A voltage regulator is a circuit that generates a fixed output voltage of a preset magnitude that remains constant regardless of changes to its input voltage or load conditions. It converts an unstable dc voltage into a stable dc voltage. Its power supply composed of discrete components has the advantages of large output power and wide adaptability. In recent years, integrated regulated power supplies have been widely used. Among them, three-terminal series regulators are the most common for low-power regulated power supplies. The commonly used integrated voltage regulators in the circuit mainly include 78xx series, 79xx series, adjustable integrated voltage regulator, precision voltage reference integrated voltage regulator, etc.
Voltage regulators are generally divided into linear voltage regulator and switching voltage regulator. Linear voltage regulator is a circuit used to maintain a steady voltage, which is divided into low dropout type and general dropout type. Switching voltage regulator is a type of switch mode power supply circuit that is designed to efficiently reduce dc voltage from a higher voltage to a lower one, which is divided into step-down type, step-up type and integrated type with opposite input and output polarity.
According to the number of outlet terminals and usage of the voltage regulator, it can be roughly divided into three-terminal fixed type, three-terminal adjustable type, multi-terminal adjustable type and single-chip switch type.
The three-terminal fixed type voltage regulator integrates sampling resistors, compensation capacitors, protection circuits, high-power adjustment tubes, etc. on a chip. So that the entire integrated circuit block has only 3 terminals: input, output and public. It is very convenient to use. Its disadvantage is that the output voltage is fixed, so a series of products with various output voltages and current specifications must be produced to match.
The three-terminal adjustable integrated voltage regulator only needs two external resistors to obtain various output voltages.
The multi-terminal adjustable type is an early integrated voltage regulator. With small output power and many pins, it is not convenient to use, but the precision is high and the price is cheap.
The monolithic switch type integrated regulated power supply develops in recent years, and its efficiency is particularly high. Its working principle is different from the above three types. It is a converter that converts DC to AC (high frequency) and then DC. Usually there are two types of pulse width modulation and pulse frequency modulation, and the output voltage is adjustable.
2 Main Parameters
1) Voltage Regulation Rate
It is an important indicator that characterizes the voltage regulation performance of the integrated voltage regulator, also known as the voltage regulation coefficient or stability. It represents how stable the output voltage V0 of the regulator is when the input voltage V1 changes.
2) Current Regulation Rate
It is also known as current stability coefficient, and shows the ability of the regulator to suppress output voltage fluctuations caused by changes in load current (output current) when the input voltage remains unchanged.
3) Ripple Rejection Ratio
It reflects the ability of the regulator to suppress the mains ripple voltage introduced at the input.
4) Output Voltage Temperature Coefficient
It is also known as the output voltage temperature change rate, and refers to when the input voltage and output current (load current) remain unchanged, the output voltage of the regulator changes with temperature.
5) Long-term Stability of the Output Voltage
It refers to the magnitude of the change in the output voltage value over time (when the output current, input voltage and ambient temperature remain unchanged). It is usually the maximum amount of change in the output voltage of the regulator within a specified time.
6) Output Noise Voltage
Its absolute value represents the noise performance of the regulator directly. There is also a percentage value of the output noise voltage Vn and the output voltage V0 of the regulator to characterize the noise performance.
7) Thermal Stability
It refers to the thermal stability of the voltage regulator. It is usually the percentage value of the relative change in the output voltage caused by its unit power consumption.
8) Temperature Stability
It is the percentage value of the relative change of the regulator’s output voltage within the specified maximum change range of operating temperature.
3 Application Notes
① There are many types of integrated voltage regulator. According to the adjustment method there are linear and switch type. Based on the output method, there are fixed and adjustable types. Because of the obvious advantages of the three-terminal voltage regulator, it is more convenient to use and operate.
② Before connecting to the circuit, it is necessary to distinguish the pins and their functions to avoid damage to the integrated block. The input and output ends of the three-terminal integrated voltage regulator with an output voltage greater than 6v need to be connected with protective diodes to prevent the rapid discharge of the output capacitor, which will cause damage to the three-terminal integrated voltage regulator when the input voltage drops suddenly.
③ In order to ensure the stability of the output voltage, the minimum input voltage difference should be guaranteed. For example, the minimum pressure difference of the three-terminal integrated voltage regulator is about 2v, and it should be kept above 3v during general use. At the same time, it should be noted that the maximum voltage difference of input and output does not exceed the specified range.
④ In order to expand the output current, the three-terminal integrated voltage regulator is allowed to be used in parallel.
⑤ When using, the welding should be firm and reliable. If a heat dissipation device is required, it should meet the required size.
If you have a bad regulator, it may cause many components such as the fuel pump, ignition system, or other parts which require a minimum amount of voltage to not function correctly. You may experience the engine sputtering, a rough idle, or simply a lack of acceleration when you need it.
4 Typical Examples: LM317 & LM7805
The LM317 chip is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an output-voltage range of 1.25V to 37V. It serves a wide variety of applications including local, on card regulation. This device can also be used to make a programmable output regulator, or by connecting a fixed resistor between the adjustment and output, the LM317 can be used as a precision current regulator.
- LM317 Specifications:
Adjustable output voltage as low as 1.2V
Guaranteed 1.5A output current
Typical linear adjustment rate: 0.01%
Typical load regulation rate: 0.1%
Ripple rejection ratio: 80dB
Output short circuit protection
Over-current, overheat protection
Adjusting tube safe working area protection
Output voltage: 1.25-37V DC
Output current: 5mA-1.5A
Max input-output voltage difference: 40V DC
Min input-output voltage difference: 3V DC
Operating temperature: -10± 85℃
Storage temperature: -65± 150℃
The linear voltage regulator LM7805 has over-voltage protection, over-current protection, and over-heat protection functions, which makes its performance very stable. It is a 5V regulator, and is able to achieve output current above 1A, and has a good temperature coefficient. So the product has a wide range of applications.
- LM7805 Specifications:
5V positive voltage regulator
Min input voltage: 7V
Max input voltage: 25V
Operating current(IQ): 5mA
Internal thermal overload and short circuit current limiting protection
Max Junction temperature: 125℃
Available chip package: TO-220 and KTE
If you want to make a 5V power supply with a 7805, output currents up to 1A can be drawn from the IC provided that there is a proper heat sink. A 9V transformer steps down the main voltage, 1A bridge rectifies it and capacitor C1 filters it and 7805 regulates it to produce a steady 5Volt DC. Then you can test it, turn on the DC power supply and adjust the output voltage of about 8V or slightly larger. Or alternatively you can use a battery 9V-12V as voltage source. Look at the voltmeter panel when you set the voltage. Prepare a DC voltmeter readings on voltage range 50V to measure the output voltage of the IC 7805.