You may have often wondered whether it was feasible to get higher voltages than 12 V from a 7812 IC? In this short post we learn how to configure the IC 7812 with a BJT stage, so that its output can be made variable for achieving any desired voltage higher than 12 V, without compromising the performance of the 7812 IC.
A 7812 IC is a 3 terminal fixed voltage regulator device which is able to produce a constant 12 V output in response to an input Dc between 15 V and 30 V.
It is sometimes essential to set up this type of 3-terminal voltage regulator IC to provide an increased output voltage than the fixed value through the regulator itself.
How the Circuit Works
The standard method of getting a higher voltage output from a 7812 IC is to hook up the ''common'' terminal of the IC to the junction of a resistive divider installed across the regulated output supply positive and ground.
The regulator voltage at this point appears over the upper divider resistor; thus, in case for example identical divider resistors are used, the output voltage becomes two times more than the value managed by the regulator across its common terminal and output terminal.
The issue using this technique is that a lot of IC regulators (eg the 78XX series) include a tiny quiescent current (around 10mA) moving out through their common terminal towards ground. The value of this current is not tightly governed, and therefore the total output voltage tends to become a little bit erratic because of this extra current streaming within the lower half of the resistor divider.
Low value resistive divider seems to solve the problem, but this may up end up with more complications such as heat dissipation and reduced efficiency.
The circuit above eliminates the challenge through the use of transistor Q1 to crank out a low impedance on the regulator common terminal through its emitter-follower configuration.
The transistor emitter transfers the voltage derived from a relatively high-resistance divider network connected across the base of the transistor.
The value of R3 is not crucial, however should be sufficiently small in order to enable the maximum possible quiescent current from the ground terminal of the 7812 IC, without resulting in Q1 to switch off. The circuit exhibits a functional 24 Volt supply through a 7812 regulator.
A desired higher output voltage from the IC 7812 can be adjusted by altering the values of R1 and R2 accordingly.