A diode voltage regulator is a voltage regulator in which a single diode acts as the voltage regulating device. Just like a conventional integrated circuit (IC) voltage regulator, the diode serves in the circuit to take a varying input voltage and convert it down to a specific, constant direct currant (DC) voltage. Diodes function considerably well as voltage regulator devices because of their logarithmic characteristic of having very minimal changes in voltage in spite of relatively substantial current changes, thus providing good voltage stability in a circuit when design constraints are met.
The most commonly used diode that functions as a regulator device is the zener diode, which achieves voltage regulation most commonly through a method called reverse bias. This is a method in which the diode’s anode, its positive lead, is connected to the power supply’s ground, and the cathode, its negative lead, is connected to the positive side of the power supply. When this occurs, the power supply drops a voltage across the diode equal to the maximum voltage rating of the diode. The maximum voltage rating is the voltage that is dropped across the zener diode when it is connected in reverse bias in a circuit, as long as the power supply voltage is of greater voltage. This voltage rating is crucial when designing the voltage regulation of a circuit because it decides the regulated voltage output.
If, for example, there is a power supply of 10 volts and a zener diode with a maximum voltage rating of 5.1 volts, the zener diode will sink 5.1 volts. A component placed in parallel will also receive this same regulated voltage output, because circuits obey the rule that all components in parallel receive the same voltage. This is how diode voltage regulation is achieved for a circuit.
An additional rule when creating diode voltage regulator circuit is for one to place a resistor before the diode. A resistor can be and is usually always placed before the diode so that if there is excessive voltage, it drops across the resistor and doesn’t burn out the zener diode, which can be rendered defective if excessive power supply voltage is fed to it. In the example of a power supply of 10 volts and a zener diode with a maximum voltage rating of 5.1 volts, the zener diode would sink 5.1 volts, and the remaining 4.9 volts would sink across the resistor, so that not all 10 volts sink across the diode. Thus, the resistor before the zener diode functions as a safety device to drop excessive voltage across it so that the zener diode doesn’t receive more voltage than what is necessary.
Another way of connecting a zener diode in a circuit for voltage regulation, though less commonly used, is connecting it in a method called forward bias. This is a method in which the diode’s anode is connected to the positive side of the power supply and its cathode is connected to the ground of the power supply. In an arrangement such as this, the diode will drop its operating voltage drop, which typically is about 0.7 volts. This is not as popular of a method because it drops only a slightly lower voltage than the voltage given off by the power supply and isn’t as flexible in differing value ranges as the maximum voltage ratings are.
A diode voltage regulator can work well as a voltage regulator device, but when precision is key, a better choice is a IC voltage regulator, which contains more built-in regulating mechanisms. If a diode voltage regulator contains large enough shifts in current, it can produce differing voltage. When accuracy isn't too important, however, diode voltage regulators can be a good choice.