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Adjustable Voltage Regulators

Adjustable voltage regulator

Some­times variable voltage is required and under such a situation adjustable voltage regulator is used. A number of IC regulators, like LM 317, LM 338, and LM 350, are adjustable regulators. They have maximum load currents from 0.10 to 1.5 A. For example, the LM317 is a three terminal positive voltage regulator, which can be oper­ated with the output voltage regulated at any setting over the range of 1.2 V to 57 V. Figure illustrates how the regulated output voltage of an LM317 can be set.

Resistors R_x and R₂ set the output to any desired voltage over the adjustment range (1.2 to 57 V). The output voltage desired can be determined using equation.

V_out = V_REF [1+ R₂/R₁] + I_ADJ R₂

where V_REF = 1.25 V, the reference voltage between the output and adjustment terminals.

However, the current I_ADJ is very small (100 micro Amperes) and constant. So the voltage drop across R₂ due to I_ADJ is also very small and can be neglected so that equation  can be written as

V_out = 1.25 [1+ R₂/R₁]

The current set resistor R_x is usually 240 Ohms, and to achieve good load regulation it should be tied directly to the output of the regulator rather than near the load.

The load regulation is 0.1 percent while the line regulation is 0.01 percent per volt. This means that the output voltage varies only 0.01 percent for each volt of input voltage. The ripple rejection is 80 db, equivalent to 10,000.

The LM 337 series of adjustable voltage regulators is a complement to the LM 317 series devices. The negative adjustable voltage regulators are available in the same voltage and current options as the LM 317 devices.

More Circuits on Adjustable Voltage Regulators:

1. 13 volts adjustable voltage regulator using LM 338 IC

2. 25 volts adjustable regulator using LM 117 IC