Battery Charger Ckt Using Scr

8/2/2019 Battery Charger Ckt Using Scr

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BY

A.SAI SHASHANK


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Most modern electrical appliances receive theirpower directly from the utility grid, a growingnumber of everyday devices require electrical

power from batteries in order to achievegreater mobility and convenience.

The system used to draw energy from the grid,store it in a battery, and release it to power a

device is called a battery charger system. Battery charger systems often maximize the

energy efficiency of their devices to ensure longoperation times between charging.


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A battery charger is a device used to put energyinto a secondary cell or rechargeable battery byforcing an electric current through it.

The charge current depends upon the technology

and capacity of the battery being charged.A series of power conversion steps must be

performed to shape the high-voltage ac electricityfrom the utility into low-voltage dc electricity thatcan be accepted by the battery, and the chargingprocess must be controlled so that the batteryreceives the appropriate amount of current.


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Block diagramBlock diagramshowing the generalshowing the generalconfiguration batteryconfiguration batterycharger system.charger system.

The first two stages are

functions typically

incorporated into ac-dc

power supplies.

The addition of the thirdstage controlling the

rate of charge of the

battery with charge

control circuitry is

typically what

distinguishes a batterycharger from an

ac-dc power supply


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Four-layer solid state device thatcontrols current, An SCR consists of fourlayers of alternating

P and N type semiconductor materials. The planar construction is used for low

power SCRs


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In the normal "off" state, the device restrictscurrent to the leakage current. When the gate-to-cathode voltage exceeds a certain threshold, thedevice turns "on" and conducts current. The devicewill remain in the "on" state even after gate currentis removed so long as current through the deviceremains above the holding current. Once currentfalls below the holding current for an appropriate

period of time, the device will switch "off". If thegate is pulsed and the current through the deviceis below the holding current, the device willremain in the "off" state.


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The feedback control design of the charger makes itpossible for one SCR in the SCR assembly to notfunction and the charger to still have some output. This

condition can be detected by noting thetemperature of the separate SCRs and replacing the

complete assembly if one of the SCRs feels cold

and the other hot. This condition may also suggest

its presence by continual melting of the DC fuse


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If the applied voltage increases rapidly enough, capacitivecoupling may induce enough charge into the gate to trigger the deviceinto the "on" state; this is referred to as "dv/dt triggering." This isusually prevented by limiting the rate of voltage rise across thedevice, perhaps by using a snubber. "Dv/dt triggering" may not

switch the SCR into full conduction rapidly and the partially-triggered SCR may dissipate more power than is usual, possiblyharming the device.

SCRs can also be triggered by increasing the forward voltage beyondtheir rated breakdown voltage (also called as break over voltage), butagain, this does not rapidly switch the entire device into conduction

and so may be harmful so this mode of operation is also usuallyavoided. Also, the actual breakdown voltage may be substantiallyhigher than the rated breakdown voltage, so the exact trigger pointwill vary from device to device. This device is generally used inswitching applications.


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A transformer is a device that transfers electrical energy fromone circuit to another through inductively coupled conductorsthe transformer's coils. A varying current in the firstor primary winding creates a varying magnetic flux in thetransformer's core and thus a varying magnetic field through

the secondaryw

inding. This varying magnetic field induces avarying electromotive force (EMF), or "voltage", in the secondarywinding. This effect is called mutual induction.

If a load is connected to the secondary, an electric currentwill flow in the secondary winding and electrical energy will betransferred from the primary circuit through the transformer tothe load. In an ideal transformer, the induced voltage in the

secondary winding (Vs) is in proportion to the primary voltage(Vp), and is given by the ratio of the number of turns in thesecondary (Ns) to the number of turns in the primary (Np) asfollows


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The transformer is based ontwo principles: first, thatan electric current canproduce a magneticfield (electromagnetism),

and, second that a changingmagnetic field within a coilof wire induces a voltageacross the ends of the coil(electromagnetic induction).Changing the current in the

primary coil changes themagnetic flux that isdeveloped. The changingmagnetic flux induces avoltage in the secondary coil.


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The BC148 equivalent is BC548. The BC548 is a general

purpose silicon NPN BJT transistor found commonly inEuropean electronic equipment; the part number isassigned by Pro Electron, which allows manymanufacturers to offer electrically and physicallyinterchangeable parts under one identification. TheBC548 is commonly available in European Union andCommonwealth Countries and is often the first type of

bipolar transistor young hobbyists encounter. TheBC548 is often featured in circuit diagrams and designspublished in Electronics Magazines such as "SiliconChip" and "Elektor".

As a representative of the large family of bipolartransistors the BC548 provides a "stepping off point" tothe use of more esoteric, higher voltage, current or

frequency devices for beginners. If the TO-92 package is held in front of one's face with

the flat side facing toward you and the leadsdownward, (see picture) the order of the leads, from leftto right is collector, base, emitter.


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A simple battery charger based on SCR is shown . Here the SCR rectifiestheAC mains voltage to charge the battery. When the battery connected to thecharger gets discharged the battery voltage gets dropped. This inhibits theforward biasing voltage from reaching the base of the transistor Q1 through R4and D2.This switches off the transistor.

When the transistor is turned OFF, the gate of SCR (H1) gets the triggeringvoltage via R1 & D3.This makes the SCR to conduct and it starts to rectify theAC input voltage. The rectified voltage is given to the battery through theresistor R6(5W).This starts charging of the battery.

When the battery is completely charged the base of Q1 gets the forward biassignal through the voltage divider circuit made of R3,R4,R5 and D2.This turnsthe transistor ON. When the Q1 is turned ON the trigger voltage at the gate of

SCR is cut off and the SCR is turned OFF. In this condition a very small amountof charge reaches the battery via R2 and D4 for trickle charging. Since thecharging voltage is only half wave rectified ,this type of charger is suitable onlyfor slow charging.For fast charging full wave rectified charging voltage isneeded.


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Topology TypicalEfficiencyRange

ExampleProducts

MarketSegment

Relative Costper Watt

Linear 2 % - 35% Cordlessphones,

power tools

Residential,Commercial

Low

Switch Mode 40% - 60% Laptopcomputers,cell phones

Residential,Commercial

High

Ferro resonant 25% - 50% Golf carts,forklifts

Commercial,Industrial

Low

SCR 30% 55% Recreational

vehicle battery

chargers,

forklifts

Commercial,

Industrial

Medium


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Battery charger using SCR accomplish all thisthrough three functions:

1) Reducing voltage from the utility level to the

lower voltage at which batteries operate,

2) Rectifying ac electricity into dc electricity, and

3) Controlling the low-voltage dc current into the

battery


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1.Someone should be within range of your voice or closeenough to come to your aid when you work near abattery.

2. Have plenty of fresh water and soap nearby in case

battery acid contacts skin, clothing or eyes.3. Wear complete eye and clothing protection.

Avoid touching eyes while working near batteries.

4. NEVER smoke or allow a spark or flame in the vicinity

of batteries.5. Be extra cautious to reduce risk of dropping a metal tool

onto battery. It might spark or short circuit battery orother electrical part that may cause explosion.


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There are significant opportunities to improve theefficiencies of battery charger systems in use today. Atpresent, inefficiencies in the charger and battery oftenconsume more electricity than the product they power.There are millions of battery charger systems in operation

worldwide, and therefore substantial energy savings areachievable by reducing or eliminating these inefficiencies. Several methods can be used to achieve higher efficiency in

battery charger systems, including: Higher voltage systems

Use of switch mode power supplies Use of synchronous rectification Use of improved semiconductor switches Use of lithium-ion batteries Charge and discharge at lower current rate


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Battery Charger Ckt Using Scr

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