Among radio amateurs, especially beginners, ohmmeters with a linear scale are very popular, which do not require replacement or calibration of the dial indicator scale. The relatively simple design of such an ohmmeter was developed using an operational amplifier. An ohmmeter allows you to measure resistance from 1 ohm to 1 megohm, which is quite sufficient for many practical purposes.

The principle of operation of an ohmmeter on an operational amplifier is illustrated in Fig. 1. Measuring resistor RX included in the feedback circuit between the output of the amplifier and its inverting input. There is also a reference resistor in the same circuit. R 3 . The non-inverting input is supplied with a reference voltage from the source G1. In this mode, the output voltage of the operational amplifier will depend on the resistance ratio Rx And R 3 feedback circuits. It is measured relative to the reference voltage by a voltmeter PV, the readings of which are directly proportional to the resistance Rx.

Rice. 1. Functional diagram of an ohmmeter with a linear scale

The schematic diagram of the ohmmeter is shown in Fig. 2. The reference voltage + 2 V at the non-inverting input of the amplifier is created by a resistor divider R10 and a current stabilizer on the transistor VI. The exact value of the reference voltage is selected using a variable resistor R12. Since when measuring small resistances, the current in the measuring circuit, and therefore the output current of the amplifier, may exceed what is permissible for an op-amp, an emitter follower on a transistor is inserted into the ohmmeter V3. To protect the dial indicator from overloads in the event of an accidental increase in the output voltage of the amplifier due to the incorrect position of switch S1, a diode is connected parallel to the indicator terminals V2,

A voltmeter consists of a milliammeter PA1 and resistors R13, R14. In the position of the button shown in the diagram S2 The voltmeter is designed to measure voltages up to 2 V. When the button contacts are closed, the resistor R14 is shunted and the voltmeter measures the voltage up to 0.2 V.

Reference resistors are connected to the inverting input of the op-amp using a switch S1. The resistance of the reference resistor determines the measurement subrange of the ohmmeter. So, when you turn on the resistor R1 The device can measure resistance from approximately 100 kOhm to 1 MOhm. At the next switch position, the maximum measured resistance can reach 300 kOhm, and at further positions these values ​​will correspond to 100 kOhm, 30 kOhm, 10 kOhm, 3 kOhm, 1 kOhm, 300 Ohm, 100 Ohm. This results in nine measurement subranges.

Thanks to the button S2 the limits of measured resistances can be reduced by 10 times. It is used only on the last two subbands. Thus, more are added to the existing subranges two: up to 30 Ohm and up to 10 Ohm.

Rice. 2. Schematic diagram of an ohmmeter with a linear scale

In order to more economically consume the energy of the power source, it is connected to the device with the S3 button only during measurement.

Rice. 3. Placement of parts on the front panel of the case

The ohmmeter parts are housed in a small housing. On a removable front panel made of getinax measuring 190 X 130 mm (Fig. 3), an indicator and a subrange switch are mounted S1 and push-button switches S2, S3, calibration resistor R12 and terminals for connecting the power source and the resistor being tested (or other part with ohmic resistance).

The reference resistors are soldered directly to the switch blades, and the operational amplifier and transistors are mounted on a fiberglass board (you can getinaks) measuring 35 X 30 mm, which can be attached, for example, to the front panel from the inside.

Resistors R1 - R9 can be MLT-0.125, MLT-0.25 or others, selected with an accuracy of ±1% - the accuracy of measurements largely depends on this. Variable resistor R12 - SPZ-4a or other. Diode V2 It may be, in addition to what is indicated in the diagram, D226 with any letter index or another with a direct voltage of 0.3...0.6 V. Transistors are any of the K.T312, KT315 series. The dial indicator can have a total needle deflection current of 1 mA and an internal resistance of 82 Ohms. Then the resistor R.I.3 must have a resistance of 118 ohms, a R14 - 1.8 kOhm. An M24 microammeter with a full needle deflection current of 100 μA and an internal resistance of 783 Ohms is also suitable. (such an indicator is shown in Fig. 3), it is convenient because it has a scale of 100 divisions, making it easier to read the measured resistances. But in this case, it is necessary to bypass the indicator with a resistor with a resistance of about 92 Ohms so that the indicator needle deviates by the final division at a current of 1 mA. Resistor values R13, R14 for this option remain unchanged. If you use an indicator with a different internal resistance, you will have to recalculate the resistance of the resistors so that with the resistor R14 the indicator arrow deviated by the final scale division at a voltage of 0.2 V, and with resistors connected in series R13, R14 - n.p.And voltage 2 V.

Setting up the device begins with checking the correct installation. Then a 9 V source is connected to the power terminals, for example two 3336L batteries connected in series. The terminals of a precisely measured resistor, for example, with a resistance of 100 kOhm, are connected to the “Rx” terminals. Variable resistor motor R12 set to the middle position, and the switch handle S1 - to position “.300 k.” Only then do you press the button S3. The indicator needle should deviate by about a third of the scale. This is achieved with a variable resistor R12 "Caliber". Then the switch sets the subrange "100 k" and a variable resistor achieve precise deflection of the indicator needle by the final scale division. Check calibration on other subranges by connecting to the terminals « Rx» resistors with a resistance of 30 kOhm, 10 kOhm, 3 kOhm and so on. If there are significant discrepancies in the indicator readings and the resistance of the measured resistor, you should select a more accurate reference resistor.

To avoid the indicator needle going off scale when working with an ohmmeter, you should always start measurements in the switch position “1 M", and then, as the indicator arrow deviates, gradually move to other subranges.

DC ohmmeter circuits are divided into two main groups.

• a) Consistent. Ohmmeters with a series circuit are used to measure resistances greater than 1 kOhm.
• b) Parallel. Ohmmeters with a parallel circuit are used to measure resistances not exceeding 1 kOhm.

In our case, we need to measure a resistance of a maximum of 100 Ohms, therefore, we will use the second type of circuit. The simplest diagram of this ohmmeter is shown in Figure 1.1

Rice. 1.1

In parallel circuits, the measured resistance Rx is connected in parallel with the inductor. When terminals 1 and 2 are closed, the greatest current flows through the indicator, which should be equal to the total deflection current In.

To obtain the required current value, the additional resistance is selected equal to:

where is additional resistance, Ohm;

U - power source voltage, V;

Indicator resistance, Ohm.

The calculated value includes the internal resistance of the power supply. When connected to an ohmmeter, resistance Rx shunts the indicator, reducing the angle of deflection of its needle. When the terminals are short-circuited, the indicator is short-circuited and the current through it is zero.

The resistance between terminals 1 and 2 is called the input resistance of the ohmmeter Ri. For the simplest circuit

The operating conditions of the ohmmeter may differ from the normal conditions under which it was calibrated. This causes additional measurement error. Therefore, if the supply voltage is different, then the indicator readings will have an additional error. To increase accuracy in ohmmeters that use a single-frame indicator, a special “infinity” regulator is introduced.

Adjusting the “infinity” consists of checking before starting the measurement with the clamps open and setting the indicator arrow to the extreme position opposite the division with the ? mark.

In ohmmeters, the “infinity” adjustment is made using a magnetic shunt or an electrical “infinity” regulator.

Our device will use an electrical “infinity” regulator, which is a trimming resistor connected in series to the power source. The value of the electrical “infinity” regulator is determined from the formula

Rвmax =, (1.4)

where Rvmax is the maximum resistance of the electrical “infinity” regulator, Ohm.

Umax - maximum voltage of the power source, V.

Umin - minimum voltage of the power source, V.

The input resistance of the parallel circuit is mainly determined by the resistance of the indicator and can be approximately considered Ri?Ru.

If the input resistance should exceed the resistance of the indicator frame, then the ohmmeter is assembled according to the diagram in Figure 1.2

Scheme 1.2 Ohmmeter with sequential connection of the “infinity” regulator at Ri>Ru

In this case, the total resistance of the Ru+x indicator increases, which is achieved by connecting in series with the resistance indicator

Ru = Ru+x -Ru (1.5)

Increasing the input resistance of the ohmmeter as a result of increasing the resistance of the indicator circuit is not always beneficial, since it can lead to an increase in the supply voltage required for a given accuracy.

If the required input resistance is less than the resistance of the indicator, then the ohmmeter is assembled according to the diagram in Figure 1.3

Scheme 1.3 Ohmmeter with sequential connection of the “infinity” regulator at Ri

In this circuit, a shunt Rsh is connected parallel to the indicator, reducing the total resistance of the indicator and shunt circuit Ru+sh to the value

Turning on the shunt reduces the sensitivity of the indicator and increases the current in the power circuit required to deflect the indicator needle to the full scale, to the value

where: Iu+sht is the current flowing through the indicator and the shunt, A.

Reducing the input resistance by shunting and an indicator does not require increasing the supply voltage.

To expand the measurement limits of ohmmeters, a combination of these two circuits in one device is used. The transition from one measurement limit to another is carried out by measuring the input resistance of an ohmmeter. A general “infinity” regulator is also used, this means that the indicator arrow needs to be adjusted to the “infinity” value only once, this value will be saved when moving to any measurement limit.

The shunt resistance in such ohmmeters is determined from the condition of obtaining the lowest input resistance Ri=Rimin. Hence,

The maximum supply voltage is selected from the condition of ensuring the required measurement accuracy with the highest input resistance Ri=, the total deviation current in such a circuit will be equal to

Checking resistors

Constant resistors are checked with an ohmmeter
by measuring their resistance and comparing with
nominal value, which is indicated on the resistor itself and
on the schematic diagram of the device. When measuring
resistor resistance polarity of connection to it
The ohmmeter doesn't matter. It must be remembered that
actual resistor resistance may vary
from the nominal value to the tolerance value. Therefore, for example,
if a resistor with a nominal value is checked
100 kOhm resistance and ±10% tolerance, actual
The resistance of such a resistor can range from 90 to
110 kOhm. In addition, the ohmmeter itself has a certain
measurement error (usually about 10%). So
Thus, if the actual measured value deviates
resistance by 20% of the nominal value of the resistor
should be considered correct.
When checking variable resistors, it is measured
resistance between the extreme terminals, which should
correspond to the nominal value taking into account the tolerance
and measurement errors, and it is also necessary to measure
resistance between each of the extreme terminals and
middle output. These resistances when the axis rotates from
from one extreme position to the other should be smooth, without
jumps change from zero to the nominal value.
When checking a variable resistor soldered into the circuit,
two of its three pins must be desoldered. If
the variable resistor has additional taps,
it is permissible for only one pin to remain soldered to
the rest of the diagram.

Checking capacitors

Capacitors may have the following defects: open circuit,
fight and increased leakage. Capacitor breakdown
is indicated by the presence of a short circuit between its terminals
that is, zero resistance. Broken capacitor
any type can be easily detected with an ohmmeter by checking
ki resistance between its terminals.
The capacitor does not pass direct current, it is
the resistance measured by an ohmmeter should be
infinitely great. However, this turns out to be true
only for an ideal capacitor. In fact
between the plates of the capacitor there is always some kind of
dielectric having a finite value
resistance, which is called leakage resistance. It's him
measured with an ohmmeter. Depending on the used
dielectric capacitor criteria are established
serviceability based on leakage resistance. Mica,
ceramic, film, paper, glass and air
capacitors have a very high leakage resistance,
and when checking them, the ohmmeter should show indefinitely
great resistance. However, there is a large group
capacitors whose leakage resistance
relatively small. This includes all polar capacitors,
which are designed for a specific polarity
voltage applied to them, and this polarity is indicated by
their buildings. When measuring the leakage resistance of this
groups of capacitors must maintain polarity
connection of the ohmmeter (the positive terminal of the ohmmeter should
connect to the positive terminal of the capacitor), in
otherwise the measurement result will be incorrect. To this
The group of capacitors includes all electrolytic and
oxide semiconductor capacitors. Resistance
There should be no leakage of such serviceable capacitors
less than 100 kOhm, the rest not less than 1 MOhm. When checking
high-capacity capacitors, it must be taken into account that when
connecting an ohmmeter to the capacitor if it was not charged,
its charging begins and the ohmmeter needle throws in
side of the zero value of the scale. As the shooter charges
moves in the direction of increasing resistance. The more
the capacitance of the capacitor, the slower the needle moves.
The leakage resistance should only be measured
after it almost stops. When checking
capacitors with a capacity of about 1000 μF can do this
it will take a few minutes.
Internal break or partial loss of capacity
capacitor cannot be detected by an ohmmeter. For this
A device is needed to measure capacitance. However, the cliff
capacitors with a capacity greater than 0.2 µF can be detected
ohmmeter to determine the absence of an initial jump in the needle during
charging. Re-checking the capacitor for open circuit may
be carried out only after removing the charge, for which the conclusions
The capacitor must be short-circuited.
Variable capacitors are checked
ohmmeter to check for short circuits. For this purpose an ohmmeter
connects to each section of the unit and turns slowly
axis from one extreme position to another. Ohmmeter
should show infinitely high resistance in
any axis position.

Checking inductors

When checking inductors with an ohmmeter
only the absence of a break in them is monitored. Resistance
single-layer coils should be equal to zero,
multilayer resistance is close to zero. Sometimes in passport
device data indicates the resistance of multilayer
coils to direct current, and its value can be
guide you when checking them. If the coil breaks, ohmmeter
shows infinitely large resistance. If
the coil has a tap, you need to check both sections by connecting
ohmmeter first to one of the outer terminals of the coil and to its
tap, and then - to the second extreme pin and tap.

Checking low-frequency chokes
and transformers

As a rule, in the passport data of the equipment or in
the instructions for its repair indicate resistance values
DC windings that can be used
when checking transformers and chokes. Winding break
is fixed by an infinitely large resistance between
its conclusions. If the resistance is significantly less
nominal, this may indicate the presence of a short circuit
chucked turns. However, most often short-circuited turns
occur in small quantities when a change occurs
friction between adjacent turns and winding resistance
changes slightly.
The absence of short-circuited turns can be checked
as follows: the winding of the transformer is selected
with the largest number of turns, to one of the terminals
to which an ohmmeter is connected using an alligator clip,
the second is touched with a slightly damp left finger
hands. Holding the metal tip of the second probe
ohmmeter with your right hand, connect it to the second terminal
winding without lifting the finger of your left hand from it. Arrow
the ohmmeter deviates from its initial position, indicating
winding resistance. When the arrow stops, withdraw
right hand with a probe from the second winding terminal. If
the transformer is working properly, then at the moment the circuit breaks you feel
mild electric shock. Due to the fact that energy
discharge is insignificant, there is no danger from such a check
is. In this case, you need to use an ohmmeter
smaller measurement limit, which corresponds to
the highest measuring current.

Diode check

Semiconductor diodes are characterized by sharply nonlinear
current-voltage characteristic, therefore their direct and
reverse currents at the same applied voltage
are different. This is the basis for checking diodes with an ohmmeter. Direct
resistance is measured when positive is connected
the ohmmeter lead to the anode, and the negative lead to the cathode of the diode. U
of a broken diode, the forward and reverse resistances are equal
zero. If the diode is open, both resistances are infinite
great. Specify the forward and reverse values ​​in advance
resistances or their ratio is impossible, since they depend on
applied voltage, and this is the voltage for different vehicles
meters and at different measurement limits are not the same. However
For a working diode, the reverse resistance should be less
be more direct. Reverse resistance ratio
to direct for diodes designed for low reverse
voltage is high (can be more than 100). For diodes,
designed for large reverse voltages, this ratio
turns out to be insignificant, since the reverse voltage,
applied to the diode with an ohmmeter is small compared to that
reverse voltage for which the diode is designed.
The method for checking zener diodes and varicaps is not
differs from the above. As you know, if to the diode
voltage equal to zero is applied, the diode current will also be
equal to zero. To obtain direct current it is necessary
apply some small threshold to the diode
voltage, which is provided by any ohmmeter. However, if
several diodes are connected in series (in one direction),
threshold voltage required to unlock all
diodes increases and may turn out to be more than
voltage at the ohmmeter terminals. For this reason, measure
forward voltages of diode columns or selenium columns
impossible with an ohmmeter.

Thyristor testing

Uncontrolled thyristors (dinistors) can be
tested in the same way as diodes, if the voltage
unlocking the dinistor is less than the voltage at the ohmmeter terminals.
If it is larger, the dinistor does not
unlocks and the ohmmeter shows in both directions
very high resistance. However, if the dinistor
broken, the ohmmeter registers this as zero readings
direct and reverse resistance.
To test controlled thyristors (thyristors)
the positive terminal of the ohmmeter is connected to the anode of the thyristor,
and the negative terminal goes to the cathode. The ohmmeter should
show very high resistance, almost equal
infinite. Then the terminals of the anode and control are closed
SCR electrode, which should lead to a sharp
reducing resistance. If you turn it off after that
control electrode from the anode without breaking the circuit, for many
types of SCRs the ohmmeter will continue to show
low resistance of the open thyristor. This is happening
dits when the anode current of the trinistor is greater than that:
called holding current. In this case, the thyristor is both
necessarily remains open. This requirement is
tactful, but not mandatory. Three individual copies
nistors of the same type can have the same values
retention is significantly less than guaranteed. In it"
in the case of a trinistor when the control electrode is disconnected;
from the anode remains open. But if at the same time he locks
and the ohmmeter shows a high resistance, it is impossible to calculate
It means that the thyristor is faulty.

Checking transistors

The equivalent circuit of a bipolar transistor is represented by
It consists of two diodes connected towards each other. Dl*
pnp transistors, these equivalent diodes are connected as
tods, and for p-p-p transistors - anodes. Thus,
checking a transistor with an ohmmeter comes down to research
both p-n junctions of the transistor: collector-base and emitter
base. To check the forward resistance of the junctions
pnp transistor, the negative terminal of the ohmmeter is connected to
base, and the positive terminal of the ohmmeter - alternately to the collector
and emitter. To check reverse resistance
transitions, the positive terminal of the ohmmeter is connected to the base.
When checking n-p-n-transistors, connection
is done the other way around: forward resistance is measured at
connection to the base of the positive terminal of the ohmmeter, and the reverse
resistance - when connected to the negative base
output. When a transition breaks out, its forward and reverse
resistance turns out to be zero. If the crossing breaks
direct resistance is infinitely great. In serviceable
low-power transistors reverse resistance
transitions are many times greater than their direct resistance. U
powerful transistors, this ratio is not so great, however
Less than an ohmmeter allows them to be distinguished.
From the equivalent circuit of a bipolar transistor
it follows that using an ohmmeter you can determine the type
conductivity of the transistor and the purpose of its terminals. At first
determine the type of conductivity and find the base terminal of the transistor.
To do this, the first terminal of the ohmmeter is connected to the terminal
transistor, and the other terminal of the ohmmeter touches two
other terminals of the transistor. Then the first lead of the ohmmeter
connected to another terminal of the transistor, and the other terminal
touch the free terminals of the transistor. After which the same
the ohmmeter lead is connected to the third terminal of the transistor, and
Another conclusion concerns the rest. After that they change
place the ohmmeter leads and repeat the indicated measurements.
You need to find a position of the ohmmeter at which the connection
its second terminal to each of the two terminals of the transistor, not
connected to the first terminal of the ohmmeter, corresponds to
low resistance (both junctions are open). Then
the terminal of the transistor to which the first terminal is connected
ohmmeter is the base terminal. If the first terminal of the ohmmeter
is positive, which means the transistor belongs to the p-p-p-pro-
conductivity, if - negative, then to p-p-p-conductivity.
Now we need to determine which of the remaining two
The transistor terminals are the collector terminals. For this
ohmmeter is connected to these two terminals, base
connects to the positive terminal of the ohmmeter with an n-p-n transistor or
with the negative terminal of the ohmmeter with a pnp transistor and
resistance is noticed and measured with an ohmmeter. Then
the ohmmeter leads are swapped (the base remains
connected to the same ohmmeter terminal as before), and again
resistance is noticed on the ohmmeter. In the case when
the resistance turns out to be less, the base was connected to
collector of the transistor.

Hi all! Today we are reviewing Kelvin Clamps from Ebay. In amateur radio engineering, it is often necessary to measure small resistances, so I dreamed of buying a Milliohmmeter for this purpose. From time to time I search for the phrase “milliohm meter” on Ali and Ebay, read the options found and leave the computer with a sigh, because... the prices for these devices are not encouraging, especially during a crisis, where money is already tight. Actually, my requirements for measuring small resistances are not high; I don’t need to measure microohms or something similar with an accuracy of 6 decimal places. But sometimes there is a need to measure the resistance of the switch contacts, select a shunt for an ammeter, and often it is simply necessary to select the most suitable resistor from a bunch of similar ones... Therefore, the idea arose to make your own budget measuring device capable of measuring, quite accurately, resistance in the range from 0.001 Ohm to 2 Ohm. For anyone who is interested, please, under Cat... Attention: Lots of photos (traffic)!!!

For those who like to find fault with words, metrologists and those who are simply in a bad mood

Right at the beginning of the review, I want to dot some i’s. The review will not describe a single precision measuring instrument that has a certificate of verification of the Measuring Instrument. To some, my review may seem pointless, or a “review for the sake of review.” Well, you can’t please everyone... But maybe my review will be useful to someone. With my reviews, I pursue only 2 goals: 1. To popularize amateur radio equipment. Suddenly, someone also gets itchy hands and wants to collect something. 2. I just like to share what I have done, so I write reviews for my own pleasure, too. If you don't like my reviews, blacklist me and read more interesting lingerie reviews. Moreover, it’s spring now and the girls, I hope, will delight us with beautiful photographs more than once!)))

All spare parts were purchased with my own money, point 18 doesn’t even smell here... To all “homemade” people and those who like to read reviews in the “Handmade” topic, Welcome (we kindly ask, kosh keldiniz)... Ask questions in the comments, constructive criticism is welcome, spelling Please indicate any errors in a personal message and I will try to correct them...

Initially, it was planned that the homemade milliohmmeter would be powered by an 18650 lithium battery, and, accordingly, a bunch of Chinese boards, which have already been reviewed more than once on our website: a charging module, an overdischarge protection module and a booster board (popularly “booster”), because it’s a millivoltmeter operates at voltages from 8 to 12V. Therefore, I decided to test whether the voltage of the lithium battery is enough so that the current stabilizer on the Lm317 is guaranteed to produce a current of 100 mA. I quickly screwed a resistor with a resistance of about 12 ohms onto the legs of the LM317 and assembled a test circuit. The connection diagram is very simple, I will give a picture illustrating the connection of radio components, only instead of the measured resistor we will have an ammeter connected:

As can be seen in the series of photographs (gif), current stabilization starts at approximately 4V and the current is stable over a wide voltage range. Thus we see that the current stabilizer is working.

During the initial tests, regarding the possibility of using a lithium battery, I was seriously disappointed... The current stabilizer consistently gave a stable current, starting from 4-4.5V... Thus, when the battery was discharged to 3V, the current became 80mA, which means no accuracy measurements, when using power from a lithium battery, there is no need to speak. We'll have to move on to plan B... If we can't implement the idea on battery power, we'll do it on mains power.

It was ordered from Banggood, with two independent channels for 12 and 5 Volts. I was captivated by 2 things in this block: independent channels of 5 and 12 volts, which is very important given the chosen circuit design, because The current stabilizer and millivoltmeter must be powered from galvanically independent power supplies. And the presence of at least some kind of filter at the input of the SMPS, which is rare for inexpensive Chinese power supplies. Thanks to the discount that I learned about on our website “Muska”, the magic word “elec”, this board cost me 4.81 USD, instead of the original price of 5.66 USD (I hope this discount does not apply to step 18)))) The board is already on its way to Kazakhstan, we just have to wait for it... At the same time, we’ll test this switching power supply.

While the package is traveling from China, let's draw a block diagram of our homemade Milliohmmeter. The circuit is very simple and can be repeated even by a novice radio amateur or simply anyone whose hands grow from the right place, even if he does not understand anything about radio engineering)))) The circuit can be assembled simply by looking at the picture and using any multimeter as a millivoltmeter on the 200mV range.

The only thing you need to do is find the positive (+) terminal of the 5 Volt power supply yourself and connect it to pin 3 of the LM317 chip. In the diagram I indicated the connection to the power source purely schematically, without indicating the polarity, because It is not known in advance where the positive output of the Chinese SMPS will be. If you make a milliohmmeter - an attachment for a multimeter, then you can use any 5V power supply from a cell phone, etc. Power supply for the millivoltmeter is then not needed, because The multimeter has its own battery power.

We are assembling a test bench where we will check the performance of our milliohmmeter. Since the power supply has not arrived yet, we use 2 laboratory power supplies instead. 5 volts to power the LM317 and 12V to power the millivoltmeter:


We are assembling a current stabilizer, I simply soldered 2 resistors (constant and trimming, connected in parallel) on the Lm legs. The result is this “collective farm”:


We connect a multimeter to the resistors in resistance measurement mode and use a trimmer to set the resistance to approximately 12.5 Ohms. Let's more accurately adjust the resistance using the ammeter:


We are preparing test resistors... We will have 3 Chinese wire resistors, they have the index “J”, which indicates that the accuracy of the resistor is ±5% and 2 Soviet resistors C5-16, with an accuracy of ±1%. More precisely, I don’t have one, I think that this will be quite enough...


We connect a 0.13 Ohm ±1% resistor to the Kelvin probes, connect the entire structure to the power supplies, the ammeter showed a current of 98 mA, first of all we bring the current to 100 mA using a trimming resistor:


Let's see, the value of the voltage drop across the resistor is 0.13 Ohm, I also connected a multimeter to check the correctness of the readings of the millivoltmeter purchased in China. As we can see, the readings are the same, no adjustments need to be made... The voltage drop across the resistor is 13 mV, which equals a resistance of 130 mOhm, or 0.13 Ohm. (according to the rules, milliohms are written with a small letter “m”, and megaohms with a capital letter “M”)


As you can see, our homemade milliohmmeter works and has sufficient accuracy for amateur radio. I will hide the rest of the measurements under a spoiler, for those who are interested you can take a look, but for the rest I will save a little traffic))))

Low Resistor Measurements

Resistor measurement 0.3 ohm ±1%


Resistor measurement 0.1 ohm ±5%


Resistor measurement 0.22 ohm ±5%


Finally, measure the 1 ohm resistor ±5%

Conclusions: Using a multimeter (or millivoltmeter), Kelvin probes and a small pile of radio components, in an hour, on your knees, you can assemble a quite decent milliohmmeter attachment, which allows you to measure small resistances accurately enough for amateur radio practice. On this optimistic note I end the review. Peace, goodness and spring in your soul to everyone!!!

An incorruptible metrologist from the Quality Control Department

A practically incorruptible metrologist and representative of the quality control department nicknamed Fox always monitored my work.

UPD: Due to debates in the comments, I decided to add an experiment with replacing a 4-wire circuit with a 2-wire one...
Option 1. Kelvin scheme...

Option 2 We close the contacts in the Kelvin probes with wire jumpers (the wire jumpers are clearly visible in the photo. The resistor resistance has increased by 1 mOhm

And now we change the 4-wire circuit to a 2-wire one... The wires are 1.5 mm thick, the clamps are soldered... We look at the resistance of the 0.13 Ohm resistor... We draw our own conclusions...


UPD2: Thanks to our comrade mikas, I re-soldered the decimal point jumper on the Millivoltmeter. Now the resistance is shown immediately in the required format. The picture shows a 0.13 Ohm resistor


And this is a 1 ohm resistor

UPD3: I finally made a homemade milliohm meter work from two 18650 batteries. (It didn’t work with one, even though there were 2 converters, but the voltmeter readings strongly depended on the resistance of the resistor being tested. Therefore, it won’t work with one power supply)
This is what happened... This is the power supply for the current stabilizer. Chain: 18650 battery - charging and protection board (two in one) - booster (booster with a frequency of 1 MHz) up to 5V.


Let's put it together:

Next, we add another 18650 battery - a booster (increase) to 10V to power the millivoltmeter. This is how the “hell” design turns out...

Without a photo of the device itself, it seems like the review is not complete. The case was made from scrap materials (an adapter for two rectangular pipes for a kitchen hood, bought at a hardware store for 550 tenge), it’s a little crooked, but it’s self-made))) The filling has not yet been inserted, the IIP has not yet arrived.

UPD4: I finished assembling the device. The device runs on 2 batteries of 18650 and 14500 format (high power current, low power supply for the millivoltmeter). It costs 2 charging boards with battery protection, and 2 boosting modules: 5V for the current source and 10V for powering the millivoltmeter. Next are just photos of what happened...








The last photo shows charging... For now the channels are separate, then I will connect 2 channels to one input.

That's it for sure now!!! I completed my mission to review a homemade milliohmmeter to the end. Beaver everyone!!!))))