TSU Advisor: regulators

Voltage regulator integrated circuits are widely used in electronics. There are two main types of regulator: linear and switching. This page concerns the linear variety.

The idea of a regulator is simple: you place it between an irregular voltage source (such as a battery, a solar cell, or an 'unsmoothed DC' supply) and a circuit which requires a constant supply voltage. Even if your circuit can work from an unsmoothed supply it may still be a good idea to use a regulator because built into it is a current limiting feature which will protect the supply when you accidently short it on your circuit.

The simplest regulator of all is called a three terminal regulator . There is one connection for the unsmoothed input, one for the regulated output, and one for a common. They are obtainable for outputs of 5, 12, 15 or 24 volts, either positive or negative. You also have a choice of permissable current capability: most often 100mA or 1A.

For experimental or prototype work it may be too expensive to stock all 16 types mentioned above. The solution is to keep just four types -

Adjustable regulators
Polarity Current
LM317L Positive 100mA
LM337L Negative 100mA
LM317 Positive 1A
LM337 Negative 1A

circuit with LM317 and
pinouts Adjustable regulators operate by increasing drive to the internal series pass transistor until they see the voltage between the output pin and the feedback or 'adjustment' pin increase as far as the reference voltage. The disadvantage of these types is that they require two external resistors in order to set the output voltage which they give. The formula relating the output voltage, Vo, to the value of R1 and R2 is -

Vo = Vref(1 + R2/R1) + IadjR2    volts
Equation RGA

For the 100mA types Iadj (the current flowing from the 'adjust' pin) will be about 50 micro amps. Vref is 1.25 volts. R1 should be made about 270 ohms. Substituting these values in the formula above and re-arranging gives -

R2 = (Vo - 1.25) / 0.00468    ohms
Equation RGB

If this is too much effort then consult the table below -

Choosing R2
Vo (volts) 3.3 5 9 10 12 15 24
R2 (ohms) 438 801 1660 1870 2300 2938 4861
E24 series 430 820 1600 1800 2400 3000 4700

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E-mail: R.Clarke@surrey.ac.uk
Last modified: 2007 February 23rd.