Prelude
While somehow exploring the vast expanses of the Internet for Chinese utilities, 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 didn’t quite fit into my power supply (the readings are not from zero - but this is the price to pay for the power from the circuit being measured), but it’s inexpensive.
I decided to take it and figure it out on the spot.
Voltmeter module diagram
In fact, the module turned out to be not so bad. I unsoldered the indicator, drew a diagram (the numbering of parts is shown conventionally):Unfortunately, the chip remained unidentified - there are no markings. Perhaps it's some kind of microcontroller. The value of capacitor C3 is unknown; I did not measure it. C2 - supposedly 0.1 microns, I didn’t solder it either.
File in place...
And now about the modifications that are necessary to bring this “show meter” to fruition.
1. In order for it to start measuring voltage less than 3 Volts, you need to unsolder the jumper resistor R1 and apply a voltage of 5-12V from an external source to its right (according to the diagram) contact pad (higher is possible, but not advisable - 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 modification according to item 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 input divider ratio is about 33, which gives us a maximum of 3 volts at the DD1 input at 99.9V at the divider input. I supplied a maximum of 56V - I don’t have any more, nothing burned :-), but the error also increased.
4. To move or completely turn off the point, you need to unsolder the R13 10 kOhm CHIP resistor, which is located next to the transistor, and instead solder a regular 10 kOhm 0.125 W resistor between the contact pad farthest from the trimming CHIP resistor and the corresponding control segment pin DD1 - 8, 9 or 10.
Normally, the dot lights up at the middle digit and the base of transistor VT1 is connected to the pin via a 10kOhm CHIP. 9 DD1.
The current consumed by the voltmeter was about 15 mA and varied depending on the number of illuminated segments.
After the described modification, all this current will be consumed from an external power source, without loading the measured circuit.
Total
And finally, a few more photos of the voltmeter.
Factory condition
With desoldered indicator, front view
With desoldered indicator, rear view
The indicator is tinted with automotive tint film (20%) to reduce brightness and improve the visibility of the indicator in the light.
I highly recommend tinting it. You will be happy to be given scraps of tinting film for free at any auto repair shop that does tinting.
There are also other modifications of this module on the Internet, but the essence of the modifications does not change - if you come across the wrong module, simply adjust the circuit diagram on the board by removing the indicator or ringing the circuits with a tester and off you go!
Those who like to do it themselves are offered a simple tester based on the M2027-M1 microammeter, which has a measurement range of 0-300 μA, internal resistance of 3000 Ohms, accuracy class 1.0.
Required Parts
This is a tester that has a magnetoelectric mechanism to measure current, so it only measures DC current. The moving coil with an arrow is mounted on guy wires. Used in analog electrical measuring instruments.
Finding it at a flea market or buying it at a radio parts store won’t be a problem. There you can also purchase other materials and components, as well as attachments for the multimeter. In addition to the microammeter you will need:
If a person decides to make himself a multimeter with his own hands, it means that he has no other measuring instruments. Based on this, we will continue to act.
Selecting measurement ranges and calculating resistor values
Let us determine the range of measured voltages for the tester. Let's choose the three most common ones, covering most of the needs of radio amateurs and home electricians. These ranges are from 0 to 3 V, from 0 to 30 V and from 0 to 300 V.
The maximum current passing through a homemade multimeter is 300 μA. Therefore, the task comes down to selecting an additional resistance at which the needle will deflect to full scale, and a voltage corresponding to the limit value of the range will be applied to the series circuit Rd + Rin.
That is, on the 3 V range Rtot=Rd+Rin= U/I= 3/0.0003=10000 Ohm,
where Rtot is the total resistance, Rd is the additional resistance, and Rin is the internal resistance of the tester.
Rd = Rtot-Rin = 10000-3000 = 7000 Ohm or 7 kOhm.
On the 30 V range the total resistance should be 30/0.0003=100000 Ohm
Rd=100000-3000=97000 Ohm or 97 kOhm.
For the 300 V range Rtot = 300/0.0003 = 1000000 Ohm or 1 mOhm.
Rd=1000000-3000=997000 Ohm or 997 kOhm.
To measure currents, we will 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 to the microammeter in parallel. That's why
Rtot=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=Ush=0.0003*3000=0.9 V.
From here in the range 0...3 mA
Rtotal=U/I=0.9/0.003=300 Ohm.
Then
Rsh=Rtot*Rin/(Rin-Rtot)=300*3000/(3000-300)=333 Ohm.
In the range of 0...30 mA Rtot=U/I=0.9/0.030=30 Ohm.
Then
Rsh=Rtot*Rin/(Rin-Rtot)=30*3000/(3000-30)=30.3 Ohm.
From here, in the range of 0...300 mA Rtot=U/I=0.9/0.300=3 Ohm.
Then
Rsh=Rtot*Rin/(Rin-Rtot)=3*3000/(3000-3)=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. Let's prepare the board for installation. To do this, you need to draw it into squares measuring a centimeter by a centimeter or a little smaller.
Then, using a shoemaker's knife or something similar, the copper coating is cut along the lines to the fiberglass base. The result was isolated contact pads. We noted where the elements would be located, and it looked like a wiring diagram right on the board. In the future, tester elements will be soldered to them.
In order for a homemade tester to give correct readings with a given error, all its components must have accuracy characteristics that are at least the same, or even higher.
We will consider the internal resistance of the coil in the magnetoelectric mechanism of the microammeter to be equal to 3000 Ohms stated in the passport. The number of turns in the coil, the diameter of the wire, and the electrical conductivity of the metal from which the wire is made are known. This means that 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 battery voltage
In order to find out the actual battery voltage yourself, you will need at least one accurate resistor with a nominal value of 2 or 2.2 kOhm with an error of 0.5%. This resistor value was chosen due to the fact that when a microammeter is connected in series with it, the total resistance of the circuit will be 5000 Ohms. Consequently, the current passing through the tester will be about 300 μA, and the needle will deflect to full scale.
I=U/R=1.5/(3000+2000)=0.0003 A.
If the tester shows, for example, 290 µA, then the battery voltage is
U=I*R=0.00029(3000+2000)=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 of the shunts and additional resistors.
Assembling the power supply
The power supply for the multimeter is assembled from two 1.5 V batteries connected in series. After this, a microammeter and a 7 kOhm resistor pre-selected at nominal value are connected to it in series.
The tester should show a value close to the current limit. If the device goes off scale, then a second, small value resistor must be connected in series to the first resistor.
If the readings are less than 300 μA, then a high-value resistance is connected in parallel to these two resistors. This will reduce the total resistance of the additional resistor.
Such operations continue until the needle reaches the scale limit of 300 μA, which signals an accurate fit.
To select the exact 97 kOhm resistor, select the closest one that matches the nominal value, and follow the same procedures as with the first 7 kOhm one. But since a 30 V power supply is required here, the multimeter’s power supply will need to be reworked from 1.5 V batteries.
A unit is assembled with an output voltage of 15-30 V, as long as it is enough. For example, if it turns out to be 15 V, then all adjustments are made on the basis that the needle should tend to read 150 μA, that is, half the scale.
This is acceptable, since the tester scale when measuring current and voltage is linear, but it is advisable to work with full voltage.
To adjust the 997 kOhm additional resistor for the 300 V range, you will need DC or voltage generators. They can also be used as attachments to a multimeter when measuring resistance.
Resistor values: R1=3 Ohm, R2=30.3 Ohm, R3=333 Ohm, R4 variable at 4.7 kOhm, R5=7 kOhm, R6=97 kOhm, R7=997 kOhm. Selected by fit. Power supply 3 V. Installation can be done by hanging elements directly on the board.
The connector can be installed on the side wall of the box into which the microammeter is embedded. The probes are made of single-core copper wire, and the cords for them are made of stranded copper wire.
The shunts are connected using a jumper. As a result, a microammeter turns into a tester that can measure all three main parameters of electric current.
I received from AliExpress a couple of electronic built-in voltmeters model V20D-2P-1.1 (DC voltage measurement), the price is 91 cents each. In principle, you can now find it cheaper (if you look hard enough), but it’s not a fact that this will not be to the detriment of the build quality of the device. 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 (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 came across information about the possibility of improving them - adding a current measurement function.
Digital Chinese voltmeter I prepared everything I needed: a two-pole toggle switch, output resistors - one MLT-1 for 130 kOhm and a second wire resistor for 0.08 Ohm (made from a nichrome spiral with a diameter of 0.7 mm). And all evening, according to the found circuit and instructions for its implementation, I connected this equipment with wires to a voltmeter. To no avail. Either there was not enough insight in understanding what was left unsaid and incompletely drawn in the material found, or there were differences in the schemes. The voltmeter didn't work at all.
Connecting the digital voltmeter module I had to unsolder the indicator and study the circuit. What was needed here was not a small soldering iron, but a tiny one, so it took quite a bit of fiddling. But over the next five minutes, when the entire scheme became available for review, I understood everything. In principle, I knew that this was where I needed to start, but I really wanted to solve the issue “easy.”
V-meter modification scheme
Refinement scheme: ammeter to voltmeter This is how this scheme for connecting additional electronic components with those already existing in the voltmeter circuit was born. The standard resistor of the circuit marked in blue must be removed. I’ll say right away that I found differences from other circuits given on the Internet, for example, the connection of a tuning resistor. I didn’t redraw the entire voltmeter circuit (I’m not going to repeat it), I only drew the part that was necessary for modification. I think it’s obvious that the voltmeter’s power supply needs to be separate; after all, the starting point in the readings should start from zero. Later it turned out that power from a battery or accumulator will not work, because the current consumption of the voltmeter at a voltage of 5 volts is 30 mA.
Board - Chinese voltmeter module After assembling the voltmeter, I got down to the essence of the action. I won’t split hairs, I’ll just show and tell you what to connect with what to make it work.
Step-by-step instruction
So, action one– an SMD resistor with a resistance of 130 kOhm is removed from the circuit, standing at the input of the positive power wire, between the diode and the 20 kOhm trimming resistor.
We connect a resistor to a voltmeter-ammeter Second. On the freed contact, on the side of the trimmer, a wire of the desired length is soldered (for testing, conveniently 150 mm and preferably red)
Unsolder the SMD resistor Third. A second wire (for example, blue) is soldered to the track connecting the 12 kOhm resistor and the capacitor from the “ground” side.
Testing a new circuit
Now, according to the diagram and this photo, we “hang” an addition to 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 But here there are more wires, although everything is simple:
» — a pair of connecting wires connects the e/motor« separate power supply for voltmeter"- battery with two more wires
« power supply output"- a couple more wires
After applying power to the voltmeter, “0.01” was immediately displayed; after applying power to 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 install a button without locking, it will be safer for the circuit and more convenient for operation. I was pleased that everything worked on the first try. However, the ammeter readings differed from the multimeter readings by more than 7 times.
Chinese voltmeter - ammeter after modification Here it turned out that the wirewound resistor, instead of the recommended resistance of 0.08 Ohm, has 0.8 Ohm. I made a mistake in the measurements during its manufacture in the counting of zeros. I got out of the situation like this: the crocodile with the 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 ampere-voltmeter coincided and became the moment of truth. The resistance of the involved section of the nichrome wire was 0.21 Ohm (measured with a multimeter attachment at the “2 Ohm” limit). So it didn’t even turn out bad that instead of 0.08 the resistor turned out to be 0.8 Ohm. Here, no matter how you count, according to the formulas, you still have to adjust. For clarity, I recorded the result of my efforts on a video.
Video
I consider the purchase of these voltmeters a success, but it’s just a pity that their current price in that store has increased significantly, almost 3 dollars apiece. Author Babay iz Barnaula.
Hello dear reader. Sometimes it becomes necessary to have a small, simple voltmeter “on hand.” Making such a voltmeter with your own hands is not difficult.
The suitability of a voltmeter for measuring voltages in certain circuits is judged by its input resistance, which is the sum of the resistance of the pointer frame and the resistance of the additional resistor. Since at different limits the additional resistors have different values, the input resistance of the device will be different. More often, a voltmeter is evaluated by its relative input resistance, which characterizes the ratio of the input resistance of the device to 1V of the measured voltage, for example 5 kOhm/V. This is more convenient: the input resistance of the voltmeter is different at different measurement limits, but the relative input resistance is constant. The lower the current of the total deflection of the needle of the measuring device Ii used in the voltmeter, the greater its relative input resistance will be, the more accurate the measurements it makes will be. In transistor designs, it is necessary to measure voltage from fractions of a volt to several tens of volts, and in tube designs even more. Therefore, a single-limit voltmeter is inconvenient. For example, a voltmeter with a 100V scale cannot accurately measure even voltages of 1-5V, since the deviation of the needle will be barely noticeable. Therefore, you need a voltmeter that has at least three or four measurement limits. The circuit of such a DC voltmeter is shown in Fig. 1. The presence of four additional resistors R1, R2, R3 and R4 indicates that the voltmeter has four measurement limits. In this case, the first limit is 0-1V, the second 0-10V, the third 0-100V and the fourth 0-1000V.
The resistance of additional resistors can be calculated using the formula following from Ohm's law: Rd = Up/Ii - Rp, here Up is the highest voltage of a given measurement limit, Ii is the total deflection current of the measuring head needle, and Rp is the resistance of the measuring head frame. So, for example, for a device with a current Ii = 500 μA (0.0005 A) and a frame with a resistance of 500 Ohms, the resistance of the additional resistor R1, for the 0-1V limit should be 1.5 kOhm, for the 0-10V limit - 19.5 kOhm, for the 0 limit -100V - 199.5 kOhm, for the limit 0-1000 - 1999.5 kOhm. The relative input resistance of such a voltmeter will be 2 kOhm/V. Typically, additional resistors with values close to the calculated ones are installed in the voltmeter. The final “adjustment” of their resistances is made when calibrating the voltmeter by connecting other resistors to them in parallel or in series.
If a DC voltmeter is supplemented with a rectifier that converts AC voltage into DC (more precisely, pulsating), we get an AC voltmeter. A possible circuit of such a device with a half-wave rectifier is shown in Fig. 2. The device works as follows. At those moments in time when there is a positive half-wave of alternating voltage at the left (according to the diagram) terminal of the device, the current flows through diode D1 and then through the microammeter to the right terminal. At this time, diode D2 is closed. During the positive half-wave at the right terminal, diode D1 closes, and the positive half-waves of the alternating voltage are closed through diode D2, bypassing the microammeter.
The additional resistor Rd is calculated in the same way as for constant voltages, but the result obtained is divided by 2.5-3 if the rectifier of the device is half-wave, or by 1.25-1.5 if the rectifier of the device is full-wave - Fig. 3. More precisely, the resistance of this resistor is selected experimentally during calibration of the instrument scale. You can calculate Rd using other formulas. The resistance of additional resistors of the rectifier system voltmeters, made according to the circuit in Fig. 2, is calculated using the formula:
Rd = 0.45*Up/Ii – (Rp + rd);
For the circuit in Fig. 3, the formula looks like:
Rd = 0.9*Up/Ii – (Rp + 2rd); where rd is the resistance of the diode in the forward direction.
The readings of the rectifier system devices are proportional to the average rectified value of the measured voltages. The scales are calibrated in rms values of sinusoidal voltage, so the readings of rectifier system devices are equal to the rms voltage value only when measuring sinusoidal voltages. Germanium diodes D9D are used as rectifier diodes. These voltmeters can also measure audio frequency voltages up to several tens of kilohertz. A scale for a homemade voltmeter can be drawn using the FrontDesigner_3.0_setup program.
To digitally monitor the voltage and current in the power supply, it is not necessary to make an ADC and indicator yourself. For this purpose, a Chinese multimeter costing 3-4 dollars is quite suitable, which is comparable in price to the cost of manufacturing your own digital display.
The popular M830B was chosen for the conversion. Below we describe in detail, in pictures, the modification of a multimeter to indicate the voltage and current in your power supply.
The main purpose of the modification was to reduce the size of the board with the indicator, i.e. I just had to cut off part of the board. For the conversion, the simplest and cheapest Chinese multimeter M830B was purchased. The M830B multimeter circuit diagram can be downloaded from our file archive. The voltage measurement limit of our design will be 200 V, and the current limit will be 10 A. To select the “Voltage” - “Current” measurement mode, switch S1 with two groups of contacts is used. The diagram shows the switch position in voltage measurement mode.
First you need to disassemble the multimeter and remove the board. You can see the view of the board from the parts side in the photo.
And here is a photo of the board from the indicator side.
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 9-volt stabilizer. 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 designation on the board of the M830B multimeter. The diagram also provides additional explanations. The letters in 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 winding of the transformer.
Let's actually get started. Solder R18, R9, R6, R5. We save resistors R6 and R5 for the input part of our design. We cut off the upper contact R10 from the circuit and cut out part of the track (marked with crosses in the photo). Solder R10. Solder R12 and R11.
R12 and R11 are connected in series. And solder one end to the top contact of R10, and the other to the track cut off from R10. Unsolder R20 and solder it in place of R9. We desolder R16 and drill new holes for it (see photo)
Solder R16 to a new place.
And here is a view of the R16 soldering from the indicator side.
Take metal scissors and cut off part of the board.
Turn the board over with the indicator facing you. Contact R9 (now R20) closest to the indicator is cut off from the circuit (marked with a cross). We connect the contacts R9 (now R20) and R19 farthest from the indicator together (on the indicator side), indicated in the photo by a red jumper. We connect the upper contact R10 (there are now R11 and R12) with the lower contact R13, indicated in the photo with a red jumper. We delete some of the tracks marked with crosses. And we solder a 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 patches for wiring to the second board, indicated by arrows in the photo.
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, let's start checking. We connect it to a 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 value of the flowing current is displayed on the indicator.
