current is usually used in temperature compensation devices. Naturally, any change in the indicated resistance will violate the temperature compensation and, in extreme cases, can be tolerated only within small limits. It should also not be forgotten that a change in the resistance of this resistor, associated with the removal or addition of turns of wire, must be accompanied by a long, but obligatory operation of aging the manganin wire.
In order to maintain the nominal internal resistance of the voltmeter, any changes in the resistance of the shunt resistor must be accompanied by a change in the additional resistance, which further complicates the adjustment and makes it undesirable to use this method.
Then the voltmeter turns on according to the usual scheme for it and is verified. With proper current and resistance adjustments, additional adjustments are usually not required.
Adjusting voltmeters whose internal resistance is not indicated on the dial
The voltmeter is switched on, as usual, in parallel with the measured electrical circuit and adjusted to obtain the deviation of the pointer to the final numerical mark of the measurement range at the nominal voltage for a given measurement range. The adjustment is performed by changing the position of the plate when moving the magnetic shunt, or by changing the additional resistance, or by replacing the coil springs (stretch marks). All the remarks made above are valid in this case as well.
Often, the entire electrical circuit inside the voltmeter - the frame and the wire resistors - is burnt out. When repairing such a voltmeter, first remove all burnt parts, then thoroughly clean all the remaining unburned parts, install a new moving part, short-circuit the frame, balance the moving part, open the frame and, turning on the device according to the milliammeter scheme, that is, in series with a model milliammeter, determine the total deflection current of the movable part, make a resistor with additional resistance, magnetize the magnet if necessary, and finally assemble the device.
Adjustment of single-limit ammeters with internal shunt
In this case, there can be two cases of repair operations:
1) there is an intact internal shunt, and it is required, by replacing the resistor with the same frame, to switch to a new measurement limit, that is, to re-calibrate the ammeter;
2) during the overhaul of the ammeter, the frame was replaced, in connection with which the parameters of the moving part changed, it is necessary to calculate, manufacture a new one and replace the old resistor with additional resistance.
In both cases, the current of the complete deflection of the device frame is first determined, for which the resistor is replaced with a resistance box and, using a laboratory or portable potentiometer, the resistance and the current of the complete deflection of the frame are measured by the compensation method. The resistance of the shunt is measured in the same way.
Adjustment of multi-limit ammeters with internal shunt
In this case, a so-called universal shunt is installed in the ammeter, that is, a shunt, which, depending on the selected upper measurement limit, is connected parallel to the frame and a resistor with additional resistance in whole or in part of the impedance.
For example, a shunt in a three-limit ammeter consists of three series-connected resistors Rb R2 and R3. For example, the ammeter can have any of three measurement ranges - 5, 10 or 15 A. The shunt is connected in series to the measuring electrical circuit. The device has a common terminal "+", to which the input of the resistor R3 is connected, which is a shunt at the measurement limit of 15 A; resistors R2 and Rx are connected in series to the output of resistor R3.
When the electrical circuit is connected to the terminals marked "+" and "5 A", the voltage is removed from the series resistors Rх, R2 and R3 to the frame through the resistor R add, that is, completely from the entire shunt. When the electrical circuit is connected to the "+" and "10 A" terminals, the voltage is removed from the series-connected resistors R2 and R3, and the resistor Rx turns out to be connected in series with the resistor R add, when connected to the "+" and "15 A" terminals, the voltage into the frame circuit is removed from the resistor R3, and the resistors R2 and Rx are included in the R add.
When repairing such an ammeter, two cases are possible:
1) the measurement limits and the shunt resistance do not change, but in connection with the replacement of the frame or a defective resistor, it is necessary to calculate, manufacture and install a new resistor;
2) the ammeter is calibrated, that is, its measurement limits change, in connection with which it is necessary to calculate, manufacture and install new resistors, and then adjust the device.
In case of emergency, which happens in the presence of high-resistance frames, when temperature compensation is needed, a circuit with temperature compensation by means of a resistor or thermistor is used. The device is verified on all limits, and with correct adjustment of the first measurement limit and correct manufacture of the shunt, additional adjustments are usually not required.
Adjustment of millivoltmeters without special temperature compensation devices
The magnetoelectric device has a frame wound from copper wire and spiral springs made of tin-Inca bronze or phosphor bronze, the electrical resistance of which depends on the temperature of the air inside the device: the higher the temperature, the greater the resistance.
Considering that the temperature coefficient of tin-zinc bronze is rather small (0.01), and the manganin wire from which the additional resistor is made is close to zero, the temperature coefficient of the magnetoelectric device is approximately assumed:
where Xp is the temperature coefficient of the copper wire frame, equal to 0.04 (4%).It follows from the equation that in order to reduce the effect on the instrument readings of deviations of the air temperature inside the case from its nominal value, the additional resistance should be several times greater than the resistance of the frame. The dependence of the ratio of the additional resistance to the resistance of the frame on the accuracy class of the device has the form
where K is the accuracy class of the measuring device.
It follows from this equation that, for example, for devices of accuracy class 1.0, the additional resistance should be three times the resistance of the frame, and for accuracy class 0.5 - already seven times more. This leads to a decrease in the useful voltage on the frame, and in ammeters with shunts - to an increase in the voltage on the shunts. The first causes a deterioration in the characteristics of the device, and the second - an increase in the power consumption of the shunt. Obviously, the use of millivoltmeters that do not have special temperature compensation devices is advisable only for panel instruments of accuracy classes 1.5 and 2.5.
The readings of the measuring device are adjusted by selecting an additional resistance, as well as by changing the position of the magnetic shunt. Experienced repairmen also use permanent magnet bias of the device. When adjusting, include the connecting wires supplied with the measuring device or take their resistance into account by connecting to a resistance box millivoltmeter with the corresponding resistance value. When repairing, sometimes they resort to replacing the coil springs.
Adjustment of millivoltmeters with temperature compensation device
The temperature compensation device allows you to increase the voltage drop across the frame without resorting to a significant increase in the additional resistance and power consumption of the shunt, which sharply improves the quality characteristics of single-limit and multi-range millivoltmeters of accuracy classes 0.2 and 0.5, used, for example, as ammeters with a shunt ... With a constant voltage at the terminals of the millivoltmeter, the measurement error of the device from a change in the air temperature inside the case can practically approach zero, that is, be so small that it can be ignored and ignored.
If, during the repair of the millivoltmeter, it is found that there is no temperature compensation device in it, then such a device can be installed in the device to improve the characteristics of the device.
olsa, Olsa. With all due respect - not right! There are also light indicators. I don't need arrows for them 
But 5066, 5068, 69.71, etc. with arrows. Glass. Where can you buy?
We bought devices at the factory, but for a long time, illegally, for cash.
You can search in metrological laboratories, sometimes supplied in spare parts.
Is 10 pieces enough? I will give 
Come in 
But then you need to balance.
ponitechLook for someone who is going to Truskavets to treat kidneys - all trains go through Lviv, I will give 10 pieces at the station. 
Unfortunately, the skiing season is already closing.
ponitech, download the Instrument and Regulator Repair Handbook. (Smirnov A.A. 1989) I have such a book. I had to use the advice from this book.
Nabi, Thank you. Smirnov has been around for a long time. Desk book.
olsa, Thank you for the kind words. There is no messenger yet.
Please write to me. There is a question.
Now I'm repairing it.
that big device that is higher.
Frame in the cliff
Turned out rusted and fell off
Well, I broke the arrow 
It is a glass sabak, it's good that it is hollow.
I inserted a vein from the wire inside
Aligned
And a supermoment
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- Cutting out of sheets of fiberglass laminate of the necessary size blanks and their cleaning;
- Making a photomask for each of them with its subsequent application;
- Etching of these boards in ferric chloride solution;
- Stuffing them with radio components;
- Soldering all placed components.
- It is imperative to use an active flux that promotes good spreading of liquid solder throughout the landing site;
- Try not to hold the sting in one place for too long, which excludes overheating of the mounted part;
- After completing the soldering, be sure to rinse the PCB with alcohol or any other solvent.
KonstantinXX (08 March 2013 - 23:41) wrote:
Happens.
2166985131.html
2087117861.html
(And so, in our flea markets, we come across Soviet Ts-eshki for 40.50 UAH)
It's a master's business, if not a pity for its time.
The spring should be flat, like in a watch.
The ambush can still be in the position of the magnet in relation to the frame, the scale turns out to be non-linear if it is incorrect.ZY. So that this device would measure the post. current with the limits indicated on the scale, it needs an appropriate external shunt.
Post has been editedaluma on: 09 March 2013 - 02:21
And although we have long been accustomed to digital voltmeters, dial gauges are still found in nature.
In some cases, their use can be more convenient and practical than the use of modern digital ones.
If a dial voltmeter has fallen into your hands, then it is advisable to find out its main characteristics. They can be easily identified by the scale and the inscriptions on it. A built-in voltmeter fell into my hands M42300.
Below, under the scale, as a rule, there are several icons and the model of the device is indicated. So, the icon in the form of a horseshoe (or curved magnet) means that this is a device of a magnetoelectric system with a movable frame.
In the next picture, you can see such a horseshoe.
A horizontal bar indicates that this measuring device is designed for operation with direct current (voltage).
It is also worth clarifying why we are talking about direct current. It is no secret that not only voltmeters can be analogue meters, but also a huge number of other measuring instruments, for example, the same analog ammeter or ohmmeter.
The operation of any pointer device is based on the deflection of the coil in the field of the magnet when a direct current passes through this very coil. To display with an arrow readings on the scale of the device, the current must be constant.
If it is variable, then the arrow will deviate left and right with the frequency of the alternating current that flows through the coil winding. To measure the magnitude of an alternating current or voltage, a rectifier is built into the measuring device.
That is why, under the scale of the device, the type of current with which it is capable of working is indicated: direct or alternating.
Further, on the scale of the device, you can find an integer or fractional number, like 1,5; 1,0 and the like. This is the accuracy class of the instrument, expressed as a percentage. It is clear that the lower the number, the better - the readings will be more accurate.
You can also see such a sign - two intersecting lines at right angles. This symbol indicates that the operating position of the instrument is vertical.
Readings may be less accurate when positioned horizontally. In other words, the device can “lie”. It is better to install a pointer voltmeter with such an icon in the device vertically and exclude a significant tilt.
But such a sign indicates that the working position of the device is horizontal.
Another interesting sign is a five-pointed star with a number inside.
This sign warns that the voltage between the device body and its magnetoelectric system should not exceed 2kV (2000 volts).It is worth paying attention to this when using a voltmeter in high-voltage installations. If you plan on using it in a 12 - 50 volt power supply, then don't worry.
For those who see the scale of the device for the first time, a quite reasonable question arises: "But how to read the readings?" At first glance, nothing is clear
.
In fact, everything is simple. To determine the minimum division of the scale, you need to determine the nearest number (digit) on the scale. As you can see on the scale of our М42300 it is 2.
Next, we count the number of spaces between the lines up to the first number or digit - in our case, up to 2. There are 10. Then we divide 2 by 10, we get 0.2. That is, the distance from one small line to the next one is 0.2 volts.
So we found the minimum scale division. Thus, if the arrow of the device deviates by 2 small divisions, then this will mean that the voltage is 0.4V (2 * 0.2V = 0.4V).
The already familiar built-in voltmeter model M42300 is available. The device is designed to measure direct voltage up to 10 volts. The measurement step is 0.2 volts.
We fasten two wires to the terminals of the voltmeter (respect the polarity!), and connect a dead 1.5 volt battery or any available one.
These are the readings I saw on the scale of the device. As you can see, the battery voltage is 1 volt (5 divisions * 0.2V = 1V). While photographing, the voltmeter needle stubbornly moved to the beginning of the scale - the battery was giving off its last "juices".
In addition, I became interested in what current the dial voltmeter itself consumes. Therefore, instead of a battery, I connected the power supply and set the output to 10 volts - so that the arrow of the device deviated to the full scale. Next, I connected a digital multimeter to the open circuit and measured the current.
It turned out that the current consumed by the dial voltmeter was only 1 milliampere (1 mA). It is enough for the arrow to deviate to the full scale. This is very small. Let me explain my hint.
It turns out that a dial voltmeter is more economical than a digital one. Judge for yourself, any digital meter has a display (LCD or LED), a controller, and buffer elements to control the display. And this is only part of his scheme. All this consumes current, drains the battery or accumulator. And if in the case of a voltmeter with a liquid crystal display, the current consumption is small, then in the presence of an active LED indicator, the current consumption will already be significant.
So it turns out that for portable devices with autonomous power supply, it is sometimes wiser to use a classic dial voltmeter.
When connecting a voltmeter to a circuit, there are a few simple rules to keep in mind.
Firstly, a voltmeter (any, even digital, even a pointer) must be connected in parallel with the circuit or element, the voltage on which is planned to be measured or monitored.
Secondly, the working range of measurements should be taken into account. It is easy to recognize it - just look at the scale and determine the last number on the scale. This will be the boundary voltage for measurement with this voltmeter. Naturally, there are universal voltmeters with a choice of a measurement limit, but now we are talking about a built-in pointer voltmeter with one measurement limit.
If you connect a voltmeter, for example, with a measurement scale of up to 100 volts, into a circuit where the voltage exceeds these 100 volts, then the arrow of the device will go beyond the scale, “off scale”. This state of affairs will sooner or later lead to damage to the magnetoelectric system.
Thirdly, when connecting, it is worth observing the polarity if the voltmeter is designed to measure DC voltage. As a rule, the polarity is indicated on the terminals (or at least one) - plus “+” or minus “-”. When connecting voltmeters designed to measure AC voltage, the polarity of the connection does not matter.
I hope that now it will be easier for you to determine the main characteristics of a dial voltmeter, and most importantly, to apply it in your homemade products, for example, by integrating it into a power supply with an adjustable output voltage.
... And if you make LED illumination of its scale, then it will look generally gorgeous! Agree, such a pointer voltmeter will look stylish and impressive.
When working with various electronic products, there is a need to measure the modes or distribution of alternating voltages on individual circuit elements. Conventional multimeters turned on in AC mode can record only large values of this parameter with a high degree of error. If it is necessary to take small readings, it is desirable to have an AC millivoltmeter that allows measurements to be made with millivolt accuracy.
Homemade digital voltmeter
In order to make a digital voltmeter with your own hands, you need some experience with electronic components, as well as the ability to handle an electric soldering iron well. Only in this case can you be sure of the success of the assembly operations carried out independently at home.
Before making a voltmeter, experts recommend that you carefully study all the options offered in various sources. The main requirement for such a selection is the extreme simplicity of the circuit and the ability to measure alternating voltages with an accuracy of 0.1 Volts.
The analysis of many circuit solutions showed that for the independent manufacture of a digital voltmeter, it is most advisable to use a programmable microprocessor of the PIC16F676 type. For those who are new to the technique of reprogramming these chips, it is advisable to purchase a microcircuit with ready-made firmware for a homemade voltmeter.
When purchasing parts, special attention should be paid to the selection of a suitable indicator element on LED segments (the variant of a typical dial ammeter in this case is completely excluded). In this case, preference should be given to a device with a common cathode, since the number of circuit components in this case is noticeably reduced.
Additional Information. Conventional commercial radioelements (resistors, diodes and capacitors) can be used as discrete components.
After purchasing all the necessary parts, you should go to the wiring of the voltmeter circuit (manufacturing its printed circuit board).
Before making a printed circuit board, you need to carefully study the electronic meter circuit, taking into account all the components on it and placing them in a convenient place for unsoldering.
Electronic device diagram
Important! If you have free funds, you can order the production of such a board in a specialized workshop. The quality of its execution in this case will undoubtedly be higher.
After the board is ready, you need to "fill" it, that is, place all the electronic components (including the microprocessor) in their places, and then solder them with low-temperature solder. Refractory compounds are not suitable in this situation, since high temperatures are required to heat them up. Since all the elements in the assembled device are miniature, their overheating is extremely undesirable.
In order for the future voltmeter to function normally, it will need a separate or built-in DC power supply. This module is assembled according to the classical scheme and is designed for an output voltage of 5 volts. As for the current component of this device, which determines its design power, half an ampere is quite enough to power the voltmeter.
Based on these data, we prepare ourselves (or give it to a specialized workshop for manufacturing) a printed circuit board for a power supply.
Note! It would be more rational to immediately prepare both boards (for the voltmeter itself and for the power supply), without spreading these procedures in time.
If you make it yourself, this will allow you to perform several operations of the same type at once, namely:
In the case when boards are sent for production on proprietary equipment, their simultaneous preparation will also allow you to benefit both in price and in time.
When assembling a voltmeter, it is important to ensure that the microprocessor itself is installed correctly (it must already be programmed). To do this, it is necessary to find the marking of its first leg on the body and, in accordance with it, fix the body of the product in the mounting holes.
Important! Only after you have complete confidence in the correct installation of the most critical part, you can proceed to its soldering ("solder fit").
Sometimes, to install a microcircuit, it is recommended to solder a special socket under it into the board, which greatly simplifies all working and setting procedures. However, this option is beneficial only if the socket used is of high quality and provides reliable contact with the microcircuit legs.
After sealing the microprocessor, all other elements of the electronic circuit can be stuffed and immediately soldered. In the soldering process, the following rules should be followed:
In the event that no mistakes were made during the assembly of the board, the circuit should start working immediately after connecting power to it from an external source of stabilized voltage of 5 volts.
In conclusion, we note that its own power supply unit can be connected to a ready-made voltmeter after completing its adjustment and verification, carried out according to the standard method.
Novice radio amateurs can be recommended to make a simple device that is most often used in the repair or tuning of radio devices. The autometer combines a multi-range ammeter and a voltmeter of direct and alternating current, an ohmmeter, and sometimes also a tester of low-power transistors.
A schematic diagram of such a simplified measuring device is shown in Fig. below. It measures DC currents up to 100mA, DC voltages up to 30 V and resistances from 50 Ohm to 50 kOhm. Switching of types and limits of measurement is carried out by connecting one of the probes to the sockets Гн1-Гн10. The second probe, inserted into the socket Гн11 "General", is common for all types and ranges of measurement.
Single-limit ohmmeter. It includes: microammeter IP1, power supply E1 with a voltage of 1.5 V and additional resistors R1 “Set. 0 "and R2. Before measuring, the probes of the device are connected, and the microammeter arrow is set to the end mark of the scale, which is the zero of the ohmmeter, with a variable resistor R1. Then the probes touch the terminals of the resistor, the winding of the transformer or the conductors of the circuit section, the resistance of which must be measured, and the measurement result is determined on the ohmmeter scale.
The four-limit voltmeter is formed by the same IP1 microammeter and additional resistors R3 — R6. With resistor R3 (when the second Probe is connected to socket Gn2), the full-scale deflection of the microammeter needle corresponds to a voltage of 1 V, with a resistor R4—3 V, with a resistor R5 — 10 V, with a resistor R6—30 V.
Milliammeter five-range: 0-1, 0-3, 0-10, 0-30 and 0-100 mA. It is formed by a universal shunt made up of resistors R7 — R11, to which an IP1 microammeter is connected with the Kn1 button.This is done so that when measuring, the microammeter is connected to a shunt through which most of the measured current flows, and not vice versa.
The design of the recommended combination meter is shown in Fig. Microammeter type M49 for a total current deflected arrows 300 μA with a frame resistance of 300 ohms. The variable resistor R1 (SPO-0.5), the KN button (KM1-1) and all the sockets of the device are fixed directly on the front panel, cut from sheet PCB 2 mm thick. The role of Gn1-Gn11 sockets is played by the socket part of the ten-pin connector. Low-resistance resistors R9-R11 of the MOI type (or wire-wound), the rest are MLT for a dissipation power of 0.5 or 0.25 W. The required resistances of the resistors are selected when adjusting by replacing them, by connecting several resistors in parallel or in series. In the described device, each of the resistors R3 and R6, for example, is composed of two series-connected resistors, each of the resistors R5 and R11 also of two resistors, but connected in parallel.
Calibration of the voltmeter and milliammeter consists in adjusting the resistances of the additional resistors and the universal shunt to the maximum voltages and currents of the corresponding measurement limits, and the ohmmeter to the scale markings for exemplary resistors.
Calibrate the voltmeter according to the diagram shown in fig. In parallel with the B1 battery with a voltage of 13.5 V (or from a power supply unit), connect a variable resistor Rp with a resistance of 2-3 kOhm, which will act as a regulating resistor, and between its slider and the lower (according to the diagram) output, parallel connected self-made calibrated (VK) and exemplary (V) voltmeters. The voltmeter of the factory avometer can be exemplary. First, put the slider of the adjusting resistor in the lowest (according to the diagram) position, and turn on the calibrated voltmeter to the first measurement limit - up to 1 V. Gradually increasing the voltage supplied from the battery to the voltmeters, set the voltage on them according to the reference voltmeter, exactly equal to 1 V. If at the same time the arrow of the voltmeter being calibrated does not reach the end point of the scale, this will indicate that the resistance of the additional resistor R3 turned out to be more than necessary, and if it goes beyond the scale, then it is less. When choosing this resistor, ensure that at a voltage of 1 V, the voltmeter needle is set exactly against the end point of the scale.
In the same way, but at voltages of 3 and 10 V, recorded with a reference voltmeter, adjust the additional resistors R4 and R5 of the following two measurement limits. To calibrate the fourth measurement limit, it is not necessary to apply a voltage of 30 V to the voltmeters. You can supply 10 V and, by selecting the resistor R6, set the arrow of the voltmeter to be calibrated to the mark corresponding to the first third of the scale. In this case, the deviation of its arrow on the entire scale will correspond to a voltage of 30 V.
To calibrate a milliammeter, you will need: a milliammeter for a current of up to 100 mA, a fresh element 343 or 373 and two variable resistors - a film (SP, SPO) with a resistance of 5-10 kOhm and a wire resistance of 50-100 Ohm. The first of these adjusting resistors will be used when adjusting the resistors R7 — R9, the second when adjusting the resistors R10 and R11 of the universal shunt.
Adjust the shunt resistor R7 first. To do this, connect in series (Fig. B): an exemplary mA milliammeter, calibratable mAToconnected to the first measurement limit (up to 1 mA), element E1 and variable resistor Rp... Press the button Kn1 "/" (see Fig. 17) of the autometer and, smoothly decreasing the input resistance of the adjusting resistor Rv, set the current in the circuit to 1 mA. The resistance of the resistor R7 should be such that with such a current in the circuit, the arrow of the calibrated milliammeter is against the end of the scale.
Adjust in the same way: resistor R8 is at the 3 mA limit, resistor R9 is at the 10 mA limit, and then, replacing the film adjusting resistor with a wire one, resistor R10 is at the 30 mA limit and, finally, R11 is at the 100 mA limit. When selecting the resistance of the next shunt resistor, do not touch the already fitted ones - you can knock down the calibration of the device at the first measurement limits.
The easiest way to mark the ohmmeter scale is to use fixed resistors with a tolerance of ± 5% or more. Do it like this. First, short-circuit the Probes and the adjusting resistor R1 “Set. О »set the microammeter arrow to the final mark of the scale corresponding to zero of the ohmmeter. Then open the probes and connect resistors with nominal resistances to them: 50, 100, 200, 300, 400, 500 Ohm, 1 "Ohm, etc. up to about 50-60 kOhm, noticing each time on the scale the point to which it deviates arrow of the device. And in this case, make up the resistors of the required resistances from resistors of other ratings. For example, a 40 ohm resistor can be made up of two 20 ohm resistors, a 50 k ohm resistor made up of 20 and 30 k ohm resistors. At the points of deviations of the arrow, corresponding to different resistances of the reference resistors, mark (graduate) the ohmmeter scale.
The scales of a homemade combined measuring device should be as shown in Fig.
The upper one is the ohmmeter scale, the lower one is the general scale of the voltmeter and milliammeter. They should be drawn as accurately as possible on thick varnished paper in the form of a microammeter scale. Then carefully remove the magnetoelectric system of the device from the case and stick a new scale, precisely aligning the arc of the ohmmeter scale with the old scale. In order not to disassemble the microammeter, the scales of a home-made device can be drawn on thick paper in an appropriate scale in straight lines and glued to the front or front side wall of the device drawer.
In the described combined device, a microammeter for current Iand= 300 μA with a frame resistance Ri equal to 300 Ohm. With such parameters of the microammeter, the relative input resistance of the voltmeter does not exceed 3.5 kOhm / V. It is possible to increase the relative input impedance and thereby reduce the influence of the voltmeter on the mode in the measured circuit only by using a more sensitive microammeter. So, for example, with a microammeter for current I = 200 μA, the relative input resistance of the voltmeter will be 5, and with a microammeter for current I = 100 μA - 10 kOhm / V. With such devices, the limit of measurement with an ohmmeter will also expand. But when replacing the microammeter with a more sensitive one, it is necessary, taking into account its parameters I and K, to recalculate the resistance of all resistances of the avometer.
In this way, you can check or calibrate any dial or digital voltmeter (ammeter). It is recommended to use a factory-made digital device as an exemplary one.
Such a device can also be placed in the glove compartment of a car. On a trip, it can be useful for finding damage to electrical wiring, unusable lamps, and matching the vehicle's on-board voltage.
Video (click to play). Literature: V.G. Borisov. Radio engineering circle and its work.
The main breakdown of such devices (unless the frame is damaged by excessive current) is mechanical damage to the frame mount.
In this case, you first need to ensure that the frame turns freely, without jamming on needles, without unnecessary backlash.
Then, with weights, they make sure that the arrow remains stationary from overturning the device, only after that the spring is adjusted.
The thing that sets the device to “0” is called the lock.
The description of what to screw where really takes a lot of time, it is better to find a photo.PS Not all details are shown in the photo.
There are no magnet fixing screws and outer contact nuts.Post has been editedAl_ex: 09 March 2013 - 00:21
