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Constant-Current B a t t e r y C h a r ge r~M

onoj Das

here 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 a 4.5Ah battery, constant charging cur-rent 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 potmeter and multimeter in series with the battery.

remove the battery from the circuit. 4. If the battery is discharged be-low a limit, it will give deep-discharge indication. 5. Quiescent current is less than 5 mA 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 volt-age (PRV) of 60V at 5A or a 1N5822 diode having 40V PRV at 3A. Normally, the minimum DC source volt-age should be D1 drop+Full charged battery voltage+VDSS+ R2 drop, which is approximately Full charged battery voltage+5V. For example, if we take full-charge voltage as 14V for a 12V battery, the source voltage should be 14+5=19V. For the sake of simplicity, this con-

T

stant-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, transistor T1, diodes D1 and D2, resistors R1, R2, R10 and R11, and potmeter 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 +70C. Gate-source voltage (VGS) of T5 is set by adjusting VR1 slightly above 4V. By setting VGS, charging current canbe fixed depending on the battery

capacity. First, decide the charging current (one-tenth of the batterys Ah capacity) and then calculate the nearest standard value of R2 as follows: R2 = 0.7/Safe fault current

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

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ideasR2 and T1 limit the charging cur-rent if something fails or battery terminals get short-circuited accidentally. To set a charging current, while a multimeter is connected in series with the battery and source supply is present, adjust potmeter 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 potmeters 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 VG S 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 optocoupler also glows and the internal transistor con-ducts. As a result, gate-source voltage (VGS) of MOSFET T5 becomes zero and charging stops. Normally, zener diode ZD2 con-ducts to drive transistor T3 into conduction and thus make transistor T4 cut-off. If the battery terminal voltage drops to, say, 11V in case of a 12V battery, adjust potmeter 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 and ZD2 will be the same for 6V, 9V and 12V batteries. For other voltages, you need to suitably change the values of ZD1 and ZD2. Charging current provided by this circuit is 1 mA to 1 A, and no heat-sink is required for T5. If the maximum charging current required is 5A, put another LM236-5 in series with diode D2, change the value of R11 to 1 kilo-ohm, replace D1 with two SB560 devices in parallel and provide a good heat-sink for MOSFET T1. TO-220 pack-age 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 potmeters VR1, VR2 and VR3 on the front panel of the box.z

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circuit

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Generator rooM liGht~M

anuj Paul

A t night when power fails, one finds it difficult to reach the generator to start it. Here is the circuit for a generator room light that automatically turns on at night, facilitating easy access to the generator. During daytime, the light remains off. Fig. 1 shows the circuit for gen-

erator room light, while Fig. 2 shows the battery charger circuit, which is optional and can be omitted if the generator is self-start type and has built-in battery. At the heart of the generator room light circuit (Fig.1) is a light-dependent resistor (LDR1) that senses the ambient light as well as light from glowing LED1.

IC1 remain high, making output pin 3 of IC1 low and transistor T2 cut-off. So lamp L1 connected between the collector of T1 and the positive terminal of 12V supply does not glow. As the ambient light fades during sunset, the resistance of LDR1 increases. As a result, the voltage drop across LDR1 increases and npn transistor T1 conducts. Pins 2 and 6 of IC1 go low to make its output pin 3 high, and lamp L1 glows. You can replace incandescent lamp L1 with bright white LEDs using proper current-limiting resistors. Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Install the unit near the generator. Arrange LED1 and LDR1 such that during the availability of mains, light emitted from LED1 falls directly on LDR1. Also, make sure that during daytime the ambient light falls on the LDR. For powering the battery charger circuit (Fig. 2), 15V AC secondary voltage is derived from step-down transformer X1. For fast charging of the battery, you may increase the current rating of transformer X1. The charger charges the battery through a thyristor (SCR1) when the battery voltage is low. The thyristor gets a regulated gate voltage from the zener diode, and goes to tickle charging mode when the battery voltage nears the zener voltage. Assemble the charger circuit on a general-purpose PCB and enclose in a suitable cabinet. Use two crocodile clips (red for positive and black for negative) for connecting the battery terminal to the charger circuit. z

During daytime, sun-light or light from LED1 reduces the resistance of LDR1. As a result, the voltage drop across LDR1 decreases and npn transistor T1 does not conduct. The collector of T1 and therefore pins 2 and 6 of

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PC POWer ManaGer~T.K.

Hareendran

V ery often we forget to switch off the connected peripherals like monitor, scanner and printer while switching off our PC. This leads to needless energy consumption and possible shortening of the life of the peripheral. PCs with an ATX switch-mode power supply (SMPS) unit are not provided with a mains switch outlet. It is therefore not possible to achieve automatic switching (on/off) of peripheral units with the computer power switch. Here is a simple circuit that turns the connected peripherals on/off along with your PC. It consists of a regulated power supply, a simple USB interface and two electromagnetic relays used as power switches. The power supply for the circuit is derived from the AC mains via trans-former X1. The 15V AC available at the secondary winding of transformer X1 is first rectified by a bridge recti-

fier comprising diodes D1 through D4, smoothed by capacitors C1 and C2, and regulated by IC LM7812 (IC1). The regulated 12V DC is used to energise relay RL1. LED1 works as a power-active indicator. To set up the circuit, first connect the input socket (SOC1) of the circuit to a proper AC mains wall outlet using a three-core power cable. Now connect one end of a standard USB cable to the B-type USB input socket and the other end of the cable to any

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ideasvacant USB port (A-type) of the PC. Finally, plug one standard four-way switchboard (extension cord) into the supply output socket (SOC2) of the circuit and take power from this switchboard to activate all loads like monitor, scanner, printer and even your PC.

To activate the PC manager circuit, proceed as follows: Press start switch S1 and hold it in this position for a few minutes. When power-active indicator LED1 lights up, relay RL1 energises and the 230V mains power supply from SOC1 is fed to output socket SOC2 through the contacts of relay RL1.

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ideasNow start your computer as usual, by pressing the power button on the fro