Voltage Regulation, Part 1 of 3

Voltage Regulation

Introduction

Voltage regulation is a way of adapting a source voltage to a voltage that can be used by the circuit that needs to be powered. The circuit may require 3.3 or 5 VDC, but your power supply delivers 12V. What to do? Luckily there are various ways to lower, or step down, that original 12VDC to a more usable 3.3 or 5 VDC, depending on your circuit’s needs.

Of the many different ways to step down the voltage there are some that could be more adequate than others. This also depends on the needs of the circuit that needs to be powered. Besides the obvious question of how much voltage the circuit needs, another important question to ask is “How much current?” For example, if a circuit needs 500 mA of current and your power supply delivers 300 mA then I think you can see that this is going to be a problem. The power supply must always deliver more current than the circuit needs.

Although the power supply can deliver more current than is necessary it does not have to be a lot more current. If the circuit requires 500 mA then a power supply that delivers 1 A is good enough and a power supply that delivers 2 A is overkill. Knowing the requirements of the circuit and keeping the power supply at a reasonable level helps twofold: First the circuit may be smaller in size. This is a benefit especially when circuit “real estate” is scarce. Second, the power supply may be more inexpensive. Since a power supply that delivers 2 A must be more robust than one that only delivers 1 A, its components must be of a higher rating and most likely more expensive.

Interestingly enough, it is true that the power supply should deliver more current than the circuit needs; it is not true of voltage. When voltage is concerned, the power supply must deliver exactly the voltage, with a small tolerance, that the circuit needs. Voltage regulation exists precisely for this purpose so that the circuit will receive the voltage that it needs even though the power supply delivers more.

Another aspect that needs to be taken into consideration when discussing voltage regulation concerns the noise, or “ripple” that the voltage may have as it is delivered to the circuit. The ripple may be caused by various factors that won’t permit the voltage to completely stabilize or that add unwanted characteristics to the output voltage. In some cases depending on the ripple and the nature of the circuit, this may not affect the circuit’s operation, but in other cases, it can wreak havoc and make it hard to troubleshoot the circuit. The last section of this text adresses this unwanted component.

This write up concentrates on four switching regulators. The term “switching” refers to the way that the regulator works. The voltage is regulated by having a control circuit which activates (and deactivates) a switch which in turn moves a small amount of energy from the input to the output. Along with other components that store energy and the modulated pulse that controls the energy transfer a switching regulator is able to provide the desired voltage given a higher voltage to work with. This type of functionality allows the regulator be efficient, losing only about 15% (as compared to non-switching regulators that can lose around 50 – 60% of the input energy).

The venerable 7805

The 7805 linear voltage regulator. The metal tab at the top serves as a heat sink.

Efficiency is an important factor when selecting a regulator and the reason why switching regulators are often a better choice than their linear counterparts. Typically, a linear regulator steps down the voltage by “burning” the excess off as heat. This is why many linear regulators incorporate or require a heat sink. The process of stepping down the voltage by dissipating it off as heat is inefficient. If your project is battery powered you are “wasting” part of the battery’s energy supply by stepping down the voltage this way. In this case, the efficiency is around 40 to 50 %.

On the other hand, a switching regulator’s efficiency is typically around 80% and depending on the input voltage and power needs, it may go up as high as 90%. This is very good because this tells us that of the power originally supplied by the batteries (or other source) 80 to 90 % is being used by the circuit and the rest is taken up (lost) by the regulation process. Contrast that with the 40 to 50 % mentioned above. I emphasize that the efficiency of the regulator in part depends on the input voltage and in the voltage and current needs of the circuit (load) connected to the regulator. This means that the efficiency may vary for 2 circuits using the same input voltage and the same regulator if the first circuit consists of powering an Arduino (for example) with no peripherals attached and the second circuit consists of controlling and powering motors or solenoids.

As a side note, I should add that what is mentioned above about efficiency does not mean that using linear regulators is bad. There are cases when a linear regulator is preferred, but these are usually cases where the circuit has low power requirements. It is advisable to investigate each case individually so that the best regulation device is selected.

Hands on

To give the four switching regulators a try I decided to establish a series of hands on “work shops” where I used a laptop power brick and a few circuits to use as loads. Where possible I will mention the source of the components that I used. In some cases this was stuff bought from a commercial source, but in other cases this was as fringe as a garage sale.

At this time I would like to thank Eddy Wright of Wright Hobbies Robotics for providing guidance and the regulators in addition to some discrete components that were used to write up this series.

Power supplies:

  • DELL Laptop adapter, 20 VDC, 3.5A output

Circuits:

  • “Hello World” LED
  • Flood light
  • Motor (PC fan)
  • Voice module

The following are diagrams that illustrate how I connected the devices listed above to the regulators using the above mentioned power supply. 

The above is the classical “light an LED” circuit. I provided 8 VDC using the regulators.

The above circuit is how I connected the halogen lamp. The lamp was rated at 12 VDC. I fed it 8 VDC.

Next up was a PC fan. I fed it 6 VDC and then 12 VDC. The fan was rated at 12 VDC.

For the last test I connected a voice module recorder that accepted somewhere between 3 and 4.5 VDC. I fed it 4.5 VDC.

The next part of this series will describe each of the above circuits as they were connected to the four regulators. Special thanks to Wright Hobbies Robotics for providing most of the electronic components needed for this article.

Where do you want to go? Part 2 Part 3

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