Constant Current Battery Charger

                               

1. INTRODUCTION

            Constant current battery charger is a popular method of charging lead-acid and Ni-Cd batteries. Here battery is charged with a constant current, generally 1/10 th of battery capacity in ampere hour. Batteries of 6v, 9v and 12v could be charged and also others are charged by changing values of zenerdiodes. Indication is shown for fully charged condition and for deep discharge condition. Automatic swithch off and short circuit protection is also provided.

                                                                

2. BLOCK DIAGRAM

3. BLOCK DIAGRAM EXPLANATION.

3.1 POWER SUPPLY

             Almost all electronic equipments include a circuit that converts AC voltage of main supply in to DC voltage. This part of the equipment is called power supply. In general, at the input of the power supply, there is a power transformer. A diode circuit called Rectifier follows it. The output of the rectifier

goes to a smoothing filter, and then to a voltage regulator circuit. The rectifier is the heart of the power supply.

           

TRANSFORMER

RECTIFIER

FILTER

IC REGULATOR

LOAD

 

            The ac voltage, typically 220Vrms, is connected to a transformer, which  then provides a full-wave rectified voltage that is initially filtered by a simple capacitor filter to produce a dc voltage. This resulting dc voltage usually has some ripple or ac voltage variation.

A regulator circuit removes the ripples and also remains the same dc value even if the input dc voltage varies, or the load connected to the output dc voltage changes. This voltage regulation is usually obtained using one of the popular voltage regulator IC units. 

3.2 ADJUSTABLE VOLTAGE REGULATOR(ZENER)

     A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an

electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

     A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this electrical property.

     A conventional solid-state diode will not allow significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by circuitry, the diode will be permanently damaged due to overheating. In case of large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations

3.3 OVER CHARGE PROTECTION CIRCUIT:

     Zener diode, NPN transistor and opto coupler are used in this circuit.

     In electronics, an opto-isolator, also called an optocoupler, photocoupler, or optical isolator, is "an electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output".^[1] The main purpose of an opto-isolator is "to prevent high voltages or rapidly changing voltages on one side of the circuit from damaging components or distorting transmissions on the other side."^[2] Commercially available opto-isolators withstand input-to-output voltages up to 10 kV^[3] and voltage transients with speeds up to 10 kV/μs.^[4]

3.4  MOSFET

     IRF540 is used here as the MOSFET

           

            The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first proposed by Julius Edgar Lilienfeld in 1925. In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other

contacts called source and drain. The channel can be of n-type or p-type (see article on semiconductor devices), and is accordingly called an nMOSFET or a pMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common.

3.5 VISUAL INDICATIONS

     Different colored LEDs are used here as the visual indicators

   

            A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices, and are increasingly used for lighting. Introduced as a practical electronic component in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet and infrared wavelengths, with very high brightness

3.6 DEEP DISCHARGE   PROTECTION CIRCUIT

                This circuits contains zener diodes, resistor networks and NPN transistors 2N2222 NPN transistors are used here.

4. CIRCUIT DIAGRAM

5. CIRCUIT DIAGRAM EXPLANATION:

              There are many ways of battery charging but constant-current Charging, in particular, is a popular method for lead-acid and Ni-Cd batteries. In this circuit, the battery is charged with a constant current that is generally one-tenth of the battery capacity in ampere-hours. So for a4.5Ah battery, constant charging current would be 450 mA.

This battery charger has the following features:

1.      It can charge 6V, 9V and 12V batteries. Batteries rated at other voltages can be charged by changing the values of zener diodes ZD1 and ZD2.

2.      Constant current can be set as per the battery capacity by using a pot meter and multi meter in series with the battery.

3.      Once the battery is fully charged, it will attain certain voltage level (e.g.13.5-14.2V in the case of a 12V battery),give indication and the charger will switch off automatically. You need not remove the battery from the circuit.

4.      If the battery is discharged below a limit, it will give deep-discharge indication.

5.      Quiescent current is less than 5mA and mostly due to zeners.

6.      DC source voltage (VCC) ranges from 9V to 24V.\

           7. The charger is short- circuit protected

            D1 is a low-forward-drop schottky diode SB560 having peak reverse voltage (PRV) of 60V at 5A or a 1N5822diode having 40V PRV at 3A. Normally, the minimum DC source voltage should be ‘D1 drop+ Full charged battery voltage+ VDSS+ R2 drop,’ which is approximately ‘Full charged batteryvoltage+5V.’ For example, if we take full-charge voltage as 14V for a 12Vbattery, the source voltage should be14+5=19V.

             For the sake of simplicity, this constant-current battery charger circuit is divided into three sections: constant current source, overcharge protection and deep-discharge protection sections.The constant-current source is built around

MOSFET T5, transistorT1, diodes D1 and D2, resistors R1, R2,R10 and R11, and pot meter VR1. DiodeD2 is a low-temperature-coefficient, highly stable reference

diode LM236-5.LM336-5 can also be used with reduced operating temperature range of 0 to+70°C. Gate-source voltage (VGS) of T5is set by adjusting VR1 slightly above4V. By setting VGS, charging current can be fixed depending on the battery capacity. First, decide the charging current (one-tenth of the battery’s Ah capacity) and then calculate the nearest standard value of R2 as follows:

R2 = 0.7/Safe fault current

             R2 and T1 limit the charging current if something fails or battery terminals get short-circuited accidentally. To set a charging current, while a multi meter is connected in series with the battery and source supply is present,  adjust pot meter VR1 slowly until the charging current reaches its required value.

            Overcharge and deep-discharge protection have been shown in dotted areas of the circuit diagram. All components in these areas are subjected to a maximum of the battery voltage and not the DC source voltage. This makes the circuit work under a wide range of source voltages and without any influence from the charging current value. Set overcharge and deep-discharge voltage of the battery using pot meters VR1  and VR2 before charging the battery. In overcharge protection, zener diode ZD1 starts conducting after its breakdown voltage is reached, i.e., it conducts when the battery voltage goes beyond a prefixed high level. Adjust VR2 when the battery is fully charged (say, 13.5V in case of a 12V battery) so that VGS of T5 is set to zero and hence charging current stops flowing to the battery. LED1 glows to indicate that the battery is fully charged. When LED1 glows, the internal LED of the opto coupler also glows and the internal transistor conducts. As a result, gate-source voltage(VGS) of MOSFET T5 becomes zero and charging stops. Normally, zener diode ZD2 conducts to drive transistor T3 into conduction and thus make transistor T4cut-off. If the battery terminal voltage drops to, say, 11V in case of a 12V battery, adjust pot meter VR3 such that transistor T3 is cut-off and T4 conducts.LED2 will glow to indicate that the battery voltage is low.

            Values of zener diodes ZD1 andZD2 will be the same for 6V, 9V and 12V batteries.  For other voltages, you need to suitably change the values ofZD1 and

ZD2. Charging current provided by this circuit is 1mA to 1 A, and no heat-sink is required for T5. If the maximum charging current required is5A, put another LM236-5 in series with diode D2, change the value of R11 to 1kilo-ohm, replace D1 with two SB560devices in parallel and provide a good heat-sink for MOSFET T1. TO-220 package of IRF540 can handle up to 50W.

     Assemble the circuit on a general-purpose PCB and enclose in a box after setting the charging current, overcharge voltage and deep-discharge voltage. Mount pot meters VR1, VR2and VR3 on the front panel of the box.

6.    PCB LAYOUT AND FABRICATION METHOD

6.1. PCB PREPERATION

         You need to generate a positive (copper black) UV translucent art work film. You will never get a good board without good art work, so it is important to get the best possible quality at this stage. The most important thing is to get a clear sharp image with a very solid opaque black. Art work is done using ORCAD software. It is absolutely essential that your PCB software prints holes in the middle of pads, which will act as centre marks when drilling. It is virtually impossible to accurately hand-drill boards without these holes. If you are looking to buy PCB software at any cost level and want to do hand-prototyping of boards before production, check that this facility is available when defining pad and line shapes, the minimum size recommended (through-linking holes) for reliable result is 50 mil, assuming 0.8mm drill size; 1 mil=(1/1000)^th  of an inch. You can go smaller drill sizes, but through linking will be harder. 65mil round or square pads for normal components.

Copper clad laminate:

        

         ICs, with 0.8 mm hole, will allow a 12.5mil, down to 10mil if you really need to. Center-to-centre spacing of 12.5 mil tracks should be 25 mil-slightly less may b possible if your printer can manage it. Take care to preserve the

correct diagonal track-track spacing on mitered corners; grid is 25 mil and track width  12.5mil. The art work must be printed such that the printed side is in contact with PCB surface when exposing, to avoid blurred edges. In practice, this means that if you design the board as seen from the component side, the bottom (solder side) layer should be printed the ‘correct’ way round, and top side of the double-sided board must be printed mirrored.

6.1.1. ETCHING

         Ferric chloride etchant is a messy stuff, but easily available and cheaper than most alternatives. It attacks any metal including stainless steel. So when setting up a PCB etching area, use a plastic or ceramic sink, with plastic fitting and screws wherever possible, and seal any metal screws with silicon. Copper water pipes may be splashed or dripped-on, so sleeve or cover them in plastic; heat-shrink sleeve is great if you are installing new pipes. Fume extraction is not normally required, although a cover over the tank or tray when not in use is a good idea. You should always use the hex hydrate type of ferric chloride, which should be dissolved in warm water until saturation. Adding a teaspoon of table salt helps to make the etchant clearer for easier inspection. Avoid anhydrous ferric chloride. It creates a lot of heat when dissolved. So always add the powder very slowly to water; do not add water to the powder, and use gloves and safety glasses. The solution made from anhydrous ferric chloride doesn’t etch at all, so you need to add a small amount of hydrochloric acid and leave it for a day or two. Always take extreme care to avoid splashing when dissolving either type of ferric chloride, acid tends to clump together and you often get big chunks coming out of the container and splashing into the solution. It can damage eyes and permanently stain clothing. If you are making PCBs in a professional environment where time is money you should get a heated bubble-etch tank.

With fresh hot ferric chloride, the PCB will etch in well under 5 minutes. Fast etching produces better edge-quality and consistent line widths. If you aren’t using a bubble tank, you need to agitate frequently to ensure even etching. Warm the etchant by putting the etching tray inside a larger tray filled with boiling water.

6.1.2. DRILLING

     If you have fiber glass (FR4) board, you must use tungsten carbide drill bits. Fiber glass eats normal high-speed steel (HSS) bits very rapidly, although HSS drills are alright for older larger sizes (> 2mm). Carbide drill bits are available as straight-shank or thick-shank. In straight shank, the hole bit is the diameter of the hole, and in thick shank, a standard size (typically about 3.5 mm) shank tapers down to the hole size. The straight-shank drills are usually preferred because they break less easily and are usually cheaper. The longer thin section provides more flexibility. Small drills for PCB use usually come with either a set of collets of various sizes or a three-jaw chuck. Sometimes the 3-jaw chuck is an optional extra and is worth getting for the time it saves on changing collets. For accuracy, however, 3-jaw chucks are not brilliant, and small drill sizes below 1 mm quickly formed grooves in the jaws, preventing good grip. Below 1 mm, you should use collets, and buy a few extra of the smallest ones; keeping one collect per drill size as using a larger drill in a collet will open it out and it no longer grips smaller drills well. You need a good strong light on the board when drilling, to ensure accuracy. A dichroic halogen lamp, under run at 9V to

reduce brightness, can be mounted on a microphone gooseneck for easy positioning. It can be useful to raise the working surface above 15 cm above the normal desk height for more comfortable viewing. Dust extraction is nice, but not essential and occasional blow does the trick! A foot-pedal control to switch the drill ‘off’ and ‘on’ is very convenient, especially when frequently changing bits. Avoid hole sizes less than 0.8 mm unless you really need them. When making two identical boards, drill them both together to save time. To do this, carefully drill a 0.8 mm whole in the pad near each corner of each of the two  boards, getting the center as accurately as possible. For larger boards, drill a hole near the centre of each side as well. Lay the boards on the top of each other and insert a 0.8 mm track pin in two opposite corners, using the pins as pegs to line the PCBs up. Squeeze or hammer the pins into boards, and then into the remaining holes. The two PCBs are now ‘nailed’ together accurately and can be drilled together.

6.1.3. SOLDERING

         Soldering is the joining together of two metals to give physical bonding and good electrical conductivity. It is used primarily in electrical and electronic circuitry. Solder is a combination of metals, which are solid at normal room

temperatures and become liquid between 180 and 200 degree Celsius. Solder bonds well to various metals, and extremely well to copper. Soldering is a necessary skill you need to learn to successfully build electronics circuits. To solder you need a soldering iron. A modern basic electrical soldering iron consists of a heating element, a soldering bit (often called a tip), a handle and a power cord. The heating element can be either a resistance wire wound around a ceramic tube, or a thick film resistance element printed on to a ceramic base. The element is then insulated and placed into a metal tube for strength and  protection. This is then thermally insulated from the handle. The heating element of soldering iron usually reaches temperatures of around 370 to 400 degree Celsius (higher than need to melt the solder). The strength or power of a soldering iron is usually expressed in watts. Irons generally used in electronics are typically in the range of 12 to 25 watts. Higher powered iron will not run hotter. Most irons are available in a variety of voltages; 12V, 24V, 115V and 230V are most popular. Today most laboratories and repair shops use soldering irons, which operate at 24V. You should always use this low voltage where possible, as it is much safer. For advanced soldering work, you will need a soldering iron with temperature control. In this type of soldering irons, the temperature may be usually set between 200 and 450 degree Celsius.Many temperature control soldering iron designed for electronics have a power rating of around 40 to 50 watt. They will heat fast and give enough power for operation, but are mechanically small.

         You will occasionally see gas-powered soldering irons which use butane rather than the main electrical supply to operate. They have a catalytic element which once warmed up, continues to glow hot when gas passes over them. Gas powered soldering irons are designed for occasional ‘on the spot’ used for quick repairs, rather than for main stream construction or for assembly work.

         Currently, the best commonly available, workable, and safe solder alloy is 63/37. That is, 63% lead, 37% tin. It is also known as eutectic solder. Its most desirable characteristic is that it solids (‘pasty’) state, and its liquid state occur at

the same temperature -361 degree Fahrenheit. The combination of 63% lead and 37% tin melts at the lowest possible temperature. Nowadays there is tendency to move to use lead free solders, but it will take years until they catch on normal soldering work. Lead free solders are nowadays available, but they are generally more expensive or harder to work on than traditional solders that they have lead in them. The metals involved are not the only things to consider in a solder. Flux is vital to a good solder joint. Flux is an aggressive chemical that removes oxide  and impurities from the parts to be soldered. The chemical reactions at the point(s) of connection must take place for the metal to fuse. RMA type flux (Rosin Mildly Active) is the least corrosive of the readily available materials, and provides an adequate oxide removal. In electronics, a 60/40 fixed core solder is used. This consists of 60% lead and 40% tin, with flux cores added to the length of solder.

         There are certain safety measures which you should keep in mind when soldering. The tin material used in soldering contains dangerous substances like lead (40-60% of typical soldering tins are lead and lead is poisonous). Also the various fumes from the soldering flux can be dangerous. While it is true that lead does not vaporize at the temperature at which soldering is typically done.When soldering, keep the room well ventilated and use a small fan or fume trap. A proper fume trap of a fan will keep the most pollution away from your face. Professional electronic workshops use expensive fume extraction systems to protect their workers. Those fume extraction devices have a special filter which filters out the dangerous fumes. If you can connect a duct to the output from the trap to the outside, that would be great.Always wash hands prior to smoking, eating, drinking or going to the bathroom. When you handle soldering tin, your hands will pick up lead, which needs to be washed out from it before it gets to your body. Do not eat, drink or smoke while working with soldering iron. Do not place cups, glasses or a plate of food near your working area.Wash also the table sometimes. As you solder, at times there will be a bit of spitting or sputtering. If you look you will see tiny balls of solder that shoot out and can be found on your soldering table.                                

PCB LAYOUTS: COMPONENT SIDE:

PCB LAYOUTS: SOLDER SIDE

7.    ADVANTAGE

1. Eliminates imbalance of cells and battery connected in series

2. Most appropriate for cyclic operations.

3. Short circuit protected.

4. Provide Deep discharge indication.

5. Overcharge protection is provided.

6. It can charge 6V, 9V, 12V batteries.

7. Constant current charging prevents venting of gases during charging.

8.    DISADVANTAGE

1.      It can be only used to charge lead-acid and Ni-Cd batteries.

2.      Battery charging indication is not provided other than deep discharge , overcharge indications.

9. APPLICATION

1.      It can be used for charging batteries of digital cameras, emergency lamp etc.

2.      Battery can be charged with constant current.

3.      It can be used for cyclic operation ,where a battery is required to obtain a full charge overnight

                                                                                                                      

10. CONCLUSION

            Our project entitled  constant current battery charger is  concluded   with  the  desired  output. The desired  output  was   obtained  by  generating  the  constant  current  followed  by  an  LED  indication  showing  over- charge  indication  and  deep- discharge  indication. The  circuit  was  analyzed  and  the  output  was  verified.

             If  we  want  to  charge  the  batteries  rated  at  other  voltages  we  can  change  the  values  of  zener  diodes  ZD1  and  ZD2.

11.FUTURESCOPE

1.      Further  modification  in  the circuit  can be  used  to  charge  the  ups  battery(150A).

2.      Other  batteries  can be  charged  by  changing  the  values  of  zener  diode.

3.      It  can  be  used  to  charge  12V 200Ah 

                                                           

APPENDIX