Analog power supply

Analog Bench Power Supply

This is a bench power supply built in a computer power supply metallic housing with a pair of bread boards pasted on its top. It can provide a wide range of regulated DC voltages used mainly for laboratory use.The unit can be powered from mains 220V as well as any external AC source of maximum 25V or DC source of maximum 36V.

Working Principle

The mains AC is steped down to 20V AC which is converted to 20*√2 - (1.4V diode drop) = 26.88V DC. There is a variable supply from 3-24V/1 Amp built around LM723. Fixed supplies of 5V, 6V, 9V and 12V are also available with 1 Amp of maximum current. There is a set of 12 white LEDs whose brightness can be varied with the variable supply control. These LEDs can be used as a light source especially when soldering.

Variable Output Supply 3-24V 1 AMP

Parts List

R1 = 560R 1/4W C1 = 100nF
R2 = 1,2 K 1/4W C2 = 2200uF 35-40V
R3 = 3,9 K 1/4W C3 = 100 pF
R4 = 15K 1/4W C4 = 100uF/ 35V
R5 = 0.15R 5W

D = B40 C3300/2200, 3A rectifier bridge
P1 = 10K potentiometer TR1 = BD 135
IC = LM723 TR2 = 2N3055

Circuit diagram for the Variable output 3-24V 1 Amp

The power supply is using a well known and quite popular VOLTAGE STABILIZER IC the LM 723. The IC can be adjusted for out put voltages that vary continuously between 2 and 37 VDC and has a current rating of 150 mA which is of course too low for any serious use. In order to increase the current handling capacity of the circuit the output of the IC is used to drive a darlington pair formed by two power transistors the BD135 and the 2N3055. The use of the transistors to increase the maximum current output limits the range of output voltages somewhat and this is why the circuit has been designed to operate from 3 to 24 V DC. The resistor R5 connected in series with the output of the supply is used for the protection of the circuit from overloading. If an excessively large current flows through R5, the voltage across it increases and any voltage greater than 0.6 V across it has as a result to cut the supply off, thus effectively protecting it from overloads. This protection feature is built in the LM723 and the voltage drop across R5 is sensed by the IC itself between pins 2 and 3. At the same time the IC is continuously comparing the output voltage to its internal reference and if the difference exceeds the designated standards it corrects it automatically. This ensures great stability under different loads. The potentiometer P1 is used to adjust the out put voltage at the desired level. 2N3055 is attached to the metallic body of the box for better heatsinking.

Heat sinked power transistors

Fixed Power Supplies

Parts List

7805
7806
7809
7812

Working

The input to all the fixed voltage regulator ICs (7805,7806,7809,7812) is common at pin 1and a common ground is connected to all the ICs at pins 2.The output voltage for each IC is taken at pin 3.

78xx series ICs have built-in protection against a circuit drawing too much power. They have protection against overheating and short-circuits, making them quite robust in most applications.

The input voltage must always be higher than the output voltage by some minimum amount (typically 2 volts). As they are based on a linear regulator design, the input current required is always the same as the output current. As the input voltage must always be higher than the output voltage, this means that the total power (voltage multiplied by current) going into the 78xx will be more than the output power provided. The extra input power is dissipated as heat and therefore a heat sink and a cooling fan is installed in the unit.

Power ON Pull Switch

The is a pull switch which switches on/off the internal transformer.The input mains can be supplied with a computer power cable.

Internal VS External Power Source

The unit has the capability to use either the internal transformer for input voltage or an external source e.g battery, laptop charger, ups charger, lead acid battery charger,a bicycle dynamo or even a solar panel. To use internal transformer toggle the select switch to INT while to use external power toggle the switch to EXT. The external voltage source can be connected at the terminals labeled with ~ symbol as shown in the figure.

Lamp LEDs

There are 12 white LEDs whose brightness can be controlled from LM723 variable output. They can be switched ON/OFF with the white coloured switch labeled “light” as shown in the figure .They can serve as a lamp when you are working in low light conditions especially when soldering.