Familiarize Electronic Components Part IX – Voltage Regulators

Voltage Regulators are used to give regulated power to a circuit. Most of the circuits using CMOS devices require well regulated power supply. Zener regulated power supply is simple but less reliable than the power supply using Fixed Voltage regulator IC. 78 XX series ICs are Positive voltage regulators while 79 XX series ICs are  Negative voltage regulatorsLM317 and LM338 are Variable voltage regulators.

Voltage regulation through Fixed Voltage Regulator IC is the most suitable method.78 XX series positive and negative regulators are widely used to get fixed voltage to drive loads. A circuit using the positive regulator 7805 is shown below.

The circuit uses a Positive Voltage regulator IC 7805 to give 5 volt regulated output. The components around IC1 are meant for protecting the IC so as to give smooth output power. Zener diode ZD act as the Input Surge protecting diode. It provides 7.2 volts to the input of the regulator IC from the 12 volt power supply. Zener is not required if power surge protection is not needed. Diode D1 is the Output Surge and Input Short protecting diode.

When a surge voltage exceeding maximum voltage rating of the regulator is applied to the input or when a voltage in excess of the input voltage is applied to the output, the regulator will be destroyed. If the input terminal shorts with the ground, the output voltage increases above the input voltage(ground potential)and the charge in the capacitor connected to the output flows into the input side which is also fatal to the regulator. Both these situations can be avoided by using the Zener at the input and the diode D1 across the regulator. Capacitor C1 and C2 provides stability to the regulator and these should be soldered close to the legs of the regulator. Capacitor C3 acts as a buffer to give constant voltage in the output.
7805 IC can tolerate maximum 35 volts and its current rating is 1 Amps maximum. Resistor R1 restricts the charging current to around 330 mA (0.33A) as per the Ohms law. By increasing or decreasing the value of R1, output current can be varied. Use the formula V/ A.

Variable voltage Regulators

LM 317 is the popular voltage regulator that can vary the output voltage from 1.26 volts to 27.29 volts with 30 volt input. The output depends on the feedback from the output pin (pin 3) to the adjust pin (pin1) of LM 317.

By adjusting VR1 it is easy to set the output voltage from 1.2 V to 27.29 DC if the variable resistor is 5K. If the Wiper of VR is set at 1 Ohm, it gives 1.26 volts and if the Wiper is set at 5000 Ohms( 5K) maximum 27.29 volts will be available at the output. . The charging current depends on the values of R1 and R4 and around 300 milli amperes current is available for charging at 15 volt DC.

The circuit shown below is the variable power supply using LM317.Resistor R1 should be 220-240 Ohms and VR should be 4.7 or 5K.

See this link for LM317 voltage calculator



LM 338 is the High current ( 5 Ampere) adjustable regulator capable of supplying in excess of 5Amps over a range of 1.2 V to 32 V. The LM138 family is supplied in a standard 3-lead transistor package. A unique feature of the LM138 family is time-dependent current limiting.

The current limit circuitry allows peak currents of up to 12A to be drawn from the regulator for short periods of time. This allows the LM138 to be used with heavy transient loads and speeds start-up under full-load conditions. Under sustained loading conditions, the current limit decreases to a safe value protecting the regulator. Also included on the chip are thermal overload protection and safe area protection for the power transistor. Overload protection remains functional even if the adjustment pin is accidentally disconnected. Normally, no capacitors are needed unless the device is situated more than 6 inches from the input filter capacitors in which case an input bypass is needed. An output capacitor can be added to improve transient response, while bypassing the adjustment pin will increase the regulator’s ripple rejection.

LM338 can be connected as LM 317.

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