Regulated power supply is necessary in some electronic circuits especially in Amplifier circuits. Poorly regulated power may cause buzzing and unwanted noise in RF and amplifier circuits. There are two methods to get regulated power supply. The most common and simple one is Zener regulated power supply. A Regulator IC of 78 XX series or Variable regulator LM317 can also be used but it may increase the cost and size of the power supply circuit. Here explains the theory behind Zener regulation.

Zener Regulation is used to hold the output voltage steady, irrespective of the changes in input voltage. Zener is a sophisticated semiconductor diode. It conducts only when its breakdown voltage reaches. Zener value represents the voltage it gives. But to get easy breakdown of Zener, the input voltage must be 1-2 volts higher than that of the Zener voltage.

Low current Zener Regulation

Fig.1 shows the voltage regulation using a Zener diode.10 V input is given to a 5.6V Zener. Zener requires a minimum 5 mA current to maintain its action. This current is maintained by the series resistor R connected in series with the Zener. The value of R is important to allow at least 5 mA current for the Zener even if the load draws more current.

Zener Regulation Circuits

In the Fig.1 Imax is the current through the load. It should be maximum 250
mA.

Iz is the current passing through the Zener to maintain its action

I max + Iz is the current passing through the resistor R.

Vz is the voltage across the Zener

Using these parameters, the value of the resistor R is determined using the formula

R = (Vin-Vz) / (Imax+Iz)

That is (10-5.6) / (0.25A+0.005A) = 4.40 / 0.26 = 16 Ohms. The nearest value is 18 Ohms.

Power rating of the Zener should be enough to withstand current through the Zener in no load condition.

Power rating of the Zener is calculated using the formula

P = (I max + Iz) x Vz = P = (0.25 A+0.005A) X 5.6 = 1.4 Watts. Select a Zener with more than 1.5 W rating, if the load current is 250 mA.

Rating of the resistor R is also important to handle the load current as well as the Zener current.

Current through R is I max + I z and voltage through R is Vin – Vz

So Power dissipation in R is

P= (Imax+Iz) x (Vin-Vz) = (0.25+0.005) x 10-5.6) = 1.14 W. So a 2 watt resistor is required as R if the load current is 250 mA or 0.25 A.

High Current Regulation

The circuit shown in Fig.1can be used only if the load current is less than 250 mA. If the load requires more current, say as in battery charges, transistor based Zener regulation is necessary. Fig.2 shows how a series transistor is used in combination with a Zener to give a High current voltage regulated circuit. T1 is connected as an Emitter follower. It should be either a medium power transistor like BD 139 or TIP 122 if current is less than 1A or power transistor like 2N 3055 if current is very high. Zener voltage Vz is 0.7 V (biasing voltage of T1) more than the required output. R should give more than 5 mA to the Zener. The Zener holds the base of T1 at a steady Vz so that output from the emitter of T1 will be Vz – 0.7 Volts.

Author D.Mohankumar