Converting a multimeter to Li-Ion with charging. Li-Ion battery in a multimeter Materials and tools
Some modern low-power devices consume very little current (several milliamps), but for their power supply they require a very exotic source - a 9 V battery, which also lasts for a maximum of 30... 100 hours of operation of the device. This looks especially strange now, when Li-ion batteries from various mobile gadgets are almost cheaper than the batteries themselves. Therefore, it is natural that a real radio amateur will try to adapt batteries to power his device, and will not periodically look for “antique” batteries.
If we consider the usual (and popular) M830 multimeter, powered by a “Corundum” type element, as a low-power load, then to create a voltage of 9 V we need at least 2-3 batteries connected in series, which is not suitable for us - they simply will not fit inside the body of the device. Therefore, the only way out is to use one battery and a step-up voltage converter.
Selection of element base
The simplest solution is to use a 555 type timer (or its CMOS version 7555) in pulse converter(capacitive converters are not suitable - we have too big a difference between the input and output voltages). An additional “plus” of this microcircuit is that it has an open-collector output, and a fairly high-voltage one – capable of withstanding voltages up to +18 V at any operating supply voltage. Thanks to this, you can assemble a converter from literally a dozen cheap and common parts (Fig. 1.6).
Rice. 1.6. Simple converter circuit
Pin 3 of the microcircuit is a regular two-state output, it is used in this circuit to support oscillation. Pin 7 is an open collector output that can withstand higher voltages, so it can be connected directly to the coil, without a transistor follower. The reference voltage input (pin 5) is used to regulate the output voltage.
Operating principle of the device
Immediately after the supply voltage is applied, the capacitor SZ is discharged, no current flows through the zener diode VD1, the voltage at the input REF of the microcircuit is equal to 2/3 of the supply voltage, and the duty cycle of the output pulses is 2 (that is, the pulse duration is equal to the duration of the pause), the capacitor SZ is charged at maximum speed . Diode VD2 is needed so that the discharged capacitor SZ does not affect the circuit (does not reduce the voltage at pin 5), resistor R2 is “just in case”, for protection.
As this capacitor charges, the zener diode VD1 begins to open slightly, and the voltage at pin 5 of the microcircuit increases. As a result, the pulse duration decreases and the pause duration increases until dynamic equilibrium occurs and the output voltage stabilizes at a certain level. The value of the output voltage depends only on the stabilization voltage of the zener diode VD1 and can be up to 15...18V - at a higher voltage the microcircuit may fail.
About details
Coil L1 is wound on a ferrite ring K7x5x2 (external diameter - 7 mm, internal - 5 mm, thickness - 2 mm), approximately 50...100 turns with wire with a diameter of 0.1 mm. You can take a larger ring, then the number of turns can be reduced, or take an industrial inductor with an inductance of hundreds of microhenries (µH).
The 555 chip can be replaced with a domestic analog K1006VI1 or with a CMOS version 7555 - it has less current consumption (the battery will “last” a little longer) and a wider range of operating voltages, but it has a weaker output (if the multimeter requires more than 10 mA, it can cannot produce such a current, especially at such a low supply voltage) and it, like all CMOS structures, “does not like” increased voltage at its output.
Device Features
The device starts working immediately after assembly, the whole setup consists of setting the output voltage by selecting a zener diode VD1, while a resistor with a resistance of 3...1 kOhm (load simulator) must be connected to the output parallel to the capacitor SZ, but not a multimeter!
It is forbidden to turn on the converter with an unsoldered zener diode - then the output voltage will be unlimited and the circuit may “kill” itself. You can also increase the operating frequency by reducing the resistance of resistor R1 or capacitor C1 (if it operates on audio frequency– a high-frequency squeak is heard). When the length of the wires from the battery is less than 10...20 cm, a power filter capacitor is not necessary, or you can place a capacitor with a capacity of 0.1 μF or more between pins 1 and 8 of the microcircuit.
Identified deficiencies
Firstly, the device contains two generators (one is the master oscillator of the ADC microcircuit - the analog-to-digital converter of the device, the second is the converter generator), operating at the same frequencies, that is, they will influence each other (frequency beat) and accuracy measurements will be seriously degraded.
Secondly, the frequency of the converter generator is constantly changing depending on the load current and battery voltage (because in the PIC circuit there is a positive feedback– there is a resistor, not a current generator), so it becomes impossible to predict and correct its influence. Specifically for a multimeter, the ideal would be one common generator for the ADC and converter with a fixed operating frequency.
Second version of the converter
The circuit of such a converter is a little more complicated and is shown in Fig. 1.7.
A generator is assembled on element DD1.1; it clocks the converter through capacitor C2, and the ADC chip through C5. Most inexpensive multimeters are based on a dual ADC
Rice. 1.7. Converter circuit With fixed operating frequency
integrating the ICL7106 or its analogues (40 pins, 3.5 digits on the display), to clock this microcircuit you just need to remove the capacitor between pins 38 and 40 (unsolder its leg from pin 38 and solder it to pin 11 of the DD1.1). Thanks to feedback through a resistor between pins 39 and 40, the microcircuit can be clocked even by very weak signals with an amplitude of a fraction of a volt, so 3-volt signals from the DD1.1 output are quite enough for its normal operation.
By the way, in this way you can increase the measurement speed by 5... 10 times - simply by increasing the clock frequency. The measurement accuracy practically does not suffer from this - it deteriorates by a maximum of 3...5 units of the least significant digit. There is no need to stabilize the operating frequency for such an ADC, so a conventional RC generator is quite sufficient for normal measurement accuracy.
A standby multivibrator is assembled on elements DDI.2 and DD1.3, the pulse duration of which can be varied from almost 0 to 50% using transistor VT2. IN original condition at its output (pin 6) there is a “logical one” (high
voltage level), and the capacitor SZ is charged through the diode VD1. After the triggering negative pulse arrives, the multivibrator is “overturned”, a “logical zero” (low voltage level) appears at its output, blocking the multivibrator through pin 2 of DDI.2 and opening transistor VT1 through the inverter on DD1.4. The circuit will be in this state until the capacitor S3 is discharged - after which the “zero” at pin 5 of DD1.3 will “throw over” the multivibrator back to the standby state (by this time C2 will have time to charge at pin 1 of DD1.1 as well. will be “1”), transistor VT1 will close, and coil L1 will discharge to capacitor C4. After the arrival of the next impulse, all of the above processes will repeat again.
Thus, the amount of energy stored in coil L1 depends only on the discharge time of the capacitor S3, that is, on how strongly the transistor VT2 is open, which helps it discharge. The higher the output voltage, the more the transistor opens; Thus, the output voltage is stabilized at a certain level, depending on the stabilization voltage of the zener diode VD3.
To charge the battery, a simple converter on an adjustable linear stabilizer DA1 is used. You only have to charge the battery, even with frequent use of the multimeter, a couple of times a year, so there is no point in installing a more complex and expensive switching stabilizer here. The stabilizer is configured for an output voltage of 4.4...4.7 V, which is reduced by the VD5 diode by 0.5...0.7 V - to standard values for a charged lithium-ion battery (3.9...4.1 V). This diode is needed to prevent the battery from being discharged through DA1 in offline mode. To charge the battery, you need to apply a voltage of 6...12V to the XS1 input and forget about it for 3...10 hours. At a high input voltage (more than 9 V), the DA1 chip gets very hot, so you need to either provide a heat sink or lower the input voltage.
As DA1, you can use 5-volt stabilizers KR142EN5A, EH5V, 7805 - but then, to suppress the “excess” voltage, VD5 must be made up of two diodes connected in series. Transistors in this circuit can be used in almost any structures p-p-p, KT315B are listed here only because the author has accumulated too many of them.
KT3102, 9014, VS547, VS817, etc. will work normally. KD521 diodes can be replaced with KD522 or 1N4148, VD1 and VD2 should be high-frequency - BAV70 or BAW56 are ideal. VD5 – any diode (not Schottky!) of medium power (KD226, 1N4001). The VD4 diode is optional - the author simply had too low-voltage zener diodes and the output voltage did not reach the minimum 8.5 V - and each additional diode in direct connection adds 0.7 V to the output voltage. The coil is the same as for the previous circuit (100...200 µH). The modification diagram for the multimeter switch is shown in Fig. 1.8.
Rice. 1.8. Electrical circuit for refining the multimeter switch
The positive terminal of the battery is connected to the central track-ring of the multimeter, but we connect this ring to the “+” of the battery. The next ring is the second contact of the switch, and it is connected to the elements of the multimeter circuit by 3...4 tracks. These traces on the opposite side of the board need to be broken and connected together, as well as with the +9V output of the converter. We connect the ring to the +3 V power bus of the converter. Thus, the multimeter is connected to the output of the converter, and we turn the power of the converter on and off with the multimeter switch. Such difficulties have to be undertaken due to the fact that the converter consumes some current (3...5 mA) even when the load is turned off, and the battery will be discharged with such current in about a week. Here we turn off the power to the converter itself; the rf battery will last for several months.
A device correctly assembled from serviceable parts does not need adjustment; sometimes you only need to adjust the voltage with resistors R7, R8 ( Charger) and zener diode VD3 (converter).
Printed circuit board options are shown in Fig. 1.9.
Rice. 1.9. PCB Options
The board has the dimensions of a standard battery and is installed in the appropriate compartment. The battery is placed under the switch - usually there is enough space there; first you need to wrap it with several layers of electrical tape or at least tape. To connect the charger connector, you need to drill a hole in the multimeter body. The pin layout of different XS1 connectors sometimes differs, so the board may need to be modified slightly. To prevent the battery and converter board from “dangling” inside the multimeter, they need to be pressed with something inside the case.
The materials of this article were published in the magazine Radioamator - 2013, No. 2The article presents simple circuit and a converter design that allows the digital multimeter to be powered from a single element of a nickel-cadmium or nickel-metal hydride battery, without requiring the installation of additional switches, and allowing the battery to be recharged while the multimeter is in use.
- firstly, it does not need to use any additional switches,
- secondly, you can recharge the battery without turning off the multimeter,
- thirdly, for its operation only one battery cell with a voltage of 1.2V is enough.
Description of the device circuit
Fundamental electrical diagram device is shown in the figure:
Actually, the voltage converter circuit is borrowed from the article by A. Kavyev “Pulse power supply with an acoustic switch for a multimeter” (Radio - 2005, No. 6) and consists of transistors VT1, VT2, transformer T1 and capacitor C1. From original scheme everything unnecessary was removed and a battery charging unit from the source was added direct current voltage 9V, consisting of a current-limiting resistor R1 and a charging indicator on elements HL1, R2.
When there is no load, the converter does not operate and consumes virtually no current from the battery. When you turn on the multimeter, the converter starts up, providing power to it. When using such a circuit in simple multimeters such as DT830, there are no problems with starting the converter. Its use in more serious multimeters that have an automatic shutdown circuit when there is no user activity is associated with certain difficulties, since the auto shutdown unit does not allow the converter to enter operating mode and turns off the device. Let's look at the solution to this problem using the DT9205A multimeter as an example. The point of the solution is to bypass the automatic shutdown unit before turning on the multimeter. To do this, it is suggested to use the “HOLD” button, since in practice it is usually not necessary. The conductors leading to the “HOLD” button should be broken, and one of the closing contacts should be connected with conductors to the terminals of the “ON/OFF” button, as shown schematically with dotted lines in the figure:
Now, before turning on the multimeter, you must first press the “HOLD” button and then “ON/OFF”. The multimeter will turn on. Then you should move the “HOLD” button to its original position. If the “HOLD” button is left pressed, then automatic shutdown the multimeter will not work, which in some situations can even be useful.
Construction and details
All elements of the circuit are assembled on a printed circuit board made of one-sided foil-coated fiberglass, the size of a multimeter battery compartment. The printed circuit board drawing and the arrangement of elements are shown in the figure:
For ease of repetition, the drawing is shown from the foil side. It is very simple and designed so that the conductors can be cut with a cutter. To connect the battery, two L-shaped brass plates are soldered onto the board, one of which (going to the battery negative) is equipped with a spring to ensure reliable contact. To fix the battery on the board, it is convenient to use a bracket made from a 5 ml plastic syringe and glued to the board with hot glue.
Transformer T1 is wound on a ring magnetic core K10x6x4.5 made of 2000NM ferrite, the edges of which are blunted with a file. Additionally, the magnetic circuit is insulated with thin fluoroplastic tape. Both windings of the transformer are wound into two wires, then connecting the end of one half-winding to the beginning of the other. The primary winding contains 2x10 turns, and the secondary winding contains 2x70 turns of PEL-0.17 wire, and the secondary winding is wound first. The windings must be correctly phased according to the designation shown in the diagram. The transformer is glued to the board with hot-melt adhesive, and the terminals are connected according to the drawing.
Transistors VT1, VT2 are selected with close current transfer coefficient values. Instead of KT209, you can use other silicon direct conduction transistors such as KT203, KT208, KT501, etc.
There are no special requirements for other elements. The charger and multimeter power connectors are connected to the board with flexible conductors.
Installation and commissioning
Setting up a voltage converter comes down to selecting the number of turns of the primary winding of the transformer so that with an input voltage of 0.9V (that is, the minimum allowable for an alkaline element), its output is no more than 7.5V. This is necessary so that the multimeter promptly displays the low voltage indicator and the user is informed about the need to recharge the battery. Then you need to make sure that with a nominal battery voltage of 1.2V, the output of the converter produces a voltage of about 9V and, if necessary, re-adjust the converter.
Then, by selecting resistor R1, you need to adjust the battery charging current, which, when the multimeter is turned off, should be no more than 1/10 of the battery capacity. For example, the author used an element with a capacity of 800 mA hour, so the charging current was chosen to be 80 mA. Although the author used a 9V power source to charge the battery, it is convenient for this purpose, for example, to use a mobile phone charger with an output voltage of 5V.
The design in the photo contains a battery cell removed from an old Chinese electric razor. This “Krona” has been successfully used in my work multimeter for more than six years.
I used the DT9202A multimeter for a long time, once again the “crown” went bad, and buying a new one was a waste. I decided to buy a new multimeter. As I chose Fluke 15B+ as a replacement. Well, I threw the old multimeter into a box with junk. He lay therea couple of years until I once again came across it.
It seems like a shame to throw it away, and you can’t use it, and you can’t even bother to take it apart for spare parts, because the multimeter served me well for several years. It was decidedmake him a new power system. I wanted to approach the matter thoroughly,and don’t throw out this kind of hack:
I wanted to power the multimeter from a Li-ion battery, but a number of problems arose:
- The supply voltage of the multimeter is 9 volts, a boost converter is needed;
- The standard auto-shutdown system will stop working, you need to install your own;
- It is necessary to protect the battery from overdischarge;
- It is necessary to have a battery charging controller with an indication on board.
In addition, I wanted to assemble a structure from cheap and available parts, and most importantly - without using microcontrollers. Solve this simplest task on a microcontroller is somehow boring and not interesting. Yes, and newbie radio amateurs will not mind “pumping up” their multimeters using radio components from the trash heap;-)
After several evenings spent with a soldering iron and development board, such a monster was born:
Main characteristics:
- Output voltage 9 V
- Supply voltage 3.6...4.2 V
- Discharge protection voltage 3.6 V
- Battery charging current 250 mA
- Auto-off timer 5 min
And this is what the assembled device looks like:
On one side of the board there are SMD components, and on the other side there is a battery from an old mobile phone. Initially I wanted to putNokia BL-5C battery, but it turned out to be 2 mm longer than the compartment and did not fit in size.
I had to install a small Nokia BL-4B battery. I secured it with double-sided tape.
To implement a new power system in a multimeter, you must:
- Convert a standard switch into a tact button by removing the fixing element;
- Continue required holes, place the board in the case;
- Connect the power board to the multimeter board.
So let's get started.
1. Button modification
Since the standard power button has a lock, we had to modify it a little. To do this, you need to open the button body, remove the fixing element from there, and reassemble everything as it was ;-)
Now the button is not fixed when pressed, and works like a regular tact button.
2. Drilling holes, placing the board in the case
The power board contains a battery charging controller. Recharging is carried out via the USB-B connector, which was very conveniently placed in the multimeter body.
The height of the walls in the battery compartment had to be reduced so that they would not interfere with the board.
In the upper part of the case, holes were cut out for a USB connector and for an LED displaying the charging process.
The LED lights up during charging and goes off when charging is complete.
The board is fixed in the multimeter body without a single bolt. A step in the case prevents pushing through the USB socket. Getting the socket out is hampered by the shape of the board, which follows the inside of the case. The walls of the battery compartment prevent you from moving the board left and right. The battery prevents the board from tilting upward; the downward tilt is blocked by the wall of the battery compartment. The board sits inside tightly, like a glove.
3. Connecting the power board to the multimeter
Below is the standard auto-shutdown diagram of the multimeter. Cuts off power approximately after 10 minutes of work.
When using the multimeter together with my power board, staffing diagram needs a little modernization:
Since my board uses a DC-DC converter to power the multimeter, the auto-off timer should de-energize the power to the converter. The original auto-shutdown timer is located in the multimeter itself, that is, after the converter. When auto shutdown is triggered, the original circuit will de-energize the multimeter, and the converter will continue to operate, discharging the battery. Therefore, this option is not suitable. I had to make my own auto-shutdown system and bypass the standard one by supplying power directly to the measuring part of the circuit (V+ circuit). It is also necessary to remove the standard crown block and capacitor C19.
Place a jumper on resistor R53.
We connect the power board to the multimeter using three wires:
- MULTIMETER_9V
- MULTIMETER_ON
The implementation of the new power system was painless. I didn’t even have to cut a single track on the multimeter board. The device does not require configuration and starts working immediately after assembly.
Description of the circuit operation.
On an operational amplifier DA2.1 assembly assembled protection against battery discharge. The cut-off voltage is set denominations divider R4R7. AsThe reference voltage source uses a linear chipstabilizer DA1 (LM1117).The stabilizer is loaded with resistor R3, since it cannot operate without load.
On an operational amplifierDA2.2 contains an auto-shutdown timer. When the power is turned on, capacitor C3 is charged, then it is gradually discharged through resistor R10. The timer operation time is set by the C3R10 ratings. When the timer is triggered, transistor VT3 opens, causing the discharge protection circuit to operate.
Operational amplifier DA2 (LM358) works as a comparator, so it can be replaced with a comparator chip LM393.
The DA4 chip (MC34063) contains a pulse boost converter that produces a voltage of 9 volts to power the multimeter.
The DA3 (TP4056) chip contains an automatic battery charging unit. LED during chargingHL1 lights up and goes out when charging is complete.
There is a shutdown button on the diagram, but I didn’t use it because... The timer is enough. The power is turned off automatically by a timer, the time is set by the C3R10 ratings. Those who wish can use the “HOLD” button to turn off the power, but it’s of no use anyway.
At the end of the article you can download Excel file with all the necessary calculations.
Finally, I am attaching a video of the multimeter working with new system nutrition.
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| DA1 | Linear regulator | LM1117-N | 1 | LM1117-1.2 | To notepad | |
| DA2 | Operational amplifier | LM358 | 1 | SOIC-8 | To notepad | |
| DA3 | Charge controller | TP4056 | 1 | SOIC-8 | To notepad | |
| DA4 | DC/DC pulse converter | MC34063A | 1 | SOIC-8 | To notepad | |
| VT1 | MOSFET transistor | IRF9358 | 1 | SOIC-8 | To notepad | |
| VT2, VT3 | Bipolar transistor | BC847 | 2 | SOT-23 | To notepad | |
| VD1, VD2 | Schottky diode | MBR0540T1G | 2 | SOD-123 | To notepad | |
| R1, R6, R7 | Resistor | 10 kOhm | 3 | 0805 | To notepad | |
| R2, R8 | Resistor | 100 Ohm | 2 | 0805 | To notepad | |
| R3 | Resistor | 300 Ohm | 1 | 0805 | To notepad | |
| R4 | Resistor | 20 kOhm | 1 | 0805 | To notepad | |
| R5 | Resistor | 51 kOhm | 1 | 0805 | To notepad | |
| R9 | Resistor | 30 kOhm | 1 | 0805 | To notepad | |
| R10 | Resistor | 3.3 MOhm | 1 | 0805 | To notepad | |
| R11 | Resistor | 5.1 kOhm | 1 | 0805 | To notepad | |
| R12, R19 | Resistor | 1 kOhm | 2 | 0805 | To notepad | |
| R13 | Resistor | 180 Ohm | 1 | 0805 | To notepad | |
| R14, R15 | Resistor | 1 ohm | 2 | 0805 | To notepad | |
| R16 | Resistor | 0 ohm | 1 | 0805 | To notepad | |
| R17 | Resistor | 56 kOhm | 1 | 0805 | To notepad | |
| R18 | Resistor |
Stabilized voltage converter
In my AM-1006 multimeter from Aktakom, three 6F22 batteries, similar in size and parameters to the domestic Krona, were exhausted in one year. Among them was one from Duracell, famous for its capacity and durability. And now, when the next battery " ran out" and didn’t have a new one at hand, I came across an article about powering a multimeter from two AA batteries. I had a Li-Ion battery from mobile phone,Sony-Erickson-T-290m" and I began to attach it to my multimeter. Fortunately, the battery fits well into the niche under the cover in the upper part of the device body (Fig. 1).
For reliable fastening, it was necessary to drill only two holes with a diameter of 3 mm to hold it in this niche, similar to how it is done in a mobile phone.
Since the battery was almost ideally placed in the multimeter, all that remained was to assemble a stabilized converter with an output voltage of 8...9 V and dimensions allowing it to be placed in the battery compartment. Converter circuit shown in Fig. 2
Converter circuit
It is assembled on two transistors using an asymmetrical multivibrator circuit. Inductor L1 is used as a load for transistor VT2. Voltage pulses on the collector of this transistor with an amplitude of 15 V and a repetition frequency of 250 kHz are rectified by diode VD1, and the rectified voltage is smoothed by capacitor SZ. then it goes to the parametric stabilizer R5VD2. A voltage of 8.2 V is supplied to block X1 (removed from a failed battery of size 6F22). The converter provides the current consumed by the multimeter (up to 4 mA). To turn off the power to the converter, it was necessary to install switch SA1 (any small-sized slide switch) in the lower left corner of the multimeter (Fig. 3).
There is enough space for him there. The presence of this switch eliminated the need to use the multimeter's flip switch when turning it on or off.
In the author's version, the board was made of one-sided foil-coated fiberglass. It is cut to the size of the 6F22 battery, and the foil is divided into rectangular pads using a cutter, to which the leads of the parts are soldered. Designed for repeatability by radio amateurs printed circuit board, a drawing of which is shown in Fig. 4.
Resistors used are MYAT, S2-23, an oxide capacitor is imported, the rest are imported ceramic, a choke is DPM-0.1, a zener diode is any low-power one with a stabilization voltage of 6.5...9 V. since the multimeter remains operational when reduced supply voltage up to 6 V. Plug/socket pairs XP1, XS1 and XP2, XS2 can be any, but to eliminate the possibility of incorrect polarity connection, they must be of different diameters.
When charging the battery, plugs XP1 and XP2 are disconnected from the converter and connected to the charger. The battery I used contains a charge/discharge controller and can be charged by connecting it to a charger or power supply with an output voltage of 5 V. Most cell phone batteries contain such controllers. And if the battery is without it, you will have to make a charger. The contacts of block XI are soldered to two rigid L-shaped holders made of wire from a metal paper clip. The converter is placed in the battery compartment of the multimeter (Fig. 5).
I used the DT9205 tester for a long time and it suited me quite well. Big numbers, rotating screen, high-quality range switching, auto power off, HOLD, separate power button. In terms of its parameters, it is also not bad (not without its shortcomings, of course). And then at one fine moment I burned it. I decided to measure the voltage on a tiny converter for powering fluorescent lamps for modding computer cases. That was enough. I did not repair this tester; the ADC on the board is made in the form of a “drop” with 46 legs and it is impossible to simply solder another chip. And at a price of about $10, the time spent on repairs will not pay off. Therefore, it was decided to buy a new tester of the same model.
An inexpensive one just turned up on Ali. The purchase was made, the tester was received after the allotted time.
I immediately realized that this tester is not exactly the 9205 that I had: the screen does not rotate, the range shift knob does not turn so pleasantly, the plastic is unpleasant and tiny. The inscriptions (silk-screen printing) are of poor quality and have been worn out in some places (not printed). The black protective case is hard. The tester immediately impressed me as a low-quality product. One of the advantages is the presence of a dial indicator light (LED), which is generally not very necessary; the sound alarm is more important. The tester immediately did not behave very well: the sound signal sometimes worked, sometimes it didn’t. The ohmmeter in some ranges did not set to zero when the probes were shorted.
I open it: everything inside looks completely different from how it was in my old DT-9205.
It was like.
Inside the nose is a small board marked EX9305-7.
There are some capacitors on the long leads, the soldering is sloppy, everything is smeared with flux.
I washed off the flux and cleaned the tracks under the range switching contacts with a wash.
I replaced the wires with a piezoceramic emitter. Everything seemed to work.
I don’t like the multimeter, so I went to the market and bought an excellent DT-9208 (by the way, it turned out even cheaper, because I took the multimeter from an old delivery with a shriveled “crown” for half its price).
But something needs to be done about this terrible “Chinese”. More precisely, I have where to use it, but it’s a pity to waste expensive 9-volt Krona batteries on it, which it “eats” in a month with intensive use. You need to convert it to Li-Ion batteries.
There are a lot of different options, but almost all of them use a boost DC-DC converter, which is bad for the accuracy of measurements and interference with the circuit. I immediately thought: why not use 2 batteries connected in series? Their voltage will be exactly within the limits required for the multimeter to work. And I won’t need a protection-balancing board either, because when the voltage drops, the multimeter will inform me in advance by showing a low battery voltage icon on the screen, and I plan to charge series-connected batteries through the SkyRC E3 charger-balancing device. You will only need to remove the balancing cord from the body. Everything turns out as simple as shelling pears, you don’t need to build anything in, you don’t need to redo the power off button or the auto-shutdown circuit.
Since the board in the DT9205 case is small, there is a large empty space under the indicator.
There I placed two 18650 batteries welded in series from a laptop battery. Their residual capacity is about 2Ah. I think they should last a very long time.
The balancing cable was led into the battery compartment, where the “crown” used to be. I close the back cover of the multimeter.
I put it on charge.
Charging is complete.
I put a piece of paper with the date in the battery compartment (I still wonder how long the multimeter will work before it needs charging?).
I hide the balancing cord in the battery compartment and close it.
The tester was turned on successfully.
Everything works as it should.
For conversion to lithium. I don’t want a boost DC-DC in a multimeter, I don’t want to redo the switching circuit and the auto-shutdown circuit of the multimeter, I don’t want to plug a separate charge board into it. I really liked this modification option for its simplicity, reliability and quality of work. I used 18650 batteries because I didn't need to buy them, I had them lying around. This option is good for me and for this particular multimeter design. Not every multimeter has free space to squeeze 2 18650 batteries into it, and not everyone has charger-balancing devices (although they are very inexpensive). Alternatively, if there is little space and no desire to mess with DC-DC converter, you can put two batteries from a mobile phone, they, too, after charging, will work much longer than the “crown”.
P.S. More than 2 years have passed. The multimeter from this review has long been thrown into the trash because... I'm tired of it. During this time, I converted 4 of my multimeters in this way, which worked from Krona. All work perfectly and have not yet been discharged so that the tester shows low voltage batteries. It's been a lot more than a year now. The SkyRC E3 charger also showed its best performance.
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