When would I use a voltage regulator vs a voltage divider?

Question

When would you use a voltage regulator vs a resistor voltage divider? Are there any uses for which a resistor divider is particularly bad?

Answer 1

These two circuits types have very different applications.

A resistor divider is generally used to scale a voltage so that it can be sensed/detected/analysed more easily.

For example, say you want to monitor a battery voltage. The voltage may go as high as 15V. You are using a microcontroller's analog-to-digital converter ("ADC"), which is using 3.3V for its reference. In this case, you may choose to divide the voltage by 5, which will give you up to 3.0V at the input of the ADC.

There are a couple of drawbacks. One is that there is always current flowing through the resistors. This is important in power-constrained (battery powered) circuits. The second problem is that the divider can't source any significant current. If you start drawing current, it changes the divider ratio, and things don't go as planned :) So, it's really only used to drive high-impedance connections.

A voltage regulator, on the other hand, is designed to provide a fixed voltage regardless of its input. This is what you want to use to provide power to other circuitry.

As far as creating multiple voltage rails: For this example, let's assume that you are using switching regulators that are 80% efficient. Say that you have 9V, and want to produce 5V and 3.3V. If you use the regulators in parallel, hooking each one up to 9V, then both rails will be 80% efficient. If, however, you create 5V and then use that to create 3.3V, then your 3.3V efficiency is (0.8 * 0.8) = only 64% efficient. Topology matters!

Linear regulators, on the other hand, are assessed differently. They simply lower the output voltage, for any given current. The power difference is wasted as heat. If you have 10V in, and 5V out, then they are 50% efficient.

They have their benefits, though! They are smaller, less expensive, and less complicated. They're electrically quiet, and create a smooth output voltage. And, if there isn't much difference between the input and output voltages, then the efficiency can top a switching supply.

There are ICs which provide multiple regulators. Linear Tech, Maxim Integrated, Texas Instruments, all have a good selection. The LTC3553, for example, provides a combination of a Lithium battery charger, a switching buck regulator, and a linear regulator. They have flavors with or without the charger, some with two switchers and no linears, some with multiple linears...

One of my current products uses a 3.7V battery, and needs 3.3V and 2.5V. It was most efficient for me to a linear for the 3.3V, and a switcher for the 2.5V (fed by the battery, not the 3.3V rail). I used the LTC3553.

You'll want to spend some time on their respective website's product selector tools.

Good luck!

Answer 2

Since a voltage divider does not regulate, one would not want to use a voltage divider when one wants a regulated voltage.

A voltage regulator will, within its limits, maintain the output voltage at a fixed value even as the input voltage and load current varies.

A voltage divider will not do this. Consider the voltage divider equation:

$$v_{OUT} = v_{IN}\frac{R_2}{R_1 + R_2} - i_{OUT}\cdot R_1||R_2$$

which is manifestly dependent on \$v_{IN}\$ and \$i_{OUT}\$ so a voltage divider is not a voltage regulator.

There are, however, plenty of applications for voltage dividers, e.g., attenuation, but voltage regulation is not one of them.

Answer 3

A voltage divider is particularly bad at providing a fixed voltage to a variable or low-impedance load. Variable loads are quite common, and include most digital circuits on the planet.

Fixed, high-impedance loads can have a voltage divider in front of them. This is the case when using an ADC to measure or a comparator to fence a much larger voltage, or in the sense input of a voltage regulator.

Answer 4

Voltage dividers are not usually used to generate supply voltages because they provide no regulation. Many loads will alter their output voltage anyway, for example a resistive load to ground is essentially in parallel with R2.

Voltage dividers are usually used to provide a voltage to a high impedance input. In this case you can think of impedance as being basically the same as resistance. Having a 10M resistor in parallel with R2 won't affect it much, as long as R2 is itself orders of magnitude lower like say 10k. Of course, using low value resistors for the divider also increases the current flow through it, so cause issues for battery powered devices.

A common example of a voltage divider into a high impedance input is to divide down a high voltage to a range that an ADC can measure. Say your ADC has a 1V reference and you want to measure a 3.6V battery with it. You might use a 4:1 divider to scale that down so it is less than 1V and measurable by the ADC.

Another common example is to provide a secondary reference voltage. Say you have a 3.6V supply and need a 1.8V reference (half the supply voltage, e.g. for biasing an AC signal with a DC offset). Rather than bothering with an expensive voltage reference IC you could simply use a voltage divider to halve the supply voltage and feed that to an op-amp buffer. The op-amp has a high impedance input, and the output can be used for biasing.

A regulator can provide a certain amount of current into a load, with the voltage controlled as best it can be, so is suitable for supply voltages and the like.

Answer 5

In general avoid the use of voltage dividers to power your circuits. But, even voltage regulators may be a bad choice in many cases.

Voltage regulators change the voltage level by dissipating the excess power as heat.

Example: You have a 9v input and you are using a regulator to change it to 5v. Consider you circuit requires 1A of current.

Now, in voltage regulators, the input current is slightly higher than the output. But it is small enough to be ignored for calculations. So, we have 1A of current as output and 1A of current as input.

Output power = 1A x 5v = 5W Input power = 1A x 9v = 9W

There is a difference of 4W between the output and input. This power difference is dissipated as heat that is why often a heat sink is required. This also means that the power is getting wasted. This is a huge issue for power conscious applications such as those battery powered.

Here comes the buck converters to the rescue. For a buck converter, the output and input powers are equal if you consider 100% efficiency. However, they are generally 80-90% efficient. That means for an output of 5W, the input will be 5/0.9 (90% efficiency) = 5.55W

This is a difference of 3.45W.

Similarly, boost converters step up the output DC voltage.

Answer 6

A voltage divider is only useful when the supply and all the loads will ALL be used at all times, and none of them has a varying load.

You must only have One switch, that switches on the power, or the AC supply to the power supply. If you install other switches to turn on or off any of the various branches of the system of circuits it will fail to maintain the desired voltages you planned to provide to the remaining circuits under power.

Regulation ensures maximum voltages arent exceeded, but do not ensure the current requirement unless the supply is capable of providing an amount equal to or greater than all the possible loads.

Regulation comes in two forms, linear, where a regulator provides a buffer between a somewhat higher voltage potential and a specified design voltage that is lower. The wasted potential voltage produces heat in the linear regulator which must be dissipated via convection to air or a liquid, infrared radiation to surrounding objects, or conduction to attached larger objects. A Heatsink or Heatsink And Fan causes the heat to conduct, convect, and radiate all at the same time.

A switching power supply senses your current needs magically, sipping juice directly from the AC / Mains at a very high frequency, ensuring you dont fry something sensitive. Typically they are designed with a regulated Voltage and adjust the current needs to maintain the voltage within a small amount of the voltage setpoint. It IS possible to have a regulated Current supply, adjusting the voltage to maintain a certain amount of current, such as a battery charger, a wire EDM spark generator, an electroplating bath, an electric chair/bbq grille, or whatever dynamic voltage activity that requires a stablised current.

Switching regulators are capable of using inductors to boost or buck, that is to say they increase or decrease a voltage, and various combinations of regulation. The design parameters are Where the switch is placed in a tank circuit, whether it uses inductors or capacitors or both, the allowable input voltage types AC or DC or both, the input voltage range and the desired output types and current and voltage levels.

It is amazing to me now that devices now come with a free wall adapter that functions with 110VAV or 240VAC and also use either 50hz or 60Hz AC. This is Only possible because of switching regulators.

Linear regulators are old school, waste energy, warm to hot, and very easy to design a DC voltage regulator.

Start with a Line filter to reduce any propogated line noise from passing to your circuit. Add an AC transformer, a full wave bridge rectifier, A filter capacitor or two - one to reduce ripple another to buffer power, maybe a resistor or two in there and maybe a single inductor to help filter the DC headed towards your linear regulator. The LM317 and its negative voltage cousin the LM319 can regulate a voltage of your choice (within reason, see the data sheets) that is defined by choosing two resistors, or one fixed resistor and a variable one. The LM705, LM708, LM712 etc types regulate DC power using metal tabbed TO-220 or other package sizes both larger and smaller. LM7805 regulates 5 volt positive. LM7809 regulates 9 volts positive. LM7812 gives you 12 volts positive.

Positive linear regulators regulate a supply above ground, and negative regulators regulate power voltages below ground.

Sometimes a person might use a center tapped 24VAC transformer to make two separate 10 to 3 volt DC supplies, and they might not have both a positive and a negative regulator. They simply rectify, filter and regulate two separate supplies and use the center tap as ground for the postive supply, and one leg as the ground for the other. This works as long as the two commons are not connected accidentally when using a single heatsink. One can use insulated mounts for TO-220 case devices but then the electrical insulation limits the heat flow. Using two separated heatsinks with caution, as they are at at two different potentials. Its best to use a positive and a negative regulator when you want a +/- supply from a center tapped transformer.