For digital control of voltage and current in a power supply, it is not necessary to make the ADC and indicator yourself. For this purpose, a Chinese multimeter worth 3-4 dollars is quite suitable, which is comparable in price to the cost of making your own digital display.
The popular M830B was chosen for the conversion. Below in detail, in the pictures, the alteration of the multimeter is described to indicate the voltage and current in your power supply.
The main purpose of the rework was to reduce the size of the board with the indicator, i.e. just part of the board had to be cut off. The simplest and cheapest Chinese multimeter M830B was purchased for alteration. The M830B multimeter circuit can be downloaded in our file archive. The voltage measurement limit of our design will be 200 V, and the current limit is 10 A. To select the "Voltage" - "Current" measurement mode, switch S1 with two groups of contacts is used. The diagram shows the position of the switch in voltage measurement mode.
First you need to disassemble the multimeter and pull out the board. You can see the view of the board from the side of the details in the photo.
And here is a photo of the board from the side of the indicator.
Our design will be placed on two boards. One board with an indicator, another board with parts of the input part of the multimeter and an additional stabilizer for 9 volts. The diagram of the second board is shown in the picture. Soldered resistors from the multimeter board are used as divider resistors. Their designation in the diagram corresponds to the designations on the board of the M830B multimeter. The diagram also provides additional explanations. The letters in the circles correspond to the connection points of one board to another. To power the structure, a low-power voltage stabilizer is used, which is connected to a separate transformer winding.
Let's get started. We solder R18, R9, R6, R5. We save resistors R6 and R5 for the input part of our design. Cut off the upper R10 contact from the circuit and cut out a part of the track (marked with crosses in the photo). We solder R10. We solder R12 and R11.
We connect R12 and R11 in series. And we solder one end to the upper contact of R10, and the other to the track cut off from R10. We solder R20 and solder it in place of R9. We solder R16 and drill new holes for it (see photo)
Solder R16 to a new location.
And here is a view of the R16 soldering from the side of the indicator.
We take scissors for metal and cut off part of the board.
We turn the board over with the indicator towards us. The contact R9 closest to the indicator (now there is R20) is cut off from the circuit (marked with a cross). Contacts R9 (now there are R20) and R19, farthest from the indicator, are connected together (from the side of the indicator), in the photo it is indicated by a red jumper. The upper contact R10 (there are now R11 and R12) is connected to the lower contact R13, marked with a red jumper on the photo. Delete some of the tracks marked with crosses. And we solder the jumper to the contact R9 closest to the indicator (now there is R20), instead of the remote track.
We remove the tracks marked with a cross, and prepare the contact pads for desoldering with the second board, indicated by arrows in the photo.
We solder the jumper. We solder the contact wires from the second board, observing the correspondence of the letters (a-A, b-B, etc.)
All! The structure is assembled, we proceed to check. We connect to the power source and measure the battery voltage. Works!
In this photo, the design is built into the power supply, for which it was created. When the load is connected, by pressing the "Voltage-Current" button, the indicator shows the value of the flowing current.
Each owner of the Chinese DT830 multimeter and similar models, necessarily during operation, encountered some inconveniences that are not visible at first glance.
For example, constant battery discharge due to the fact that you forgot to put the switch to the off position. Or lack of backlighting, impractical wires, and more.
All this can be easily modified and the functionality of your cheap multimeter can be increased to the level of individual professional foreign models. Let's consider in order what is missing and what can be added to the operation of any multimeter without special capital costs.
Replacing the wire and probes of the multimeter
First of all, 99% of users of cheap Chinese multimeters are faced with the failure of low-quality measuring probes.
First, the tips of the probes can break. When touching an oxidized or slightly rusty surface to measure, this surface needs to be lightly brushed to make a good contact. Of course, the most convenient way to do this is with the probe itself. But as soon as you start scrubbing, at this moment the tip may break off.
Secondly, the cross-section of the wires included in the kit also does not stand up to criticism. Not only are they flimsy, but this will also affect the error of the multimeter. Especially when the resistance of the probes themselves plays a significant role during measurements.
Most often, the wire breaks at the connection points on the plug-in contact and directly on the soldering of the sharp tip of the probe. 
When this happens, you will be surprised how thin the wires are inside. 
Meanwhile, the multimeter must be designed to measure current loads up to 10A! How this can be done using such a wire is not clear. 
Here is the real data of measurements of current consumption for flashlights, made using standard probes included in the kit and using self-made probes with a cross section of 1.5 mm2. As you can see, the difference in error is more than significant. 
The plug-in contacts in the multimeter connectors also loosen over time and worsen the total resistance of the circuit during measurements.
In general, the unambiguous verdict of all owners of DT830 multimeters and other models is that the probes must be modified or changed immediately after purchasing the instrument. 
If you are the lucky owner of a lathe or you have a familiar turner, you can make the probe handles yourself from some kind of insulating material, for example, pieces of unnecessary plastic.
The stylus tips are made from a sharpened drill. The drill itself is hardened metal and can easily scrape off any carbon deposits or rust without risking damage to the probe. 
When replacing plug-in contacts, it is best to use these plugs used in audio equipment for speaker jacks. 
If you really have a collective farm or there are no other options at hand, then as a last resort you can use ordinary contacts from a collapsible plug.
They also fit perfectly with the connector on a multimeter. 
In this case, do not forget to insulate the ends with a thermotube, which will stick out from the outside of the multimeter, in the places where the wires are soldered to the plug.
When there is no opportunity to make the probes on your own, then the case can be left the same, replacing only the wires.
In this case, three options are possible:
After replacement, such wires will be very easy to bundle together and not get confused.
Secondly, they are designed for a huge number of bends and will break no sooner than the multimeter itself fails.
Third, the measurement error due to their larger cross-section compared to the original will be minimal. That is, there are solid pluses everywhere.
If you make long wires up to 1.5 m, taking into account all the connection points, the resistance on them can reach several ohms!Important note: when replacing wires, you do not need to strive to make them much longer than those included in the kit. Remember that the length of the wire, like its cross section, affects the total resistance of the circuit.
Those who do not want to do homemade products can order ready-made high-quality silicone probes with a variety of tips on Aliexpress. 
To make new probes with a wire take up a minimum of space, you can twist them in a spiral. To do this, a new wire is wound on a tube, wrapped with electrical tape for fixing, and the whole thing is warmed up with a construction hairdryer for a couple of minutes. As a result, you get the following result. 
In a cheap version, this trick will not work. And when used to warm up a building hair dryer, the insulation can even float.
Modification of the multimeter mount
Another inconvenience when measuring with a multimeter is the lack of a third hand. You constantly have to hold a multimeter in one hand, and work with two probes simultaneously with the other. 
If the measurements take place at the desktop, then there is no problem. Put down the tool, free your hands and work. 
But what if you measure the voltage in a panel or in a junction box under the ceiling?
The problem is solved simply and inexpensively. In order to be able to fix the multimeter on a metal surface, on the back of the device using hot glue or double-sided tape, you glue ordinary flat magnets. 
And your device will be no different from expensive foreign counterparts. 
Another option for an inexpensive modernization of the multimeter in terms of its convenient placement and installation on a surface during measurements is to make a homemade stand. To do this, you only need 2 paper clips and hot melt glue. 
And if you have no surface at all nearby where you can place the tool, what to do in this case? Then you can use an ordinary wide elastic band, for example from suspenders. 
Make a ring out of gum, pass it through the body and that's it. Thus, the multimeter can be conveniently fixed directly on the arm, like a watch.
Firstly, now the multimeter will never again fall out of hand, and secondly, the readings will always be before your eyes. 
Probe caps
The spikes at the ends of the probes are quite sharp and can hurt you. Some models come with protective caps, some do not. 
They also get lost quite often. But in addition to the danger of pricking a finger, they also protect the contacts from kinking when the multimeter is in the bag, interspersed with another tool.
In order not to buy spare every time, you can make them yourself. Take an ordinary cap from a helium pen and lubricate the tip of the dipstick with any oil. This is done so that the cap does not stick to the surface during the manufacturing process.
Then fill the inner surface of the cap with hot melt glue and put it on the sharp tip. 
Wait until the hot melt glue hardens and calmly remove the resulting result. 
Multimeter backlight
The function that the multimeter lacks in poorly lit places is the display backlight. It is not difficult to solve this problem, it is enough to apply:
Make a hole in the side of the housing for the switch. Glue the reflector under the display and solder the two wires to the crown contacts. 
They supply power to the switch and then to the LEDs. The construction is ready.
In the end, a self-made modification of the multimeter backlight will look like this: 
A backlit battery will drain much faster, so remember to turn off the switch when there is enough natural light.
Replacing the crown in the multimeter with a lithium-ion battery from the phone
In recent years, it has become very popular to modify the multimeter to replace the power supply from the original crown with a lithium-ion battery from cell phones and smartphones. For these purposes, in addition to the battery itself, you will need charging and discharging boards. They are bought on Aliexpress or other online stores. 
The overdischarge protection board for such batteries is initially built into the battery at the top of it. It is needed so that the battery is not discharged above the nominally permissible rates (approximately 3 Volts and below). 
The charging board does not allow recharging the battery over 4.2 Volts (link to aliexpress). 
In addition, you will need a board that increases the voltage from 4V to the required 9V (link to aliexpress). 
The battery itself is compactly placed on the back cover and does not interfere with its closing at all. 
Preliminarily, the step-up module must be set to an output voltage of 9 volts. Connect it with wires to a multimeter that has not yet been converted and unscrew the required value with a screwdriver. 
You will have to make a hole in the case for a micro or mini usb charging connector.
The boost module itself is located in the place where the crown should stand. 
Be sure to ensure that the cables from the module to the battery are of the required length. In the future, this will allow you to remove the cover without any problems, and by halving the case, if necessary, engage in internal revision of the multimeter.
After placing all the parts inside, it remains to solder the wiring according to the diagram and fill everything with hot glue so that nothing moves when the device is moved. 
It is advisable to pour hot melt glue not only on the case, but also on the contacts with wires in order to extend their service life. 
A significant drawback of such a multimeter on a lithium-ion battery is its operation, or rather not work at negative temperatures.
As soon as your multimeter lies in the trunk of your car or in your bag for a long time in winter, you will immediately think of the krone battery.
And you will wonder if such an alteration was useful? Ultimately, of course, it is up to you to decide, based on the operating conditions of the device.
Modification of the on and off button on the multimeter
It is advisable to improve the last version of the multimeter refinement with the transition to lithium-ion batteries by placing the shutdown button in the power circuit of the converter to the battery. 
First, the converter itself draws a small current, even in standby mode when the multimeter is not working.
Secondly, thanks to such a switch, you do not have to click the multimeter itself again to turn it off. For this reason, many devices fail ahead of time.
Some tracks are erased ahead of time, others begin to shorten among themselves. So the button to turn off the entire device at once will be very useful.
Another tip from experienced users of Chinese multimeters is to disassemble and lubricate the sliding points of the balls of the switch immediately after purchase to ensure that the switch will last a long time and properly. 
And on the board it is recommended to smear the tracks with technical vaseline. Since new devices have no lubricant and the switch wears out quickly. 
You can make a button both in the internal version, if you find free space, and in the external one. To do this, you will have to drill only two micro holes for the power wiring. 
Flashlight in a multimeter
Another innovation for the multimeter is the optional flashlight. Often you have to use the device to look for damage in the shields and electrical cabinets of the basements, wiring closures in rooms where there is no light.
An ordinary white LED is added to the circuit and a button specifically to turn it on. It is very easy to check how much luminous flux from this LED is enough. You don't even have to disassemble it for this.
Place the anode leg of the diode into connector E, and the cathode leg into connector C (the anode leg is longer than the cathode). All this is done in the connectors for the transistor measurement mode on the P-N-P block. 
The LED will light up in any switch positions and will go out only when you turn off the multimeter yourself. To mount all this inside, you need to find the necessary leads on the circuit board and solder two wires to the emitter (connector E) and the collector (connector C). A button is soldered into the wire break and mounted through a hole in the multimeter case. 
You fix everything with hot glue and you get a portable multimeter flashlight.
Prelude
While studying somehow the endless expanses of the Internet for Chinese usefulness, I came across a digital voltmeter module:The Chinese "rolled out" the following performance characteristics: 3-digit red color display; Voltage: 3.2 ~ 30V; Working temperature: -10 ~ 65 "C. Application: Voltage testing.
It did not quite fit me into the power supply (the readings are not from zero - but this is a payback for power from the measured circuit), but inexpensive.
I decided to take it and figure it out on the spot.
Voltmeter module circuit
In fact, the module turned out to be not so bad. I dropped the indicator, copied the diagram (the numbering of the parts is shown conditionally):Unfortunately, the chip remained unidentified - there is no marking. Perhaps this is some kind of microcontroller. The value of the capacitor C3 is unknown, he did not measure it. C2 - presumably 0.1mk, also did not solder.
File in place ...
And now about the improvements that are needed to bring this "display meter" to mind.
1. In order for it to start measuring voltage less than 3 Volts, you need to remove the jumper resistor R1 and apply a voltage of 5-12V to its right (according to the diagram) contact pad from an external source (it is possible above, but undesirable - the DA1 stabilizer gets very hot). Apply the minus of the external source to the common wire of the circuit. Apply the measured voltage to the standard wire (which was originally soldered by the Chinese).
2. After revision according to claim 1, the range of the measured voltage increases to 99.9V (previously it was limited by the maximum input voltage of the DA1 stabilizer - 30V). The division factor of the input divider is about 33, which gives us a maximum of 3 volts at the DD1 input at 99.9V at the divider input. I applied a maximum of 56V - I don't have more, nothing burned out :-), but the error also increased.
4. To move or completely turn off the point, you need to solder the 10kOhm R13 CHIP resistor, which is located next to the transistor and instead solder the usual 10kOhm 0.125W resistor between the contact pad farthest from the trimmer CHIP resistor and the corresponding control segment pin DD1 - 8, 9 or 10.
Normally, the point is illuminated on the middle digit and the base of the transistor VT1, respectively, is connected to the pin through a 10kOhm CHIP. 9 DD1.
The current consumed by the voltmeter was about 15mA and varied depending on the number of illuminated segments.
After the alteration described, all this current will be drawn from the external power source without loading the measured circuit.
Total
And in conclusion, a few more photos of the voltmeter.
Factory condition
With soldered-out indicator, front view
With soldered-out indicator, rear view
The indicator is tinted with a car dye film (20%) to reduce brightness and improve the visibility of the indicator in the light.
I highly recommend toning it. Tinting film scraps will be gladly given to you for free in any auto service that deals with tinting.
There are also other modifications of this module on the Internet, but the essence of the alterations does not change from this - if you come across a wrong module, just correct the circuit on the board by removing the indicator or ringing the circuit with a tester and go!
I received from AliExpress a couple of electronic built-in voltmeters model V20D-2P-1.1 (measurement of constant voltage), the issue price is 91 cents a piece. In principle, you can find it cheaper now (if you search well), but it is not a fact that this will not be at the expense of the quality of the device assembly. Here are its characteristics:
- operating range 2.5 V - 30 V
- glow color red
- overall size 23 * 15 * 10 mm
- does not require additional power supply (two-wire version)
- there is a possibility of adjustment
- refresh rate: about 500ms / time
- promised measurement accuracy: 1% (+/- 1 digit)
And everything would be fine, put it in place and use it, but I saw information about the possibility of their improvement - adding a current measurement function.
Digital Chinese Voltmeter I prepared everything you need: a two-pole toggle switch, output resistors - one MLT-1 for 130 kOhm and the second wire for 0.08 Ohm (made from a nichrome spiral with a diameter of 0.7 mm). And for the whole evening, according to the found scheme and guidelines for its implementation, he connected this economy with wires to a voltmeter. To no avail. Either shrewdness in understanding the unsaid and understated in the found material was not enough, or there were differences in the schemes. The voltmeter didn't work at all.
We connect the digital voltmeter module I had to solder the indicator and study the circuit. Here, not a small soldering iron was required, but a tiny one, so that it fiddled pretty well. But within the next five minutes, when the whole scheme became available for review, I understood everything. In principle, I knew that it was necessary to start with this, but I really wanted to solve the problem “in an easy way”.
V-meter refinement scheme
Modification scheme: ammeter to voltmeter This is how this circuit was born for connecting additional electronic components with those already existing in the voltmeter circuit. The standard circuit resistor marked in blue must be removed. I will say right away that I found the differences from other schemes on the Internet, for example, the connection of a trimmer resistor. I did not redraw the entire voltmeter circuit (I am not going to repeat it), I drew only the part that is necessary for revision. The fact that the power supply of the voltmeter needs to be done separately is considered obvious, after all, the countdown in the readings should start from zero. Later it turned out that power supply from a battery or accumulator would not work, because the current consumption of a voltmeter at a voltage of 5 volts is 30 mA.
Board - Chinese voltmeter module After assembling the voltmeter, I took up the essence of the action. I will not be wise, I will just show and tell you what to connect with so that everything works out.
Step-by-step instruction
so, first action - a 130 kOhm resistor is soldered from the circuit, which is located at the input of the positive power wire, between the diode and the 20 kOhm trimming resistor.
We connect the resistor to the voltmeter-ammeter Second... On the vacated contact, from the side of the trimmer, a wire of the desired length is soldered (for a sample, it is convenient to 150 mm and better red)
Solder SMD resistor Third... On the track connecting the 12 kΩ resistor and the capacitor, a second wire (for example, blue) is soldered from the "ground" side.
Testing a new circuit
Now, according to the diagram and this photo, we "hang" an addition on the voltmeter: a toggle switch, a fuse and two resistors. The main thing here is to correctly solder the newly installed red and blue wires, however, not only them.
We convert the voltmeter block into an A-meter And here there are more wires, although everything is simple:
"- a pair of connecting wires is connected to the electric motor« separate power supply for voltmeter"- battery with two more wires
« power supply output"- a couple more wires
After energizing the voltmeter, "0.01" was immediately displayed, after energizing the electric motor, the meter in voltmeter mode showed a voltage at the output of the power supply equal to 7 volts, then switched to ammeter mode. The switching was performed when the power supply to the load was turned off. In the future, instead of a toggle switch, I will put a button without fixing, so it is safer for the circuit and more convenient for operation. I was glad that everything worked on the first try. However, the ammeter readings were more than 7 times different from the readings on the multimeter.
Chinese voltmeter - ammeter after alteration Then it turned out that the wirewound resistor has 0.8 Ohm instead of the recommended resistance of 0.08 Ohm. I made a mistake in the measurement during its manufacture in counting the zeros. I got out of the situation like this: a crocodile with a negative wire from the load (both black) moved along a straightened nichrome spiral towards the input from the power supply, the moment when the readings of the multimeter and the now modified ammeter voltmeter coincided and became the moment of truth. The resistance of the involved section of the nichrome wire was 0.21 Ohm (measured with an attachment to the multimeter at the "2 Ohm" limit). So it didn't even work out badly, that instead of 0.08, the resistor turned out to be 0.8 Ohm. Here, whatever you count, according to the formulas, you still have to adjust. For clarity, I recorded the result of my troubles on video.
Video
I consider the purchase of these voltmeters to be successful, but it's a pity that their current price in that store has grown significantly, almost $ 3 apiece. By Babay iz Barnaula.
Those who like to do everything with their own hands are offered a simple tester based on an M2027-M1 microammeter, which has a measurement range of 0-300 μA, an internal resistance of 3000 Ohm, an accuracy class of 1.0.
Required details
This is a tester that has a magnetoelectric mechanism for measuring current, so it only measures direct current. A moving coil with an arrow is attached to the guy wires. It is used in analog electrical measuring instruments.
Finding radio parts at a flea market or buying radio parts at a store will not be a problem. There you can also purchase other materials and components, as well as attachments for the multimeter. In addition to a microammeter, you will need:
If a person decides to make himself a multimeter with his own hands, it means that he does not have other measuring instruments. Based on this, we will continue to act.
Selection of measuring ranges and calculation of resistor values
Let's define the range of measured voltages for the tester. Let's choose the three most common ones, covering most of the needs of the radio amateur and home electrician. These ranges are 0 to 3 V, 0 to 30 V, and 0 to 300 V.
The maximum current passing through a homemade multimeter is 300 μA. Therefore, the task is reduced to the selection of additional resistance, at which the arrow deviates to full scale, and a voltage corresponding to the limit value of the range will be applied to the serial chain Rd + Rvn.
That is, on the 3 V range, Rtot \u003d Rd + Rvn \u003d U / I \u003d 3 / 0.0003 \u003d 10000 Ohm,
where Rtot is the total resistance, Rd is the additional resistance, and Rvn is the tester's internal resistance.
Rd \u003d Rtot-Rvn \u003d 10000-3000 \u003d 7000 Ohm or 7kOhm.
On the 30V range, the total resistance should be 30 / 0.0003 \u003d 100000 ohms
Rd \u003d 100000-3000 \u003d 97000 Ohm or 97 kOhm.
For 300 V range, Rtotal \u003d 300 / 0.0003 \u003d 1,000,000 Ohm or 1 mOhm.
Rd \u003d 1000000-3000 \u003d 997000 Ohm or 997 kOhm.
To measure currents, we select the ranges from 0 to 300 mA, from 0 to 30 mA and from 0 to 3 mA. In this mode, the shunt resistance Rsh is connected in parallel to the microammeter. therefore
Rtot \u003d Rsh * Rin / (Rsh + Rin).
And the voltage drop across the shunt is equal to the voltage drop across the tester coil and is equal to Upr \u003d Ush \u003d 0.0003 * 3000 \u003d 0.9 V.
Hence, in the range of 0 ... 3 mA
Rtot \u003d U / I \u003d 0.9 / 0.003 \u003d 300 Ohm.
Then
Rsh \u003d Rtot * Rvn / (Rvn-Rtot) \u003d 300 * 3000 / (3000-300) \u003d 333 Ohm.
In the range 0 ... 30 mA Rtot \u003d U / I \u003d 0.9 / 0.030 \u003d 30 Ohm.
Then
Rsh \u003d Rtot * Rvn / (Rvn-Rtot) \u003d 30 * 3000 / (3000-30) \u003d 30.3 Ohm.
Hence, in the range 0 ... 300 mA Rtotal \u003d U / I \u003d 0.9 / 0.300 \u003d 3 Ohm.
Then
Rsh \u003d Rtot * Rvn / (Rvn-Rtot) \u003d 3 * 3000 / (3000-3) \u003d 3.003 Ohm.
Fitting and installation
To make the tester accurate, you need to adjust the resistor values. This part of the work is the most painstaking. We will prepare a board for installation. To do this, you need to draw it into squares measuring a centimeter by a centimeter or slightly less.
Then, with a boot knife or something similar, the copper coating is cut along the lines to the base of fiberglass. The result is isolated contact pads. We marked where the elements will be located, it turned out a semblance of a wiring diagram right on the board. In the future, the tester elements will be soldered to them.
In order for a home-made tester to give correct readings with a given error, all its components must have at least the same accuracy characteristics and even better.
The internal resistance of the coil in the magnetoelectric mechanism of the microammeter will be considered equal to the 3000 Ohm declared in the passport. The number of turns in a coil, wire diameter, electrical conductivity of the metal from which the wire is made are known. Hence, the manufacturer's data can be trusted.
But the voltages of 1.5 V batteries may differ slightly from those declared by the manufacturer, and knowledge of the exact voltage value will then be required to measure the resistance of resistors, cables and other loads with a tester.
Determining the exact voltage of the battery
In order to find out the actual battery voltage yourself, you will need at least one accurate resistor of 2 or 2.2 kΩ with an error of 0.5%. This resistor value was selected due to the fact that when a microammeter is connected in series with it, the total circuit resistance will be 5000 Ohms. Therefore, the current going through the tester will be about 300 μA and the needle will deflect to full scale.
I \u003d U / R \u003d 1.5 / (3000 + 2000) \u003d 0.0003 A.
If the tester shows, for example, 290 μA, then the battery voltage is
U \u003d I * R \u003d 0.00029 (3000 + 2000) \u003d 1.45 V.
Now, knowing the exact voltage on the batteries, having one exact resistance and a microammeter, you can select the required resistance values \u200b\u200bfor shunts and additional resistors.
Collecting the power supply
The power supply for the multimeter is assembled from two series-connected batteries of 1.5 V. After that, a microammeter and a 7 kOhm resistor pre-selected at the nominal value are connected to it in series.
The tester should show a value close to the limit current. If the device goes off scale, then in series to the first resistor it is necessary to connect the second, small value.
If the readings are less than 300 μA, then a large resistance is connected in parallel to these two resistors. This will reduce the total resistance of the series resistor.
This operation continues until the needle is at the 300 µA full scale, which signals a fine fit.
To select an accurate 97 kOhm resistor, choose the closest one that is suitable for the nominal value, and do the same procedures as with the first 7 kOhm. But since a 30 V power supply is needed here, it will be necessary to rework the multimeter power supply from 1.5 V batteries.
A block with an output voltage of 15-30 V is assembled, as long as it is enough. For example, it turned out 15 V, then the entire adjustment is made on the basis that the arrow should tend to a reading of 150 μA, that is, to half the scale.
This is permissible, since the tester's scale is linear when measuring current and voltage, but it is desirable to work with full voltage.
DC current or voltage generators are required to adjust the 997 kΩ pull-up resistor for the 300 V range. They can also be used as attachments to a multimeter when measuring resistance.
Resistor ratings: R1 \u003d 3 ohms, R2 \u003d 30.3 ohms, R3 \u003d 333 ohms, R4 variable by 4.7 kΩ, R5 \u003d 7 kΩ, R6 \u003d 97 kΩ, R7 \u003d 997 kΩ. Are matched by fit. Power supply 3 V. Installation can be done by hanging the elements directly on the board.
The connector can be installed on the side wall of the box into which the microammeter is cut. The probes are made of a single-core copper wire, and the cords for them are made of stranded wire.
The shunts are connected with a jumper. As a result, a tester is obtained from a microammeter, which can measure all three main parameters of electric current.
