Many people probably have a problem with charging a Li-Ion battery without a controller; I had this situation. I received a dead laptop, and there were 4 SANYO UR18650A cans in the battery that were alive.
I decided to replace the LED flashlight with three AAA batteries. The question arose about charging them.
After digging around on the Internet I found a bunch of diagrams, but details are a little tight in our city.
I tried charging from a cell phone charger, the problem is in charge control, you need to constantly monitor the heating, it just starts to heat up, you need to disconnect from charging, otherwise the battery will be damaged in the best case, otherwise you can start a fire.
I decided to do it myself. I bought a bed for the battery in the store. I bought a charger at a flea market. To make it easier to track the end of the charge, it is advisable to find one with a two-color LED that signals the end of the charge. It switches from red to green when charging is complete.
But you can also use a regular one. The charger can be replaced with a USB cord and charged from a computer or charger with a USB output.
My charger is only for batteries without a controller. I took the controller from an old battery cell phone. It ensures that the battery is not overcharged above a voltage of 4.2 V, or discharged below 2...3 V. Also, the protection circuit saves from short circuits by disconnecting the bank itself from the consumer at the moment of a short circuit.
It contains the DW01 chip and an assembly of two SM8502A MOSFET transistors (M1, M2). There are also other markings, but the circuits are similar to this one and work similarly.
Cell phone battery charge controller.
Controller circuit.
Another controller circuit.
The main thing is not to confuse the polarity of soldering the controller to the bed and the controller to the charger. The controller board has “+” and “-” contacts.
It is advisable to make a clearly visible indicator in the bed near the positive contact, using red paint or self-adhesive film, to avoid polarity reversal.
I put everything together and this is what happened.
Charges great. When the voltage reaches 4.2 volts, the controller disconnects the battery from charging and the LED switches from red to green. Charging is complete. You can charge other Li-Ion batteries, just use a different bed. Good luck to all.
The industry has been making screwdrivers for a long time, and many people have older models with nickel-cadmium and nickel-metal hydride batteries. Converting a screwdriver to lithium will improve the performance characteristics of the device without buying a new tool. Now many companies offer services for converting screwdriver batteries, but you can do it yourself.
Benefits of lithium-ion batteries
Nickel-cadmium batteries have a low price, withstand many charging cycles, and are not afraid of low temperatures. But the battery capacity will decrease if you charge it before it is completely discharged (memory effect).
Lithium-ion batteries have the following advantages:
- high capacity, which will ensure longer operating time of the screwdriver;
- smaller size and weight;
- Retains charge well when not in use.
But a lithium battery for a screwdriver does not withstand full discharge well, so factory tools on such batteries are equipped with additional circuit boards that protect the battery from overheating, short circuit, and overcharging to avoid explosion or complete discharge. When the microcircuit is installed directly into the battery, the circuit opens if the unused battery is located separately from the tool.
Difficulties in reworking
IN Li-Ion batteries There are objective disadvantages, such as poor performance at low temperatures. In addition, when converting a screwdriver to 18650 lithium batteries, you may encounter a number of difficulties:
- The 18650 standard means that the diameter of one battery cell is 18 mm with a length of 65 mm. These dimensions do not coincide with the dimensions of the nickel-cadmium or nickel-metal hydride elements previously installed in the screwdriver. Replacing batteries will require placing them in a standard battery case, plus installing a protective microcircuit and connecting wires;
- The output voltage for lithium cells is 3.6 V, and for nickel-cadmium cells - 1.2 V. Let's say the nominal voltage old battery– 12 V. Such a voltage cannot be provided when connecting Li-Ion elements in series. The scope of voltage fluctuations during charge-discharge cycles of an ion battery also changes. Accordingly, converted batteries may not be compatible with the screwdriver;
- Ion batteries differ in the specifics of their operation. They do not withstand overcharge voltages greater than 4.2 V and discharge voltages less than 2.7 V until they fail. Therefore, when the battery is rebuilt, a protective board must be installed in the screwdriver;
- The existing charger may not be able to be used for a screwdriver with a Li-Ion battery. You will also need to remake it or purchase another one.
Important! If a drill or screwdriver is cheap and not of very high quality, then it is better not to remodel it. This may cost more than the cost of the tool itself.
Battery selection
Screwdrivers often use 12 V batteries. Factors to consider when choosing a Li-Ion battery for a screwdriver:
- Such instruments use elements with high discharge current values;
- In many cases, the capacity of the element is inversely related to the discharge current, so you cannot select it based on capacity alone. The main indicator is current. The value of the operating current of the screwdriver can be found in the tool passport. Usually it is from 15 to 30-40 A;
- When replacing a screwdriver battery with a Li-Ion 18650, it is not recommended to use cells with different capacity values;
- Sometimes there are tips to use a lithium battery from an old laptop. This is absolutely unacceptable. They are designed for a much lower discharge current and have unsuitable specifications;
- The number of elements is calculated based on the approximate ratio - 1 Li-Ion to 3 Ni-Cd. For a 12-volt battery, you will need to replace 10 old cans with 3 new ones. The voltage level will be slightly reduced, but if you install 4 elements, then increased voltage will shorten the life of the electric motor.
Important! Before assembly, it is necessary to fully charge all elements for equalization.
Disassembling the battery case
The case is often assembled using self-tapping screws, other options are assembled using latches or glue. The glued block is the most difficult to disassemble; you have to use a special hammer with a plastic head so as not to damage parts of the body. Everything from inside is removed. You can reuse only the contact plates or the entire terminal assembly for connecting to a tool or charger.
Battery Cell Connection
CompoundLi– Ionbatteries for screwdriverperformed in several ways:
- The use of special cassettes. The method is fast, but the contacts have a high transition resistance and can quickly be destroyed by relatively high currents;
- Soldering. A method suitable for those who know how to solder, since you need to have certain skills. Soldering must be done quickly, because the solder cools quickly, and prolonged heating can damage the battery;
- Spot welding. Is the preferred method. Not everyone has a welding machine; such services can be provided by specialists.
Important! The elements must be connected in series, then the battery voltage is added, but the capacity does not change.
At the second stage, the wires are soldered to the contacts of the assembled battery and to the protective board according to the connection diagram. Wires with a cross-sectional area of 1.5 mm² are soldered to the contacts of the battery itself for power circuits. For other circuits, you can take thinner wires - 0.75 mm²;
A piece of heat shrink tubing is then placed over the battery, but this is not necessary. You can also put heat shrink on the protective microcircuit to isolate it from contact with the batteries, otherwise sharp solder protrusions can damage the shell of the element and cause a short circuit.
Further battery replacement consists of the following steps:
- The disassembled parts of the body are well cleaned;
- Since the dimensions of the new battery cells will be smaller, they need to be securely fixed: glued to the inner wall of the case with Moment glue or sealant;
- The positive and negative wires are soldered to the old terminal block, it is placed in its original place in the housing and fixed. The protective board is laid, the parts of the battery pack are connected. If they were previously glued, then “Moment” is used again.
The lithium-ion battery of the screwdriver will not be able to function properly without the BMS protection board. The copies sold have different parameters. The BMS 3S marking assumes, for example, that the board is designed for 3 elements.
What you need to pay attention to in order to choose a suitable microcircuit:
- The presence of balancing to ensure uniform charge of the elements. If it is present, the description of the technical data should include the value of the balancing current;
- The maximum value of operating current that can be withstood for a long time. On average, you need to focus on 20-30 A. But this depends on the power of the screwdriver. Low-power ones need 20 A, high-power ones – from 30 A;
- Voltage at which the batteries are switched off when overcharging (about 4.3 V);
- The voltage at which the screwdriver turns off. This value must be selected based on the technical parameters of the battery cell (minimum voltage - about 2.6 V);
- Overload protection current;
- Resistance of transistor elements (select the minimum value).
Important! The magnitude of the trip current during overload is not very important. This value is adjusted to the operating load current. In case of short-term overloads, even if the tool has turned off, you must release the start button, and then you can continue to work.
Whether the controller has an autostart function can be determined by the presence of the “Automatic recovery” entry in the technical data. If there is no such function, then in order to start the screwdriver again after the protection has tripped, you will need to remove the battery and connect it to the charger.
Charger
The lithium-ion battery of the screwdriver cannot be charged by connecting it to a conventional power supply. A charger is used for this. The power supply simply produces a stable charge voltage within specified limits. And in the charger, the determining parameter is the charge current, which affects the voltage level. Its meaning is limited. The charger circuit contains nodes responsible for stopping the charging process and other protective functions, for example, shutdown in case of incorrect polarity.
The simplest charger is a power supply with a resistance included in the circuit to reduce the charging current. Sometimes they also connect a timer that fires after a set time period has passed. All of these options are not conducive to long battery life.
Charging methodsLI Ionbatteries for screwdriver:
- Using a factory charger. Often it is also suitable for charging new battery;
- Reworking the charger circuit, with the installation of additional circuit elements;
- Purchase of a ready-made memory. A good option– IMax.
Let's say there is an old Makita DC9710 charger for charging a 12 V Ni-Cd battery, which has an indication in the form of a green LED signaling the end of the process. The presence of a BMS board will allow you to stop the charge when the specified voltage limits per element are reached. The green LED will not light up, but the red one will simply go out. The charge is complete.
The Makita DC1414 T charger is designed to charge a wide range of 7.2-14.4 V batteries. In it, when the protective shutdown is triggered at the end of the charge, the indication will not work correctly. The red and green lights flash, which also signals the end of the charge.
The cost of replacing screwdriver batteries with lithium-ion ones depends on the power of the tool, the need to buy a charger, etc. But if the drill/driver is in good functional condition and the charger does not require major alteration or replacement, then for a couple of thousand rubles you can get an improved power tool with increased battery life.
Video
For several decades, screwdrivers have been used for various jobs. These devices are powered by nickel or cadmium batteries. But progress does not stand still; scientists have found a replacement for such outdated batteries. They were replaced by lithium analogues. To use such a battery, the screwdriver must be modified. A lithium battery will improve the performance of an old tool. Moreover, it is possible to carry out such alterations independently, without resorting to the services of special companies.
The lithium battery of the screwdriver has a number of advantages that were absent in cadmium analogues.
The energy density of a Li ion screwdriver battery is much higher. The battery with lithium banks is lightweight, and the voltage of 12 volts, as well as the battery capacity, remains unchanged. Lithium batteries charge faster than ion batteries. Safe charging lasts approximately 60 minutes.
Lithium-ion batteries do not have a “memory effect”. In other words, they do not need to be completely discharged to be charged. Among the positive qualities of a lithium battery, there are also a number of disadvantages that need to be taken into account:
- Charger lithium batteries should not be higher than 4.2 volts, and the discharge should not be higher than 2.7 volts. But this is theoretical data. In real life, the interval gets even worse. If the set values are not observed, the battery will simply stop functioning. To avoid this situation, after converting the screwdriver to lithium, you need to install a special discharge controller in the screwdriver, as well as its charging.
- One Li ion has a voltage of 3.63.7 V. For a nickel battery it is no more than 1.2 volts. In other words, converting a screwdriver to li ion material causes problems associated with the assembly process of the battery, whose nominal voltage is 12 volts. Three lithium banks connected in series give a voltage of 11.1 volts, four 14.8 V. The charge voltage limits will change. In other words, reworking a battery for a screwdriver is associated with solving the problem of compatibility of the new battery with the tool.
- To remake the cadmium battery of a screwdriver, craftsmen use 18650 lithium cans. Their dimensions differ from nickel cans. Remaking the battery for a screwdriver also requires the installation of a controller, which will require additional space.
- After the modification, the charger for nickel batteries will have to be modified, or use a universal charger.
- Sub-zero temperatures have a negative impact on work ion batteries. Therefore, such a converted screwdriver cannot always be used outdoors.
- The cost of lithium batteries is much higher than their cadmium counterparts.
Algorithm for converting a battery to a lithium-ion battery
How to modify a screwdriver to get the best performance? To do this, it is necessary to strictly follow a certain technological sequence.
Selecting the right battery
The batteries are connected in series, so the voltage rating of each element is added to the next one. That is, to get 14.4 volts, you will need four elements with a voltage of 3.3 V.
To convert a cordless screwdriver, you need to buy miniature batteries only from a well-known manufacturer. For example, LiFePO4 batteries manufactured by Sistem A123. The cell capacity reaches 2,300 mAh. This value is sufficient for efficient work electric tool. Cheap batteries made in China will not have much effect. They will quickly fail.
When choosing a battery for conversion, you need to have copper strips on the terminals. Soldering such elements is much easier.
Selection of tools and materials
Soldering technology is distinguished by its specifics. The temperature of the soldering iron tip is constantly high. If the battery is exposed to such heat for a long time, it will quickly deteriorate. Therefore, the heating of the soldering iron should be minimal.
For this to happen, it is necessary to replace ordinary rosin with soldering acid. It can be purchased at a radio parts store. For such a process, you will also have to purchase a soldering iron with enough power to melt solder in the shortest possible time. The most suitable would be a household soldering iron with a power of 65 watts. At 100 watts the battery will overheat all the time.
Soldering work requires a lot of experience. For example, a 40-watt soldering iron will take a long time to heat up; you can simply “overdo it.” To start converting ion batteries, you need to purchase the following parts:
- 18650 battery.
- BMS board CF-4S30A-A/
- Wires, cross section 2.5 sq. mm.
- Soldering iron.
- Old battery housing.
A few words about the BMS board
It is designed to control the charge or discharge of the battery. The CF-4S30A-A is designed for four banks of 18650 batteries, giving a discharge current of 30A. The board is equipped with a special “balancer”. It performs charge control functions for each element separately. This completely eliminates the possibility of uneven charging. For the board to function properly, the batteries in the assembly must have the same capacity. It is desirable that they be taken from the same block.
Industry produces a large number of BMS boards, differing in their technological characteristics. For converting a screwdriver battery, a board operating at a current value of less than 30A is not very suitable. It will constantly turn on the protection mode.
To restore operation, some boards require a short-term supply of charging current. To do this, you will have to remove the battery from the case and connect the charger to it again. The CF-4S30A-A board does not have this drawback. It is enough to release the trigger of the screwdriver if there is no current causing short circuit, the board will turn on automatically.
The converted battery on this board can be charged with a universal charger. The latest models from the Interskol company are equipped with multifunctional chargers.
Lithium-ion battery installation
Of course, any installation requires preliminary preparation. It includes several very important points. Before you start soldering parts, you need to determine how the battery mounting compartment will be arranged. All the necessary elements should easily fit in it.
The new lithium batteries are then held together with tape. Since the contacts oxidize over time, they are cleaned with fine-grained sandpaper before soldering.
Nuances of the soldering process
First, the contact part of the battery is thoroughly degreased. Tinning is then carried out by heating the applied solder. POS-40 solder is most suitable for tinning.
The contact of the soldering iron with the battery contact should not exceed 2 seconds. The process of soldering the battery plus requires special attention. The most suitable are jumpers made of copper wires with a cross-section of more than 2.5 mm. sq. All wires are covered with a cambric, which acts as a good insulator.
The connection of mini-batteries must be carried out with special jumpers according to the developed diagram. Jumpers can be metal strips or thin wires.
On final stage The wires are connected to the compartment terminals intended for the battery. If installation of the prefabricated block is difficult, it is necessary to remove the stiffeners. Since they are made of plastic, they are easy to bite with ordinary side cutters.
Contact wiring diagram
To connect to the charger you need to select connectors that match specific model. Soldering of connecting cables is carried out according to the electrical diagram:
Connectors for connecting to the charger are selected depending on its model. Both connecting cables are soldered according to the diagram.
- “+” – 5 and 9.
- “–” – 1 and 6.
- Balancing contacts (ascending) – 2, 7, 3, 8 and 4.
Of course, installing lithium-ion batteries has big number positive qualities:
- Lack of "memory".
- Minimum self-charging.
- You can operate the tool at sub-zero temperatures.
- Long service life (8 years).
However, these batteries are highly sensitive to technological process charging. The voltage should always be minimum values, otherwise the Li-ion battery will quickly become unusable. To fulfill such conditions, you need another memory device, the cost of which is an order of magnitude higher. The screwdriver's native charger will not be able to charge the lithium-ion battery.
It is impossible to say unequivocally which battery is better for a screwdriver. Their service life depends on careful handling and strict adherence to the instructions supplied by the manufacturer.
Popular models
Today, batteries are produced by many manufacturers. Among such a large assortment of lithium-ion systems, the most popular are: “Bosh” 10.8, with technical characteristics:
- Capacity – 1.3 A/hour.
- Voltage – 10.8 V.
- Dimensions -110 x 54 x 52mm.
- Warranty - 1 year.
- Power – average.
If we talk about nickel-cadmium batteries, the most popular brands remain:
- "Bort".
- Hitachi.
Russian batteries are designed for low voltage; they differ from imported models only in price. They are much cheaper, but at the same time they are not inferior in their technical indicators. The most famous models are:
- "Kraton".
- "ZAKB".
Conclusion
Lithium batteries have always been considered the most technologically advanced devices. But a tool with such batteries costs much more. You can, of course, remake your device and get rid of the cadmium batteries. However, this will cause other problems. Therefore, everyone makes the decision to convert a screwdriver to lithium himself, depending on the circumstances.
Interesting videos about converting a screwdriver battery
What will be discussed in this article will help many people understand nutrition. homemade devices autonomous type. It provides a method according to which you can obtain lithium-ion batteries of any size. We know from physics textbooks that a simple battery is a device consisting of copper-zinc plates, between which there is an electrolytic solution. Such a device was created by Volt (although the issue is controversial; Luigi Galvani was the first to discover the effect, but he could not give a logical explanation for this phenomenon).
More than 200 years have passed since then, we now live in the age of digital technology, but the battery still remains an indispensable source of energy, without which not a single autonomous device can operate. Modern lithium batteries are widely used in modern technology, there are many reasons for this - a light weight, long service life, high capacity and many other parameters make batteries indispensable in portable devices.
But over time and lithium ion battery falls into disrepair. The other day the same thing happened to my phone battery. The battery is from a licensed manufacturer, so it lasted a very long time and could still serve faithfully if it weren’t for my bad idea to puncture it. The fact is that over time the battery swelled, but continued to work with a bang, so it was decided to pierce it. After a small operation, the battery was no longer what it was before, a sharp decrease in capacity in just a week.
It was replaced by another battery, but it’s a pity to throw it away (and it’s not necessary, it’s harmful to the environment!), so what to do with it? It was decided to create a new battery based on the old one. Before work, I want to warn you - some lithium compounds are toxic, so it is advisable to use gloves and do the work in the fresh air. Well, as always, I violate all safety rules; without any gloves, the battery was disassembled right in the living room. As always, the peculiar smell of lithium power supplies cannot be confused with anything. To cut the aluminum case, a regular assembly knife and pliers were used.
After a couple of minutes the aluminum capsule was removed and it was time to move on.
This is where the dirtiest work begins, you need to disassemble the battery. Lithium batteries, like any other voltage source, consist of positively and negatively charged plates, with an insulation layer between them. Now we take the paste from the gel pen and, as it were, “wind” it onto the paste.
Extreme care must be taken not to short out the plates. During the process of winding the plates, you can observe heat generation, do not be alarmed, this is how it should be. Next, you should wrap the workpiece with tape, but you need to clean the plates in advance.
Solder the contact wires to the cleaned areas. You can simply take two copper wires (stranded) and simply glue them to the contacts using the same tape.
One of the contacts was soldered to the body, the other was brought out. The body should be sealed; for this I used universal “moment” glue. Immediately after creating such a battery, we measure the voltage, it lies in the range of 2.2-2.8 volts, in the case it is already 2.8-3.3 volts. The next morning the voltage is already around 3.6-3.65 volts.
Lithium batteries are afraid of subzero temperatures; at temperatures below zero, the lithium-ion battery does not charge at all.
Nowadays lithium batteries are gaining more and more popularity. Especially finger ones, like 18650 , at 3.7 V 3000 mA. I have no doubt that in another 3-5 years they will completely replace nickel-cadmium. True, the question about their charging remains open. If everything is clear with old batteries - collect them in a battery and through a resistor to any suitable power supply, then this trick does not work here. But how then can you charge several pieces at once without using expensive branded balancing chargers?
Theory
For series connection of batteries, usually to positive electrical diagram connect the positive terminal of the first battery in series connection. The positive terminal of the second battery is connected to its negative terminal, etc. The negative terminal of the last battery is connected to the negative terminal of the unit. Resulting from a serial connection accumulator battery has the same capacity as a single battery, and the voltage of such a battery is equal to the sum of the voltages of the batteries included in it. This means that if the batteries have the same voltage, then the battery voltage is equal to the voltage of one battery multiplied by the number of batteries in the battery.
The energy accumulated in the battery is equal to the sum of the energies of the individual batteries (the product of the energies of the individual batteries, if the batteries are the same), regardless of whether the batteries are connected in parallel or in series.
Lithium-ion batteries cannot simply be connected to a power supply unit - the charging currents on each element (bank) must be equalized. Balancing is carried out when charging the battery, when there is a lot of energy and it can not be saved much, and therefore, without any significant losses, you can use the passive dissipation of “excess” electricity.
Nickel-cadmium batteries do not require additional systems, since each link, when its maximum charge voltage is reached, stops receiving energy. Signs of a fully charged Ni-Cd are an increase in voltage to certain value, and then it drops by several tens of millivolts, and the temperature rises - so that the excess energy immediately turns into heat.
The opposite is true for lithium batteries. Discharge to low voltages causes degradation of chemistry and irreversible damage to the element, with increasing internal resistance. In general, they are not protected from overcharging, and you can waste a lot of extra energy, thereby dramatically reducing their service life.
If we connect several lithium cells in a row and feed them through clamps at both ends of the block, then we cannot control the charge of individual cells. It is enough that one of them will have a slightly higher resistance or a slightly lower capacitance, and this link will reach a charge voltage of 4.2 V much faster, while the rest will still have 4.1 V. And when the voltage of the entire package reaches charge voltage, it may be that these weak links are charged to 4.3 Volts or even more. With each such cycle, the parameters will deteriorate. In addition, Li-Ion is unstable and, if overloaded, can reach high temperatures and, consequently, explode.
Most often, a device called a “balancer” is installed at the output of the charging voltage source. Simplest type The balancer is a voltage limiter. It is a comparator that compares the voltage on a Li-Ion bank with a threshold value of 4.20 V. Upon reaching this value, a powerful transistor switch is opened, connected in parallel to the element, passing most of the charge current through itself and converting the energy into heat. In this case, the can itself receives an extremely small part of the current, which practically stops its charge, allowing its neighbors to recharge. The voltage equalization on the battery cells with such a balancer occurs only at the end of the charge when the elements reach a threshold value.
Simplified diagram of a balancer for a battery
Here is a simplified circuit diagram of a current balancer based on the TL431. Resistors R1 and R2 set the voltage to 4.20 Volts, or you can choose others depending on the type of battery. The reference voltage for the regulator is removed from the transistor, and already at the border of 4.20 V, the system will begin to open the transistor slightly to prevent exceeding the specified voltage. A minimal increase in voltage will cause very fast growth transistor current. During tests, already at 4.22 V (an increase of 20 mV), the current was more than 1 A.
Basically anyone fits here. PNP transistor, operating in the range of voltages and currents that interest us. If the batteries are to be charged with a current of 500 mA. The calculation of its power is simple: 4.20 V x 0.5 A = 2.1 V, and this is how much the transistor must lose, which will probably require some cooling. For a charging current of 1 A or more, the power loss increases accordingly, and it will become increasingly difficult to get rid of the heat. During the test, several different transistors were tested, in particular BD244C, 2N6491 and A1535A - they all behave the same.
The voltage divider R1 and R2 should be selected so as to obtain required voltage restrictions. For convenience, here are a few values, after applying which we will get the following results:
- R1 + R2 = Vo
- 22K + 33K = 4.166 V
- 15K + 22K = 4.204 V
- 47K + 68K = 4.227 V
- 27K + 39K = 4.230 V
- 39K + 56K = 4.241 V
- 33K + 47K = 4.255 V
This is an analogue of a powerful zener diode loaded with a low-resistance load, the role of which here is played by diodes D2...D5. Microcircuit D1 measures the voltage at the plus and minus of the battery and if it rises above the threshold, it opens a powerful transistor, passing all the current from the charger through itself. How all this is connected together and to the power supply - see below.
The blocks turn out to be really small, and you can safely install them directly on the element. You just need to keep in mind that the potential of the negative pole of the battery arises on the transistor body, and you must be careful when installing common radiator systems - you must use insulation of the transistor bodies from each other.
Tests
Immediately 6 pieces of balancing blocks were needed to simultaneously charge 6 18650 batteries. The elements are visible in the photo below.
All elements were charged exactly to 4.20 volts (the voltage was set by potentiometers), and the transistors became hot, although there was no additional cooling - charging with a current of 500 mA. Thus, we can safely recommend this method for simultaneous charging of several lithium batteries from a common voltage source.
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