Editing Linear regulator (section)

==Simple shunt regulator==
[[Image:Zener reg.gif|left|frame|Simple shunt voltage regulator]]
The image shows a simple shunt voltage regulator that operates by way of the [[Zener diode]]'s action of maintaining a constant voltage across itself when the current through it is sufficient to take it into the [[Zener breakdown]] region. The [[resistor]] ''R''<sub>1</sub> supplies the Zener current <math>I_\mathrm{Z}</math> as well as the load current ''I''<sub>R2</sub> (''R''<sub>2</sub> is the load). ''R''<sub>1</sub> can be calculated as <math>R1 = \frac{V_\mathrm{S} - V_\mathrm{Z}}{I_\mathrm{Z} + I_\mathrm{R2}}</math>, where <math>V_\mathrm{Z}</math> is the Zener voltage, and ''I''<sub>R2</sub> is the required load current.

This regulator is used for very simple low-power applications where the currents involved are very small and the load is permanently connected across the Zener diode (such as [[voltage reference]] or [[voltage source]] circuits). Once ''R''<sub>1</sub> has been calculated, removing ''R''<sub>2</sub> will allow the full load current (plus the Zener current) through the diode and may exceed the diode's maximum current rating, thereby damaging it. The regulation of this circuit is also not very good because the Zener current (and hence the Zener voltage) will vary depending on <math>V_\mathrm{S}</math> and inversely depending on the load current. In some designs, the Zener diode may be replaced with another similarly functioning device, especially in an ultra-low-voltage scenario, like (under forward bias) several normal diodes or LEDs in series.<ref>When I{{Who|date=March 2014}} designed my AM pocket radio powered by a 3.7&nbsp;V lithium-ion battery, the 1.5–1.8&nbsp;V power supply required by the TA7642 chip was provided using a Zener regulator using a red LED (with a forward voltage of 1.7&nbsp;V) in forward in place of the Zener diode. This LED also doubled as the power indicator.</ref>