How do I convert, lower, or raise voltage?

Question

I have a project that needs a different voltage (or multiple voltages) than what I have available. Sometimes I need DC instead of AC, or I need a lower or higher voltage. How do I convert from one to another? Should I use resistors or a regulator or something else?

I've heard of converters, transformers, regulators, inverters, step-down, step-up, buck, boost, switching, linear, and other terms. Or acronyms (initialisms) like PMIC and LDO. Can someone shed some light on all this?

Answer 1

Converting voltages and current flow is among the most common of tasks in electrical engineering. Voltages are raised (stepped-up or boosted) and lowered (stepped-down or bucked) all the time to bring electrical energy to your home or business. It's also frequently necessary to change voltages in electronic projects because not all components use the same voltage.

Basics

First, it's important to understand the differences between voltage, current, and power. I won't cover it here, but there are plenty of excellent resources and books. Put simply, power \$(P)\$ in watts is the product of current \$(I)\$ in amperes (amps) and voltage \$(E)\$ in volts, or \$P=IE\$. (Easy as pie!)

Power is Conserved

When converting from one voltage to another, power is conserved. So for example, if you want to boost a voltage from 5 to 12 volts, you are limited to whatever amount of power you had to start with. If the 5 V supply is rated for 10 watts, you still only have 10 watts post-conversion. (In the real world, you have less because the conversion isn't 100% efficient. But we'll leave this out for now to make the math less complicated.) If power stays the same and voltage gets boosted, then what changes? Current. If there are 10 watts available, then at the start with 5 volts, the supply would provide a maximum of 2 amperes \$(10W = 2A\times 5V)\$. After the conversion, we have 12 volts, but only a maximum of 0.83 amperes \$(10W = 0.83A\times 12V)\$.

Step-down / Buck

There are many ways to decrease voltages. To buck voltages, the most common ways are:

1. Resistive, such as by voltage divider

   Using resistors as a voltage divider is the most crude and simple way to drop a voltage, and if all you want to provide voltage to is another resistor or a resistive load, or an analog pin on a microcontroller, this may work just fine. But many loads aren't just resistive. Often they're a combination of resistive, capacitive, and inductive. Further, complicated devices with microcontrollers or processors turn various parts on and off constantly, which means they're a variable load. A voltage divider depends on the load, so if the load has characteristics which change, then (necessarily) the voltage delivered to it will also change. For the purposes of the question about converting voltages to operate some aspect of a design or a project, this is not a viable option.

2. Linear regulator, or low-dropout regulator (LDO)

   A linear regulator employs feedback so that it can constantly adjust the output voltage based on changes. This works well for small loads, is simple to implement, doesn't require many other components, and generally inexpensive. However, they are not very efficient. In order to buck the voltage, they dissipate (get rid of) power as heat.

3. Switching regulator

   A switching regulator also uses feedback to maintain a given output voltage. It works by turning on and off (switching) at a high speed (usually 100 kHz or more) and regulating the duty cycle based on the output voltage desired and the current load conditions. External inductors and capacitors filter and smooth the on/off square wave back into DC. They can be extremely efficient and are increasingly inexpensive. They do require selecting and adding external components which can add cost, size, and complexity. (Ready-made or "off-the-shelf" DC-DC buck/boost converters are available as well, if your goal is not necessarily to incorporate a buck/boost circuit right in your design.) When looking for switching regulators, they are often categorized as a power management integrated circuit (PMIC). (PMIC's aren't limited to switching regulators, they can include other types, or multiple types.)

4. Step-down transformer, applies to AC, not DC.

   A transformer is essentially two inductors coupled so that current through one creates a current in the other. However, due to the principle of electromagnetic induction this requires a changing magnetic field. Direct Current (DC) flows in a constant direction and therefore isn't changing. Alternating Current (AC) continuously changes polarity (alternates direction), so the magnetic field is continually changing. Voltage can be stepped-down by having fewer turns on the secondary coil of the transformer. Note that there is no regulation inherent in a transformer.

Step-up / Boost

Again, there are many ways to increase voltages, including voltage multiplier circuits. However, the only two discussed here are:

1. Switching regulator

   Switching regulators can also be used to boost voltages. The same principles as described before (for bucking voltage) apply, in that external components are needed to filter and smooth the output.

2. Transformer, applies to AC, not DC.

   By having more turns in the secondary coil, a step-up transformer can increase voltage. Again, this applies only to AC, and there is no regulation without additional components/circuitry.

AC or DC?

Sometimes it can be confusing when talking about "converting" voltages. Colloquially, people will use similar terms out of convenience and not necessarily with technical accuracy. If you need to change AC to DC, that is technically called rectification. If you're changing DC to AC, it uses an inverter. (Why is it called an inverter?) Commercial products such as those found in automotive, solar, and home markets will often do all of the above. An "inverter" commonly takes a low DC voltage from a battery and "inverts" it to AC as well as boosting it to familiar mains voltage (120/240 V AC).

TL;DR

I just need to change from one voltage to another. What should I use?

The answer to this question is "it depends."

• For bucking voltage in low-power, low-noise applications, a linear regulator can still be a good, simple choice. The LM78xx series of ICs have been around a long time. Many other options exist and are often better.

• For bucking voltage when efficiency is a priority, consider a switching regulator (PMIC). You'll need to do a little more research into the passive components that support it. There is no one "go-to" IC, and many manufacturers such as Texas Instruments, Analog Devices, and Microchip offer excellent switching regulators.

• For boosting voltage, the switching regulator is again most likely your first choice.

• High-power applications require a level of knowledge and caution that this post isn't remotely intended to address. Transformers will more generally be involved here. If you're working on a high-power application, you shouldn't need this write-up. :)

• "Off-the-shelf" buck/boost converters are available if you aren't trying to incorporate something into your own design. For example, if you want to supply 48 V to something from a single 12 V battery, you can find many "DC-DC Boost Converter" already assembled and for sale online. They're usually rated in watts, and come in a variety of sizes, so make sure that you select the right one.

  Remember, stepping-up voltage means output current will be less. Likewise, stepping-down voltage means output current can be higher. For example, in the above example, if the 48 V device requires 2 A (or nearly 100 watts), a 12 V input will need to supply at least four times the current, or 12 V 8 A. Be sure your power supply or battery can meet that requirement!

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I wrote this as a means to address a common question seen on electronics.stackexchange.com: How do I convert X to Y volts? It certainly doesn't cover all cases, but it is my hope that it can be a useful reference to those who need a basic introduction to the concept.