The Regulator

A regulator is an optional stage necessary when the load is highly variable or the input source is not stable. The goal of a regulator is to produce a steady DC voltage regardless of how much current is delivered to the load.

Shunt Regulator

A shunt regulator places a zener diode in parallel with the load. The zener diode maintains a constant voltage. Since the load is in parallel with the zener diode, the load voltage is held constant.

The current-limiting resistor will pass the load current plus the zener diode current (recall Kirchhoff's Current Law). Its value is calculated by taking the rated zener current plus the maximum load current and dividing that into the expected nominal voltage across the resistor (recall Kirchhoff's Voltage Law).

In the above example, the maximum load current is 1 ampere. The zener diode is rated at 40 mA. Therefore, the current-limiting resistor will have 1.04 amperes passing through it. The nominal source voltage is 10 volts and the zener diode is rated at 6 volts. Therefore, the current-limiting resistor will have 4 volts across it. This means the resistor needs to be 3.8 ohms (with a 5-watt rating).

If the load is removed, the entire 1.04 amps will go through the zener diode (the current through the current-limiting resistor doesn't change and there is now only one path for the current). Therefore, the power rating of the diode must be sufficient to handle that current. In the above circuit, the zener diode will need to have a power rating greater than 6.25 watts.

Series Regulator

Any load on a voltage source acts like a variable impedance (i.e., a variable resistor). For example, a radio transceiver takes a relatively low current when in receive mode and therefore appears as a relatively high impedance (a high impedance input requires a low current). When the transceiver is placed in transmit mode, it requires significantly more current and therefore appears as a much lower impedance (a low impedance input requires a high current). This is just like increasing the current demand on a battery. As explained in DC circuits, the heavier current passing through the internal resistance of the battery results in a greater voltage drop across the internal resistance. This results in a lower voltage delivered to the load.

A series regulator, however, acts as a variable internal resistance. When the current demand increases, the regulator becomes a lower resistance. Therefore, the voltage drop across it remains the same. This results in no drop in voltage delivered to the load.

Series Regulator Circuit Model

A simple series regulator places a reference voltage (from a zener diode or reference diode) on the base of a transistor (called the pass transistor). The emitter of the transistor is the output to the load. Since the voltage drop from the base to the emitter is approximately 0.7 volts, the voltage at the emitter (and across the load) will be approximately 0.7 volts less than the zener diode voltage. The voltage at the collector is not critical. The transistor will adjust its resistance to whatever it takes to maintain the emitter voltage at 0.7 volts less than the voltage at the base.

An op-amp regulator

The base-to-emitter voltage of a BJT is usually assumed to be 0.7 volts. However, this is not always true. Power transistors can have several volts between the base and emitter when conducting heavily. This means that the simple regulator shown above has a limit to how well it will regulate. The output voltage may still drop if the current demand is high.

To improve regulation, an operational amplifier (configured essentially as a voltage follower) can be added to the circuit. The op-amp monitors the voltage across the load and increases the base voltage to whatever it takes to keep the output voltage equal to the reference voltage. The reference voltage does not need to be 0.7 volts greater than the output voltage because the op-amp accounts for the base-to-emitter voltage drop. If the output voltage needs to be greater than the reference voltage, the op-amp can be used as a non-inverting amplifier (see op-amp circuits above).

A typical op-amp can deliver about 40 mA. If the pass transistor requires more base current than the op-amp can deliver, something has to compensate for it. In this case, a Darlington pair can be used for the pass transistor.

A Practical Linear Power Supply

The following diagram is a schematic for a real-world linear power supply.

A practical linear power supply

The op-amp is used as a non-inverting amplifier with a gain of 2. This is because the zener diode is only 6.2 volts. The amplification is necessary to have a higher output voltage. A zener diode with a higher voltage can be used. That would reduce the required gain of the op-amp to achieve the same output voltage. Using a higher voltage zener diode, the value of R4 can be decreased accordingly. The op-amp is labeled "½ LM358" because it is one of two op-amp circuits in the LM358 package.

The 200k potentiometer across the zener diode makes the output voltage variable.

The pass transistor is made up of a Darlington pair. The 2N3055 requires about 200 mA of base current to deliver 1 amp to the load. The op-amp can deliver only about 40 mA. The op-amp drives a 2N4401 which provides enough current to drive the 2N3055 main transistor.

The second 2N4401, along with the 0.5-ohm resistor, creates a current limiter. When the output current reaches 1 amp, the voltage across resistor reaches 0.5 volts. A greater current increases the voltage above 0.5 volts which begins to turn on the second 2N4401. This pulls driving current away from the pass transistors, thus limiting the output current.

Using the LM723

Construction of a linear power supply is simplified by using an LM723 regulator circuit. This IC contains most of the critical discrete elements in the regulator above. It contains a temperature-stabilized voltage reference (better than a zener diode alone), an op-amp and two transistors. One transistor is configured for a current-limiting circuit. The other transistor can be used as the pass transistor for a low current power supply or it can be the first transistor in a Darlington pair. The LM723 also contains a zener diode configured such that it can be used as part of a fold-back current limiter (essentially shutting down the power supply when there is a short circuit rather than just limiting the current).

The practical linear power supply illustrated above can use the LM723 will little modification as in the following illustration. The IC pin numbers are shown at each connection to the IC.

The reference voltage provided by the LM723 is nominally 7.15 volts, which is close enough to the original zener diode voltage that the op-amp amplification doesn't need to be changed. A 100 pF frequency-compensation capacitor (prevents oscillations) is connected between pins 2 and 9 as recommended on the datasheet. Other than that, the circuit identical to the original.