What is the difference between voltage regulators, resistors, and buck converters?

Question

I am new to electronics and I would like to know the difference between voltage regulators, resistors and buck converters like the LM2596 and their advantages, disadvantages and use cases.

I tried using resistors, but as soon as there is a voltage drop at the input it also affects the output. I am not familiar with regulators but I have used buck converters like the XL4015 etc. It would be better if I used the regulators as they are small compared to what I use and I am expecting better results.

Answer 1

You are looking at components of very different types here. Let's break it down:

1. a resistor is a "passive component". That is, it has no "active" semiconductors. It is a two terminal component, which simply restricts the flow of current, according to the fundamental and well known Ohm's Law. This is the place to start if you want to understand electronics. (There are two other types of passive component, capacitors and inductors, which you also want to get a grasp of.) A resistor also dissipates power (i.e. it gets warmer due to the current passing through it).
2. a linear regulator is a device to take a varying input voltage as input, and give a constant voltage output, even though the current demanded by the load may vary. There are important caveats : the output voltage is LESS than the input, always, and current will have some practical maximum. These devices still obey Ohm's Law! If you like, you can think of them as "resistors that change their value automatically to keep output voltage constant". They do this by using transistors, which together with diodes, are the most fundamental types of "active component" (semiconductors). You will need to have a decent understanding of these to get much further. Linear regulators are not that efficient - they just burn off excessive power as heat (because they operate basically as "smart resistors"). These devices usually have three terminals - in, out, ground.
3. A buck regulator is another type of voltage regulator which uses a "switching" mechanism to do the same as a linear regulator, but in a much more efficient way. They do not have to burn power to adjust output voltage (at least, very little) - they use a "feedback" mechanism to keep it constant. (When you study amplifier circuits you will come across feedback a lot.)
4. There are also boost regulators, which can give out more voltage than they get in! How? using special properties of diodes, transistors, capacitors and inductors. When you understand the more basic elements well, you will be well placed to study this. Until then, a rough idea of what (buck and boost) converters do is enough.

Answer 2

Your question is about DC power conversion, specifically voltage conversion, even more specifically, converting a higher DC voltage (say 12v) into a lower one (say 5v).

The first method you mention is using a resistor divider. lets say you want to convert your 12v rail to 5v. You could create a circuit like this

^{simulate this circuit – Schematic created using CircuitLab} Which would produce a 5v signal at the output terminals, so great, mission accomplished? As you correctly note, connecting an output load to the circuit changes it, and the voltage division ratio changes. Let's assume we connect a 500\$\Omega\$ load.

^{simulate this circuit}

Now, we have an equivalent 250\$\Omega\$ resistor on the lower side of this divider, giving an output voltage of 3.158v. So there's a sag on the output voltage rail.

The next evolution of this design is to use some kind of electronic bias to keep the output voltage stable. Typically, this is done via a transistor held in its linear region, effectively acting as a variable resistor. Here's a simplified schematic

^{simulate this circuit}

Here we can see the op-amp is driving the transistor, and the op-amp itself is trying to match some ratio (Y/Y+Z) of the output voltage against the reference voltage X. Since op-amps don't draw any current, voltage reference X can be something much simpler. if the input voltage changes, or the output current changes, the op-amp will change the drive of the transistor, keeping the output voltage at 5V.

The huge disadvantage of this method is that the excess voltage (12-5=7v) is dissipated in the transistor. Let's say we draw 1A from the load, we're burning 7W, which is more than we're supplying to the load!

Switching regulators such as a buck regulator work on a somewhat different principle, chopping packets of energy up and exchanging voltage for current in a much more efficient way.

^{simulate this circuit}

As we can see, in this example, I've used a physical switch to turn on the transistor, but in reality, this is a more complicated controller which can take many forms and can include feedback from the load and line sides of the supply, current measurements, etc.. as it has a quite complicated job to time the on-off pulses of the transistor to chop the input voltage and uses the inductor to shape the pulses into higher current, lower voltage pulses which are smoothed by the capacitor. These devices can be massively more efficient than Linear regulators.

In short, Both linear and switching regulators can provide load and line compensation, but only switching regulators can do it efficiently. (there is also capacitative charge pumps but they tend to be only suitable for low current applications)

Answer 3

Linear regulators 'burn off the excess voltage'. So to bring voltage down two times, they burn half the power they pass. It is ok for low power or small change in voltage. Especially good for filters if voltage is almost as needed.

Buck converters efficiency is almost the same no matter the voltage ratio, about 90% or so. They are used if power is significant, more than 1w, and when voltage ripple is not important.

Answer 4

If your input voltage, output voltage, and load does not change, and you don’t care about power and noise, there is no difference between a buck converter, linear regulator, and voltage divider using resistors. This is a very rare case.

Here are the benefits and downsides of each.

Voltage divider using resistors

Benefits: simplicity (two resistors).

Downsides: The output voltage is a function of the input voltage and load current, so it’s best to not use a voltage divider when things are changing (like a load who’s current changes with time).

Linear regulator

Benefits: A handful of parts (an IC, and two capacitors, at a minimum). The output voltage is “cleaner” that then input voltage over about 0 Hz to 100 kHz range. A linear regulator can react quickly to changes in the load.

Downsides: The power dissipated approximately equals (input voltage – output voltage) * (output current), so you typically only use them for cases where the terms in parentheses are small. A linear regulator also has a dropout voltage of around 0.3 V, which means that the input voltage must be greater than the output voltage + the dropout voltage to work. Oftentimes a linear regulator needs a greater than 1 mA load current for the output to be stable.

Buck (step down) converter

Benefits: Most power efficient of the three (power in = power out / efficiency) where the efficiency is about 80 %. Can handle large output current (tens of Amps) and can also react quickly to changes in the load.

Downsides: The most complex of the three. The output voltage is noisy due to switching, and the buck converter radiates a lot of noise. The power efficiency goes to 0 % for small load currents (a linear regulator can be more power efficient for very small load currents).