Regulated Power Supply. Design Note 33

Most of the CMOS circuits need a well regulated power supply for proper working. Regulated power supply is the DC source that gives constant Voltage and Current in its output irrespective of the input changes. Various types of power regulation methods are available which depends on the use and type of the circuit. Here explains three Regulated power supply designs.

The most simple type of Regulated power supply is the Zener regulated one. A 9 volt Step-down transformer is used to drop 230 volt AC to low volt AC which is rectified by the full wave rectifier comprising D1 through D4. The resulting DC is smoothed by capacitor C1. LED indicates the power on status. The 9 volt DC is regulated to 5 volt DC using a Zener diode rated 5.1 volt.


Zener is special kind of diode that breaks down and conducts current when the voltage increases above its rated voltage called “Break down voltage or Avalanche voltage”. For example, if the Zener voltage is 5.1, it conducts when the voltage into it increases above 5.1 volts. Then it gives a constant 5.1 V output till the input voltage reduces below 5.1 volts.

Zener requires a current limiting resistor to restrict the current through it. The value of resistor R can be calculated using the formula

R = Vin – Vz / Iz

Where V in is the input voltage, Vz output voltage and Iz current through the Zener.

In most circuits, Iz is kept as low as 5mA. If the supply voltage is 9 V, the voltage that is to be dropped across R to get 5 V output is 4 volts. If the maximum Zener current allowed is 30 mA, then R will pass the maximum desired output current plus 5 mA. That is 35 mA. So the value of R appears as

R = 9 – 5 / 35 mA = 4 / 35 x 1000 = 114 ohms

So the nearest available value 120 Ohms is selected.
Power rating of the Zener is also an important factor to be considered while selecting the Zener diode. According to the formula
P = IV.
P is the power in watts, I current in Amps and V, the voltage.

So the maximum power dissipation that can be allowed in a Zener is the Zener voltage multiplied by the current flowing through it. For example, if a 5.1V Zener passes 5.1 V DC and 35 mA current, its power dissipation will be 175 Milli Watt. So a Zener diode rated 400 mW is sufficient.

Circuit.2 .Regulated power supply using Voltage Regulator IC

Some what robust type of Regulated power supply is the one using Voltage regulated IC. There are two types of Voltage regulator IC. Positive voltage regulators comes in the series 78XX and Negative voltage regulators as 79XX series. They provide a constant output voltage based on their voltage rating. For example 7805 gives 5 volts, 7812 gives 12 volts etc. Its output voltage remains constant until the input voltage drops below its rated voltage. Regulator IC requires 2 or more volts higher than its rated voltage for proper working. For example 7805 requires 9 or 12 volts input and 7812 requires 14 volts ot more as input. Regulator IC gives maximum 1 Ampere current from its output.
The circuit uses 7805 Regulator IC to produce 5 volts DC from a 9 volt transformer power supply. Capacitor C2 and C3 removes transients and noise from the output voltage. Diode D5 protects the IC if a short circuit occurs in the output. This diode by passes the short circuited current and prevents it from entering into the IC.
Capacitor C4 acts as a buffer for the output voltage.


Circuit 3. High Current Regulated power supply.

The Zener regulated and IC regulated power supplies give only a current less than 1 Ampere. If high current is needed, a Transistor based circuit is ideal. Here a high current NPN transistor 2N 3055 is used to give maximum current from the transformer. Here a 14 volt 2 Ampere transformer is used. The 12 volt Zener diode at the base of T1 regulates the output voltage from T1 to 12 volts. The High power transistor passes maximum current from its Emitter. 2N3055 requires a heat sink, since it becomes hot when high current passes. TIP 3055 can also be used.



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