A simple homemade voltmeter. How to make a simple voltmeter with your own hands - diagrams and recommendations Testing a new circuit
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 evaluates its relative input resistance, which characterizes the ratio input impedance 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 voltmeter DC 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 alternating voltage into direct voltage (more precisely, pulsating), we get a voltmeter AC. Possible scheme 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 at the left (according to the diagram) terminal of the device AC voltage, 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. The resistance of this resistor is selected more accurately empirically 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 graduated in root mean square values sinusoidal voltage, therefore, the readings of the rectifier system devices are equal to the root mean square 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.
For those who like to do it themselves, we offer a simple tester based on the M2027-M1 microammeter, which has a measuring range of 0-300 μA, internal resistance 3000 Ohm, 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, then others measuring instruments he doesn't have it. 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 Rtotal is 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, it looked like 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 because 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 source 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 from single-core copper wire, and the cords for them are multi-core.
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.
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 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 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 out :-), 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 unsoldering the indicator or ringing the circuits with a tester and off you go!
Every owner of a Chinese multimeter DT830 and similar models must have encountered some inconveniences during operation that are not visible at first glance.
For example, the battery constantly drains due to the fact that they forgot to turn the switch to the off position. Or lack of backlighting, impractical wires and much 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 multimeter wires and probes
The first thing that 99% of users of cheap Chinese multimeters encounter is the failure of low-quality measurement probes.
Firstly, the tips of the probes may break. When touching an oxidized or slightly rusty surface for measurement, the surface must be lightly cleaned to ensure reliable contact. The most convenient way to do this is, of course, using the probe itself. But as soon as you start scraping, at that 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, a wire break occurs at the connection points at the plug-in contact and directly at the soldering of the sharp tip of the probe. 
When this happens, you will be surprised how thin the wiring inside is really. 
Meanwhile, the multimeter must be designed to measure current loads up to 10A! It is not clear how this can be done using such a wire. 
Here are real data on current consumption measurements for flashlights, made using standard probes included in the kit and using homemade probes cross section 1.5mm2. As you can see, the difference in error is more than significant. 
The plug-in contacts in the multimeter connectors also become loose over time and worsen the overall resistance of the circuit during measurements.
In general, the unequivocal verdict of all owners of DT830 multimeters and other models is that the probes need to be modified or changed immediately after purchasing the tool. 
If you are the lucky owner lathe or you know a turner, then you can make the handles of the probes yourself from some insulating material, for example, pieces of unnecessary plastic.
The tips of the probes are made from a sharpened drill. The drill itself is a hardened metal and can be used to easily scrape off any carbon deposits or rust without the risk of damaging the probe. 
When replacing plug-in contacts, it is best to use the following plugs used in audio equipment for speaker sockets. 
If you really have to go collective farming or there are no other options at hand, then as a last resort You can use regular contacts from a collapsible plug.
They also fit perfectly into the connector on the multimeter. 
At the same time, do not forget to insulate the ends that will stick out outside the multimeter, in the places where the wires are soldered to the plug, with a heat pipe.
When it is not possible to make probes yourself, the body can be left the same, replacing only the wires.
In this case, three options are possible:
After replacement, such wires will very easily be collected into a bundle without getting tangled.
Secondly, they are designed to withstand a huge number of bends and will break no sooner than the multimeter itself fails.
Thirdly, the measurement error due to their larger cross-section compared to the original ones will be minimal. That is, there are continuous advantages everywhere.
If you make long wires up to 1.5 m, taking into account all the connections, the resistance on them can reach several Ohms!Important note: when replacing wires, you should not try to make them much longer than those that came with the kit. Remember that the length of the wire, as well as its cross-section, affects the overall resistance of the circuit.
Those who do not want to do homemade work can order ready-made high-quality silicone probes with many tips on AliExpress. 
To ensure that new probes with wire take up minimal space, you can twist them into a spiral. To do this, a new wire is wound around the tube, wrapped in electrical tape to secure it, and the whole thing is heated with a hair dryer for a couple of minutes. As a result, you get this result. 
In a cheap version, this trick will not work. And when used for heating construction hair dryer insulation may even float.
Refinement of the multimeter mount
Another inconvenience when taking measurements with a multimeter is the lack of a third hand. You constantly have to hold a multimeter in one hand and use the other to work with two probes at the same time. 
If measurements take place at your desk, then there is no problem. Put the tool down, free your hands and work. 
What should you do if you measure the voltage in a panel or in a distribution box under the ceiling?
The problem can be solved simply and inexpensively. In order to be able to mount the multimeter on a metal surface, on back side device using hot glue or double-sided tape, glue ordinary flat magnets. 
And your device will be no different from expensive foreign analogues. 
Another option for inexpensive modernization of a multimeter in terms of its convenient placement and installation on a surface for measurements is the manufacture of a homemade stand. To do this, you only need 2 paper clips and hot glue. 
And if you don’t have any surface nearby where you can place the tool, what should you do in this case? Then you can use an ordinary wide elastic band, for example from suspenders. 
You make a ring out of an elastic band, pass it through the body and that’s it. Thus, the multimeter can be conveniently mounted directly on your hand, like a watch.
Firstly, now the multimeter will never fall out of your hands again, and secondly, the readings will always be before your eyes. 
Caps for probes
The spikes at the ends of the probes are quite sharp, which 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 your finger, they also protect the contacts from breaking when the multimeter is in a bag mixed with another tool.
In order not to buy spare ones every time, you can make them yourself. Take an ordinary cap from a gel 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 glue and place it on the sharp tip. 
Wait until the hot glue hardens and calmly remove the resulting result. 
Multimeter backlight
A function that the multimeter lacks in poorly lit areas is display backlighting. Solving this problem is not difficult, just apply:
Make a hole in the side of the housing for the switch. Glue the reflector under the indication display and solder two wires to the crown contacts. 
They supply power to the switch and then to the LEDs. The structure is ready.
IN end result a homemade modification of the multimeter backlight will look like this: 
The backlit battery will be used up much faster, so do not forget to turn off the switch when there is enough natural light.
Replacing the crown in a multimeter with a lithium-ion battery from a phone
IN recent years It has become very popular to remake a multimeter by replacing 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 in its upper part. It is needed so that the battery does not discharge beyond the nominally permissible limits (approximately 3 Volts and below). 
The charging board does not allow the battery to be recharged above 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 fits compactly on the back cover and does not interfere with its closure. 
First, the output voltage on the boost module must be set to 9 Volts. Connect it with wires to a multimeter that has not yet been converted and use a screwdriver to unscrew the required value. 
You will have to make a hole in the case for a micro or mini USB charging connector.
The boosting module itself is located in the place where the crown should be. 
Be sure to ensure that the wiring from the module to the battery is of the required length. In the future, this will allow you to easily remove the cover and, having halved the body, carry out an internal inspection of the multimeter if necessary.
After placing all the parts inside, all that remains is to solder the wiring according to the diagram and fill everything with hot glue so that nothing moves when moving the device. 
It is advisable to fill not only the body with hot glue, but also the contacts with the 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 operation, at subzero temperatures.
Once your multimeter sits in the trunk of a car or in a bag in the winter for a long time, you will immediately remember the battery.
And you might think, was such a change useful? Ultimately, of course, you decide, based on the operating conditions of the device.
Refinement of the on/off button on the multimeter
It is advisable to further improve the last option for refining the multimeter with the transition to lithium-ion batteries by placing a shutdown button in the power supply circuit of the converter to the battery. 
First, the converter itself consumes a small amount of current, even in standby mode when the multimeter is not working.
Secondly, thanks to this switch, you won’t have to click the multimeter itself again to turn it off. Many devices fail prematurely because of this reason.
Some paths are erased ahead of time, others begin to shorten each other. So a button to turn off the entire device at once will be very useful.
Another tip from experienced users of Chinese multimeters is that in order for the switch to serve for a long time and properly, immediately after purchase, disassemble and lubricate the sliding areas of the switch balls. 
And on the board it is recommended to coat the tracks with technical Vaseline. Since new devices do not have lubrication, the switch wears out quickly. 
You can make a button both internally, if you find free space, and externally. To do this, you will have to drill only two micro holes for the power wiring. 
Flashlight in multimeter
Another innovation for the multimeter is the additional flashlight option. Often you have to use the device to look for damage in switchboards and distribution cabinets in basements, or short circuits in wiring in rooms where there is no light.
An ordinary white LED and a button specifically for turning it on are added to the circuit. Check how much is enough luminous flux This LED makes it very easy. You don't even have to disassemble it to do this.
Place the anode leg of the diode in connector E, and the cathode leg in 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 glow in any position of the switch and will go out only when you turn off the multimeter yourself. To mount all this inside, you need to find the necessary pins on the circuit board and solder two wires to the emitter (connector E) and collector (connector C). A button is soldered into the wire gap and mounted through a hole in the multimeter body. 
You secure everything with hot glue and you get a portable flashlight-multimeter.
The Chinese yellow tester DT-830B from Leroy-Merlin costs 75 rubles. It has an LCD display, a microcircuit like ICL7106/7106 in the form of a drop of epoxy with a strap and why not make a convenient built-in voltmeter out of it for, for example, a power supply, or some other application, simply cutting off what is unnecessary.
You need a voltmeter - remove everything unnecessary
Original
The original looked like this (yes, I forgot the cords! They are also worth something).What's in the package
What's inside
We analyze, study, draw conclusions:
Schematic diagram
Here is a schematic diagram of the “father of the family”, which can be seen in many similar devices with minor variations. Often even the markings on the board coincide with the positional designations on the diagram (R3, C6...):
The scheme, of course, does not coincide 1:1 with reality, but you get the gist enough.
PCB
Printed circuit board in “printable” form, I studied the tracks on it:
Rework
Trimming and jumpers
In general, take scissors and cut along the path above the inscription “830B.4C”.Then you will need to restore just one connection using jumper A-A and use the second jumper B-B to indicate how to display commas on the screen. See further:
Comma management
| 1. jumper from "BATT +" (upper pin R8) to lower pin R2. The result will be like this: | 
|
| 2. jumper from "BATT +" (upper pin R8) to lower pin R3. The result will be like this: |
|
| 3. jumper from "BATT +" (upper pin R8) to lower pin R4. The result will be like this: |
|
| 4. If the jumper is not installed at all, the “HV” icon will not be displayed. |
|
As you can see, commas are very easy to manage. At least a switch (if necessary, of course).
In its original case, the resulting “multimeter stub” now looks like this:
Divider for voltmeter
There are unused precision resistors left on the sides of the board - they can be used to organize the desired voltage divider for a voltmeter:| position | denomination | divider | range 1 (voltmeter resistance input) | range 2 (voltmeter resistance input) |
| R22 | 100 | 1:1 | 0 - 200 mV / 0.1 kOhm | not Spanish |
| R21 | 900 | 1:10 | 0 - 2 V / 1 kOhm | 0 - 200 mV / 1 kOhm |
| R13 | 9k | 1:100 | 0 - 20 V / 10 kOhm | 0 - 2 V / 10 kOhm |
| R14 | 90k | 1:1000 HV | 0 - 200 V / 100 kOhm | 0 - 20V / 100 kOhm |
In order to use the divider, you need to connect the lower pin R22 to the “COM” bus (for example: the upper pin C3 or the lower pin R7). Connect the input of the microcircuit to the desired divider tap (connect the upper pin R6 to the lower pin R21 if range 1 is selected or to the upper pin R21 if range 2 is selected). The difference in the choice of ranges will be in the input resistance of the resulting voltmeter. Resistors R1 100 Ohm and R2 900 Ohm cannot be touched, they are used. Resistor R9 is not used. It can even be removed; but you can't connect to it.
What happened as a result
In essence, it turned out to be a measuring head, aka digital voltmeter DC, with the following parameters:- input voltage range -199-0-199 mV (both polarities are measured with sign indication);
- overload indication;
- linearity error no more than ±0.2 units;
- zero setting error no more than ±0.2 units;
- input current no more than 1pA (typical value for ICL7106/7107), corresponding to the input resistance guaranteed to be hundreds of megaohms;
- The current consumption of the voltmeter is about 1 mA on each arm, which corresponds to hundreds of hours of operating time from the standard “Krona”.
- The low-pass filter at the input (R6 1Mohm and C3 0.1uF) provides a settling time of 0.1 sec.
If you need to power the voltmeter from the device where it will be installed, you should take into account that the voltage at the “BATT+” pin of the microcircuit (relative to “COM”, of course) will always be 3.0V because it is stabilized by the internal reference stabilizer in the microcircuit itself and cannot be exceeded; the negative voltage “BATT-” is formed as the voltage on the battery minus 3.0V. Both voltages can be formed by parametric stabilizers using two resistors and any zener diode, even a green one or, better yet, a white LED. But the best thing is to provide a galvanically independent power source for the voltmeter, especially since the current consumption is negligible.
Application
Thermometer -55...+150С with resolution 0.1С
As a sensor we use the LM35 sensor chip in the following connection:
The estimated price of the microcircuit is about 200 rubles ($6) for LM35CZ.
Schematic diagram of a thermometer
Operating temperature range, error and chip index
| marking* | temperature range | typical error at 25C** | TO-46 body | TO-92 body | SO-8 housing (SMD) | TO-220 housing |
| LM35 | -55...+155 | 0.4 | LM35H | |||
| LM35A | -55...+155 | 0.2 | LM35AH | |||
| LM35C | -40...+110 | 0.4 | LM35CH | LM35CZ | ||
| LM35CA | -40...+110 | 0.2 | LM35CAH | LM35CAZ | ||
| LM35D | 0...+100 | 0.4 | LM35DH | LM35DZ | LM35DM | LM35DT |
Note:
*index A means improved accuracy and linearity.
**at the edges of the range the error is approximately 2 times higher, for more details see