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Voltage RegulatorsšŸ”—

Beginner

This article is in the Power topic. It builds on Ohm's Law and Power, Diodes and LEDs, and Transistors, and uses the capacitors and inductors from Circuit Foundations. No other prior knowledge required.

Two parts can each turn a 12 V supply into a steady 5 V for a load that draws 3 A. One of them, a linear regulator, takes 36 W from the supply to do it and turns 21 W of that into heat. The other, a switching regulator, takes under 19 W and wastes less than 4. Both deliver exactly the same 15 W to the load.

Two ideas explain the difference:

  1. A linear regulator is a transistor that soaks up the difference. It sits between the supply and the load, constantly adjusting itself so the load sees exactly the right voltage, and every volt it removes becomes heat. Its input current always equals its output current.
  2. A switching regulator chops the supply on and off and smooths the result. A switch that's fully on or fully off wastes almost nothing, so the energy goes to the load instead of into heat, and the input current can be smaller than the output current.

The numbers in the hook come out of those two ideas, with real datasheets, in the second half.

Why Regulate at All?šŸ”—

Batteries sag as they discharge (Cells and Batteries explains why), rectified AC arrives in humps, and every supply's voltage dips when the load draws more. Many circuits don't tolerate that: a microcontroller wants 5 V or 3.3 V within a few percent, and the analog readings on Reading an Analog Sensor are only as accurate as the voltage they're measured against.

A voltage regulator turns a varying, too-high input into a fixed output that stays put while the input and the load change. The simplest one is already on this site: Diodes and LEDs showed a Zener diode holding 5.1 V. It works, but only for light loads, and it wastes current even with no load at all. Real regulators build on the same idea.

Idea One: A Transistor That Soaks Up the DifferencešŸ”—

A linear regulator puts a transistor, the pass transistor, in series between the input and the output. A small circuit inside compares the output with a fixed internal reference voltage and adjusts the transistor continuously: if the output drifts high, it turns the transistor down; if it sags, it turns it up. The transistor behaves like a resistor that keeps resetting itself to whatever value leaves exactly 5 V at the output.

A 12 volt amber column on the left and a 5 volt green column on the right, with a glowing red pass transistor between them that drops the 7 volt difference as heat. A dashed feedback loop from the output back to the transistor compares the output with a fixed reference and opens or closes the transistor to hold it at 5 volts.

The feedback loop does the regulating. The pass transistor does the dropping, and pays for it in heat.

The classic part is the 7805, a three-terminal regulator (input, ground, output) that TI's LM340 / LM7805 datasheet rates for up to 1.5 A at 5 V. It needs almost nothing around it: the datasheet's measurements use a 0.22 ĀµF capacitor at the input and 0.1 ĀµF at the output.

Schematic of a 7805 regulator: a 12 volt battery feeds the regulator's input pin, with a 0.22 microfarad capacitor from input to ground. The ground pin goes to the common negative rail. The output pin, labelled 5 volts, has a 0.1 microfarad capacitor to ground and feeds a load.

Three pins and two small capacitors. The capacitors keep the regulator's feedback loop stable.

Every Volt Removed Becomes HeatšŸ”—

The pass transistor carries the full load current and has the difference between input and output across it. By Ohm's Law and Power, it dissipates

\[ P_{\text{heat}} = (V_{\text{in}} - V_{\text{out}}) \times I \]

The input current is the same as the output current (plus a few milliamps the regulator uses itself: about 6 mA for the 7805), so the efficiency is simply the output voltage over the input voltage. From 12 V to 5 V, that's 5 Ć· 12, about 42%. Whatever the load, more than half the energy becomes heat.

That heat has to go somewhere, and the datasheet says how fast it can. A 7805 in the common TO-220 package (Package Types shows one) has a junction-to-ambient thermal resistance of 23.9 Ā°C per watt with no heat sink: every watt raises the chip 23.9 Ā°C above the room. Its maximum operating temperature is 125 Ā°C, and above 150 Ā°C it shuts itself down to survive.

A line of chip temperature against load current for a 7805 regulator in a TO-220 package with no heat sink, dropping 12 volts to 5. At 0.1 amps it dissipates 0.7 watts and reaches 42 degrees; at 0.3 amps, 2.1 watts and 75 degrees; at 0.5 amps, 3.5 watts and 109 degrees. It reaches its 125 degree maximum at about 0.6 amps, and above 150 degrees it shuts itself down.

Rated for 1.5 A, but on its own, from 12 V, it reaches its limit at about 0.6 A.

The datasheet gives the limit as Pmax = (TJ max āˆ’ Tambient) Ć· ĪøJA. In a 25 Ā°C room that's (125 āˆ’ 25) Ć· 23.9 ā‰ˆ 4.2 W, or about 0.6 A at a 7 V drop. Beyond that the regulator needs a heat sink: a finned metal block bolted to the TO-220's tab that gives the heat a larger surface to leave through. For 1 A (7 W) the whole path from chip to air would need to be under (125 āˆ’ 25) Ć· 7 ā‰ˆ 14 Ā°C/W.

That's why, in a mains power supply, the regulator is the stage that usually wears a heat sink.

Definition: Thermal Resistance

How many degrees a part's temperature rises above its surroundings for each watt it dissipates, in Ā°C/W. Lower is better. A heat sink lowers it by giving the heat more surface area to escape from.

Dropout: the Headroom a Regulator NeedsšŸ”—

A linear regulator can only remove voltage, and the pass transistor needs some voltage across it to work at all. The minimum is the dropout voltage: 2 V at 1 A for the 7805, so it needs at least 7 V in to deliver 5 V. Below that, the output simply follows the input down.

Output voltage against input voltage for two 5 volt linear regulators, simplified. Below a knee the output follows the input minus the dropout voltage; above it the output holds at 5 volts. The 7805 needs 7 volts in, with 2 volts of dropout at 1 amp. The low-dropout LM1117-5.0 needs 6.2 volts, with 1.2 volts of dropout at 800 milliamps.

Above the knee, a regulator. Below it, just a voltage drop.

A low-dropout regulator (LDO) is built to need less. TI's LM1117 drops out at 1.2 V at 800 mA, which is what lets a 5 V USB supply feed a 3.3 V circuit through a linear regulator: 5 āˆ’ 3.3 = 1.7 V of headroom, comfortably above 1.2 V.

Dropout matters most on batteries: a regulator with a small dropout keeps working further into the battery's discharge. And a low input voltage is good for efficiency too, since less headroom means less heat.

Adjustable RegulatorsšŸ”—

Fixed regulators come in common voltages (the 78xx family includes 5, 12, and 15 V parts). For anything else, an adjustable regulator such as the LM317 sets its output with two resistors: Voltage Dividers works through its formula. Its heat follows exactly the same rule as the 7805's.

Idea Two: Switch It, Then Smooth ItšŸ”—

A transistor wastes almost no power in two states: fully off, when no current flows through it, and fully on, when almost no voltage is across it. Transistors showed that the waste happens in between, and a linear regulator lives permanently in between. A switching regulator never does. It flicks a transistor fully on and fully off many thousands of times a second, and smooths the chopped result back into steady DC.

The step-down version is called a buck converter. Switched on 42% of the time, 12 V averages 12 Ɨ 0.42 = 5 V:

Two traces. Top: the voltage at a buck converter's switch, a square wave between 0 and 12 volts that is on 42 percent of each cycle, averaging 5 volts, switching 150,000 times a second on the LM2596. Bottom: after the inductor and capacitor, a steady 5 volts with a small ripple.

The controller adjusts the on-time to hold the output at 5 V as the input and load change.

The smoothing is done by an inductor and a capacitor, the two parts that store energy:

Schematic of the idea of a buck converter: a 12 volt battery feeds a switch, marked 150 kilohertz, then an inductor to the 5 volt output, which has a capacitor and a load to ground. A diode runs from ground up to the junction between the switch and the inductor.

The real controller that times the switch is left out; the power path is just these four parts.
  • Switch on: current flows from the input through the inductor to the load, and builds up in the inductor's magnetic field.
  • Switch off: an inductor resists any change in its current (Inductors), so the current keeps flowing, now drawn up through the diode from ground. The energy stored in the field feeds the load until the switch comes back on.
  • The capacitor irons out what's left, so the load sees a steady 5 V with a small ripple.

The Hook, SolvedšŸ”—

TI's LM2596 is a common buck regulator chip, switching at 150 kHz and rated for 3 A. Its datasheet gives the 5 V version an efficiency of 80% at 12 V in and 3 A out. Compare it with a linear regulator doing the same job:

Two stacked 3D bars for making 5 volts at 3 amps from 12 volts. A linear regulator takes in 36 watts: 15 go to the load and 21 become heat, 42 percent efficient, drawing the full 3 amps from the supply. The LM2596 switching regulator takes in 18.75 watts: 15 to the load and 3.75 as heat, 80 percent efficient, drawing only 1.56 amps.

Same output. The linear regulator draws twice the input current and turns more power into heat than it delivers.
Linear Switching (LM2596)
Power to the load 15 W 15 W
Power from the 12 V supply 15 Ć· 0.42 = 36 W 15 Ć· 0.80 = 18.75 W
Heat 21 W 3.75 W
Current from the supply 3 A 18.75 Ć· 12 ā‰ˆ 1.56 A

A switcher's input current is smaller than its output current when stepping down, something no linear regulator can do: power in is roughly power out, so lower voltage at the output comes with higher current. (In practice the 7805 couldn't do this job at all: its 1.5 A rating is half of the 3 A needed.)

What Switching CostsšŸ”—

Efficiency isn't free. A switching regulator needs more parts, and its fast edges make electrical noise: the 150 kHz switching and its harmonics can couple into nearby circuits and leak onto the supply wires. A linear regulator makes essentially none; the 7805 datasheet even specifies how well it removes ripple, 68 to 80 dB at 120 Hz, which turns 0.6 V of ripple into a fraction of a millivolt.

Linear Switching
Efficiency Vout Ć· Vin; poor for big drops 73ā€“90% for the LM2596's versions at full load
Heat (Vin āˆ’ Vout) Ɨ I small
Size and weight larger: heat sinks, big transformers smaller and lighter
Electrical noise very low switching noise to filter
Parts count three pins and two capacitors controller, inductor, diode, capacitors
Can step up no yes (boost converters)

So the choice follows the job. Battery-powered and portable equipment, phone chargers, and laptop supplies use switchers for their efficiency, size, and weight. Sensitive analog circuits and radio receivers, where switching noise would be heard, are where linear supplies keep their place. Switchers can also step voltage up: the camera flash in Capacitors charges its 300 V capacitor from two AA cells with a boost converter.

Putting It Together: a Linear Power SupplyšŸ”—

A power supply that runs from the wall chains together parts from all over this site. Each stage hands the next a cleaner, steadier voltage:

Four 3D blocks in a row, each with the waveform it produces drawn above it. The transformer steps the mains voltage down and isolates it: a smaller sine wave. The rectifier turns it into one-way pulses: a row of humps. The filter capacitor smooths the pulses: a high line with a small ripple. The regulator, which needs a heat sink, holds the output perfectly steady: a flat line.

Transformer, rectifier, filter, regulator: each stage fixes one problem the one before it left.

Schematic of a linear power supply: 120 volt AC mains feeds a transformer's primary; its secondary feeds the two AC corners of a four-diode bridge rectifier; the bridge's positive output has a filter capacitor to ground and feeds a regulator's input; the regulator's output is the steady DC output, with its ground pin on the common 0 volt rail.

The same four stages as a schematic.
  1. The transformer steps the 120 V mains down to a lower AC voltage and, because its two windings aren't connected, isolates everything after it from the mains (Magnetism and Electromagnetism explains how).
  2. The rectifier turns that AC into one-way pulses. A bridge of four diodes gives about 15.5 V peaks from 12 V AC (Diodes and LEDs).
  3. The filter, a large capacitor, fills at each peak and feeds the load between them. Capacitors works the numbers: 2,200 ĀµF sags only about 0.6 V between peaks.
  4. The regulator removes what's left of the ripple and holds the output constant as the load changes. A 7805 here sees about 15.5 V in, well above its 7 V minimum even at the bottom of the ripple, and at 150 mA dissipates (15.5 āˆ’ 5) Ɨ 0.15 ā‰ˆ 1.6 W: within its limit, but warm enough that many designs add a small heat sink anyway.

Bench and station supplies add protection after the regulator. An overvoltage circuit watches the regulator's output, where a failure would reach the equipment, and if the voltage climbs too high a crowbar circuit shorts the output deliberately, blowing the fuse or tripping the supply's current limit before the overvoltage can do damage (Crowbar on Wikipedia).

SafetyšŸ”—

The low-voltage side of a supply is safe to experiment with; the parts that make it are the dangerous ones.

Mains Supplies Hold Lethal Voltages

The primary side of any mains power supply is at mains voltage, and the filter capacitors on either side can stay charged after it's unplugged (Capacitors). Don't open a mains-powered supply unless you've been trained to; build regulator circuits from batteries, a USB supply, or a commercial low-voltage adapter, so the mains stays inside a sealed, certified box.

Regulators Get Hot

A linear regulator dropping a few volts at a few hundred milliamps is hot enough to burn a fingertip, and its tab is connected to one of its pins, so bolting two to the same heat sink without insulating washers can short them together. Check the datasheet's thermal numbers before building, not after something smells hot.

PracticešŸ”—

1. How Much Heat?

A 7805 supplies 200 mA at 5 V from a 9 V battery. How much power does it turn into heat, and what's its efficiency?

Solution
\[ P = (9 - 5)\ \text{V} \times 0.2\ \text{A} = 0.8\ \text{W} \]

Efficiency is 5 Ć· 9 ā‰ˆ 56%. Lowering the input voltage, as long as it stays above the 7 V minimum, is the cheapest way to make a linear regulator run cooler.

2. Does It Need a Heat Sink?

A 7805 in TO-220 (23.9 Ā°C/W, 125 Ā°C maximum) drops 12 V to 5 V for a 400 mA load in a 30 Ā°C room. Is it within its limit without a heat sink?

Solution

Power: (12 āˆ’ 5) Ɨ 0.4 = 2.8 W. Temperature rise: 2.8 Ɨ 23.9 ā‰ˆ 67 Ā°C, so the chip reaches about 30 + 67 = 97 Ā°C. That's within the 125 Ā°C limit, but hot; a small heat sink would give it margin.

3. Will It Regulate?

A 7805 (2 V dropout) is powered from four AA cells in series, which start at about 6 V. Will it hold 5 V? What about an LDO with 0.3 V of dropout?

Solution

The 7805 won't: it needs at least 7 V, so from 6 V its output follows the input down to about 4 V. An LDO with 0.3 V of dropout needs only 5.3 V, so it regulates until the cells sag below that.

4. Input Current

A buck converter delivers 5 V at 2 A with 85% efficiency, from a 12 V supply. How much current does it draw? How much would a linear regulator draw?

Solution

Output power is 10 W, so the input is 10 Ć· 0.85 ā‰ˆ 11.8 W, and 11.8 Ć· 12 ā‰ˆ 0.98 A. A linear regulator draws the full 2 A, because its input current always equals its output current.

5. Name the Stage

In a linear power supply, which stage converts AC to pulsating DC, which smooths it, which isolates the circuit from the mains, and which usually needs a heat sink?

Solution

The rectifier converts AC to pulsating DC. The filter (capacitor) smooths it. The transformer isolates it from the mains and sets the voltage. The regulator usually needs the heat sink, because it turns the excess voltage times the load current into heat.

6. Linear or Switching?

For each, which would you choose: a battery-powered sensor that must run for months; a supply for a sensitive radio receiver; a 5 V, 3 A supply from 24 V?

Solution

Switching for the battery sensor, since efficiency is battery life. Linear for the receiver, since it makes no switching noise to be heard. Switching for 24 V to 5 V at 3 A: a linear regulator would turn (24 āˆ’ 5) Ɨ 3 = 57 W into heat.

Quick RecapšŸ”—

  • A regulator holds the output

    A fixed output from a varying input, steady as the load changes.

  • Linear: the transistor soaks up the difference

    Heat = (Vin āˆ’ Vout) Ɨ I. Input current equals output current. Efficiency ā‰ˆ Vout Ć· Vin.

  • Thermal resistance sets the limit

    Ā°C per watt. A TO-220 7805 from 12 V to 5 V reaches its limit near 0.6 A without a heat sink.

  • Dropout is the minimum headroom

    2 V for a 7805, about 1.2 V for the LM1117 LDO. Below it, no regulation.

  • Switching: chop and smooth

    A transistor fully on or fully off wastes little. An inductor and capacitor smooth the result.

  • Switchers trade noise for efficiency

    Smaller, lighter, cooler, and able to step up; but they make switching noise. Linear supplies stay quiet.

  • The supply chain

    Transformer (step down and isolate), rectifier (AC to pulses), filter (smooth), regulator (hold steady, heat sink).

  • Protection after the regulator

    Overvoltage is sensed at the regulator's output; a crowbar shorts it and blows the fuse.

What's NextšŸ”—

A regulator is only as good as the supply behind it. On the bench, an adjustable one with a current limit is the safest way to power a new circuit: Bench Power Supplies covers it. Cells and Batteries covers the portable end: how cells sag, and why internal resistance decides how much current they can deliver. On the mains side, AC vs DC explains the waveform every power supply starts from.

Further ReadingšŸ”—

Datasheets

Deep Dives

Related Articles

  • Diodes and LEDs ā€” rectifiers and the Zener reference
  • Capacitors ā€” smoothing, and the energy a filter capacitor holds
  • Inductors ā€” the energy store at the heart of a switching regulator
  • Package Types ā€” the TO-220 package and its heat-sink tab