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The Definition and Working of Voltage Regulator

Author: Apogeeweb Date: 18 Jan 2021  308

Increasing the Output Current of Voltage Regulators

Ⅰ Introduction

Most Integrated ICs need a constant voltage that they can work with. They have their own operating voltage, whether it's a basic Logic Gate or a sophisticated microprocessor. 3.3V, 5V and 12V are the most common operational voltages. Although we have batteries and DC adaptors that can serve as a source of voltage, because the voltage from them is not controlled, they can not be directly linked to our circuit design most of the time.

Say, we have a 9V battery, for instance, but we need to activate a 5V relay, which obviously works on 5V. What are we doing here?

Catalog

Ⅰ Introduction

Ⅱ Definition and Using of Voltage Regulator 

Ⅲ A Closer Look at Voltage Regulators

Ⅳ Three Terminal Regulators

Ⅴ Voltage Regulators: Increase the Output Current

Ⅵ Adjustable Regulators

Ⅶ Limitations of Voltage Regulator

Ⅷ FAQ

Ⅱ Definition and Using of Voltage Regulator 

You recall your school days when we were told that voltage drops from resistors. Wouldn't it be an easy fix to only use resistors according to Ohms Law to lower the voltage? But then, depending on the current flowing through them, resistors decrease voltage. The moment your part begins to draw less current, it shoots up and destroys the voltage.

You need something better; the voltage, at least not much, does not depend on the load current. The voltage divider is the next easiest repair that comes to your head. This involves two resistors, but hey, they can also operate if they can be crammed in. Another nagging problem-the moment your part begins to draw so much current, the divider sags output-the top resistor can not keep up with the current demand. Now you're really starting to wish you had heard about this in school. By reducing the resistor values, you might resolve this, but that would make the two resistors draw too much current, likely to destroy your current budget and get too hot with the immediate risk of failure.

What could be done else? Amplifying! You had, of course, to slog through hours of lectures on that. As a voltage follower, why not add an NPN transistor? The bias of the voltage divider could be connected to the foundation, the rail input of 12V to the collector and the output to the emitter part, and bingo, you solved the problem.

The repair works, of course, but it leaves you with a nagging feeling-you've used three pieces, and you find out on checking that bugs are perfectly repeated on the performance in the 12V supply rail. This is an amplifier, of course, and it doesn't have the intellect to compensate for itself. You can replace the voltage divider's bottom resistor with a Zener diode, but the current needed to correctly bias a Zener (against things such as temperature coefficients and drift) is almost as much as your part consumes, which is pointless.

Isn't there an easier way for this to happen? Isn't there a magic black box containing anything needed to effectively drop the voltage? Similar cycles of stress (including me) have influenced millions of EEEs around the world. Of course, not all issues are correlated with falling voltages, but EEE labs are popular in similar situations everywhere!

But you're in luck—there is the exact part you need. In fact, the humble voltage regulator is one of the earliest commercial implementations of the IC technology (apart from op-amps).

Definition and using of voltage regulator

If you ever look at a voltage regulator's datasheet, you'll be amazed at the circuitry with which they have been packed to drop a voltage and keep it clean-a nice stable voltage regulator, feedback and compensation amplifiers, and a half-good power level. Of course, if we were able to cram so much technology into our own phones, why not make a nice TO-92 kit with some voltage control?

Some of them consume no more than a few nanoamps, which is a thousandth of a millionth of an amp! They keep getting stronger every day. Even better, some come with protection against short circuits and overtemperature, rendering them foolproof.

Ⅲ A Closer Look at Voltage Regulators

The primary role of a voltage regulator, as we have seen in the section above, is to drop a larger voltage to a smaller one and keep it steady, as the regulated voltage is used to power (sensitive) electronics.

As mentioned above, a voltage regulator is essentially a beefed-up emitter follower-a transistor linked to a stable reference that spits out a constant voltage, dropping the remainder.

They also have an error amplifier built-in, which samples the output voltage (through a divider again), compares it to the reference voltage, calculates the difference, and drives the output transistor accordingly. This is far from a voltage divider, which replicates the input signal faithfully, but at a smaller magnitude. You don't want to see your DC voltage rail overlaid with an AC ripple.

voltage regulators 2

A transistor with a high gain is ideal, because power transistors are a massive pain to drive, with pathetic gains in the two-digit range. By using Darlington transistors and, more recently, MOSFETs, this has been solved. As these types require less power to drive, there is a decrease in overall current consumption. This is balanced by the fact that very little current is often absorbed by the voltage reference used internally.

The current absorbed by the regulator to drive all this internal circuitry is called the quiescent current when the output is not loaded. The lower the current of silence, the stronger.

There are three transistors on the power output level, two of them in a Darlington configuration and the other as a current limiting unit, the way these regulators are designed. The successive CE junctions add up to a voltage drop across the regulator of about 2V.

This voltage is known as the voltage dropout, the voltage at which the regulator ceases controlling.

With a voltage drop of about 0.4V, you can find devices called LDOs or low dropout regulators, because they use a MOSFET switch.

Ⅳ Three Terminal Regulators

Enough speaking, now for the actual numbers of the pieces.

The 78XX series is the most common series of voltage regulators. For example, the 7805 is a 5V regulator and the 7812 is a 12V regulator. The two digits after the 78 reflect the output voltage of the regulator. A wide range from 3.3V to 24V covers the output voltages available with fixed regulators with pleasant values such as 5V, 6V, 9V, 15V and 18V available.

For most purposes, this series of regulators are outstanding, they can handle up to almost 30V at the input and up to 1A output current depending on the kit. Attach the input pin to the input voltage and the output pin to the unit that requires the lower voltage and, of course, the ground pin to ground. They are exceptionally easy to use.

Since the feedback amplifiers 'reject' input ripple and noise, ensuring that they do not move on to the output, decoupling capacitors are optional here. However, if more than a few tens of milliamps are drawn by your unit, at least 4.7uF on the input and output is recommended, preferably in ceramic.

Using these regulators, an odd thing people do is make rudimentary phone chargers. Only add a 9V battery to the input and a suitable USB connector to the output, and you've got an emergency phone charger for yourself. Thanks to the built-in thermal safety on the chip, this design is very robust.

A positive thing about these kinds of voltage regulators is that the pinouts are almost interchangeable, so it is possible to plug-in replacements. Most of the 'transistor' packages on PCBs nowadays are voltage regulators that can be picked up because they are so easy to use for other projects.

Ⅴ Voltage Regulators: Increase the Output Current

The performance current, which is heavily restricted by the package and the way the package is installed, is one limitation that easily overcomes the utility.

These regulators have high-current versions, but they are difficult to identify.

DC-DC switching converters are the only machines capable of spitting out high currents, but the performance noise figures are awful.

It is possible to build your own high current linear regulator, but inevitably you will run into all the above-mentioned issues.

Luckily, with a few extra bits, there is a way to 'hijack' a normal regulator and increase the product currently.

voltage regulator circuit

Most of these modifications include inserting a bypass transistor across the regulator and, as shown in the figure below, driving the base with the input.

Ⅵ Adjustable Regulators

It's very pleasant and simple to use three-terminal regulators, but what if you want a non-standard output voltage like 10.5V or 13V?

Of course, fixed regulators can be hijacked more or less, but the necessary circuitry is very complex and beats the primary objective of simplicity.

Devices exist that can do the job for us, with the LM317 being the most common.

voltage regulator LM317

The LM317 is just like every other linear regulator with an input and an output pin, except there's a pin named 'adjust' instead of a ground pin. This pin is intended to receive input through the output from a voltage divider such that the pin is always at 1.25V, we can obtain various voltages by changing the resistance values. The datasheet also states,' removes several fixed voltages being held,' but this only applies, of course, if you can afford to have those two resistors on board.

A good thing about adjustable regulators like this is that they can also act as continuous current supplies with a minor configuration change.

The regulator aims to maintain a constant 1.25V throughout the output resistor and thus a constant current on the output by attaching a resistor to the output pin and the adjustment pin to the other end of the resistor as shown in the figure. For the diode laser group, this simple circuit is very common.

This can also be achieved by fixed regulators, but the dropout voltages are unreasonably high (in fact, the rated output voltage). However, they can work in a pinch if you're desperate.

Ⅶ Limitations of Voltage Regulator

The greatest benefit of linear regulators is their simplicity; it is not important to say anything else. However, they come with their own set of limitations, like all good chips.

Linear regulators work with feedback like a variable resistor, falling any unneeded voltage. The same current as the load is drawn when drawing. This wasted energy is converted to heat, rendering these regulators at high currents warm and inefficient.

A 5V regulator with a 12V input that runs at 1A, for example, has a power loss of (12V-5V)*1A, which is 7W! That's a lot of wasted energy and that's just 58 percent production!

So, regulators have pathetic energy efficiency at high input-output voltage differentials or high currents.

Using more than one regulator in a series of decreasing output voltages (up to the desired voltage value), the input-output differential voltage problem can be solved so that the voltage is lowered in steps. Although the total dissipation of power is the same as having one regulator, the heat load is distributed through all devices, reducing the overall operating temperature.

By using a switching supply, the power and efficiency constraints can be resolved, but the option is application-dependent, there are no straight cut rules as to when to use which type of power supply.

Ⅷ FAQ

1. What is Dropout Voltage or headroom in Voltage regulators?

A linear regulator such as the famed 7805 outputs 5.0 volts. The dropout specification is going to be about 2 Volts typical, 2.5 maximum. That means it will regulate 5 V as long as the input unregulated voltage is above 2 to 2.5 V above the regulated output voltage of 5 V. That gives it a 2 volt (7 minus 5) headroom.

The headroom is considered to be the minimum input-output differential it can maintain. if the input falls to 6.5 volts the regulator outpupt can be expected to be about 4.5 volts. It means, counting diode drops, and ripple amplitude, you must keep above the dropout voltage or you will see the ripple in your output.

2. How does a voltage regulator work?

It works on the principle of detection of errors. The output voltage of an AC generator obtained through a potential transformer and then it is rectified, filtered and compared with a reference. The difference between the actual voltage and the reference voltage is known as the error voltage. This error voltage is amplified by an amplifier and then supplied to the main exciter or pilot exciter.

Thus, the amplified error signals control the excitation of the main or pilot exciter through a buck or a boost action (i.e. controls the fluctuation of the voltage). Exciter output control leads to the controls of the main alternator terminal voltage.

3. Can a voltage regulator convert AC to DC?

Depends on the topology and the circuit components that are used.

A circuit that converts AC to DC is called a rectifier. Additional circuits like buck-boost converters can be used to regulate the DC.

In a generic sense, most voltage regulators are marketed for AC systems. They are back to back converters which rectify AC to DC and then invert DC to AC after suitable modification to the wave shape. It is possible to take the intermediate DC output after the rectification stage and suitably modify it with further circuitry.

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