IC Voltage Regulators
Integrated circuit voltage regulators transformed power supply design from an art requiring careful transistor biasing and compensation into something any beginner can accomplish in minutes. A single three-pin IC replaces the entire regulation stage — error amplifier, pass transistor, feedback network, protection circuits — and delivers clean, stable, protected output voltage from a raw filtered DC input. The 78xx family of positive voltage regulators and the LM317 adjustable regulator are among the most widely manufactured and recognized components in electronics history. If you have a piece of commercial ham radio equipment from the past five decades, there is an excellent chance it contains one of these devices.
- Why IC Regulators Changed Everything
- The 78xx Fixed Positive Regulator Family
- Application Circuit and Capacitor Requirements
- Input Voltage and Dropout Voltage
- Package Types and Heat Sinking
- The 79xx Negative Regulator Family
- Built-in Protection Features
- The LM317 Adjustable Positive Regulator
- LM317 Output Voltage Calculator
- The LM337 Adjustable Negative Regulator
- Higher Current Variants: LM338 and LM350
- Building a 13.8 V Ham Shack Supply
- Comparison Table: Fixed vs Adjustable
- Experiment
Why IC Regulators Changed Everything
Before integrated circuit regulators appeared in the early 1970s, building a regulated supply meant designing a discrete error amplifier, carefully selecting the pass transistor, calculating the bias network, adding short-circuit protection, and hoping the whole thing was thermally stable. Even experienced engineers made mistakes. The result was that many pieces of equipment used unregulated supplies — the supply voltage varied with load and with the AC line voltage, which degraded performance.
The introduction of the LM309 (fixed 5 V) and then the 78xx series made precision regulation trivially easy. The entire regulator circuit collapsed to: connect the input capacitor, connect the IC, connect the output capacitor, done. The internal complexity — a bandgap voltage reference, a differential error amplifier, a power pass transistor, thermal shutdown, current limiting, and safe operating area protection — is all hidden inside a three-terminal package that costs less than a dollar.
For ham radio operators, IC regulators are important in two contexts: understanding and using them in original designs, and repairing equipment in which they have failed. Regulators fail surprisingly often — usually from overvoltage at the input, thermal runaway from insufficient heatsinking, or output short circuits that overwhelm the protection circuitry over time.
The 7812 application circuit — three components beyond the IC itself. The IC contains all regulation, amplification, and protection functions internally.
View LargerThe 78xx Fixed Positive Regulator Family
The "78" designates a positive output linear regulator (the "79" prefix is used for negative regulators). The two digits following "78" indicate the output voltage. The full standard range covers voltages from 5 V to 24 V:
| Part Number | Output Voltage | Typical Ham Use |
|---|---|---|
| 7805 | 5.0 V | Logic circuits, microcontrollers, display drivers |
| 7806 | 6.0 V | Relay coils, older logic supplies |
| 7808 | 8.0 V | Some IF circuits, older VHF equipment |
| 7809 | 9.0 V | Some portable transceiver sub-circuits |
| 7812 | 12.0 V | General circuits, relay drivers, keyer supplies |
| 7815 | 15.0 V | Op-amp supply rails, some PA bias |
| 7818 | 18.0 V | Higher voltage op-amp rails |
| 7824 | 24.0 V | Relay coils, some valve preamp supplies |
All standard 78xx regulators in the TO-220 package are rated for 1.5 A continuous output current. The actual maximum depends on the heat sinking — without a heatsink in still air, most can only deliver 100–200 mA before thermal shutdown. With an adequate heatsink, the full 1.5 A is available.
The output voltage accuracy is typically ±2% for quality devices, meaning a 7805 may output anywhere between 4.9 V and 5.1 V. This is better than any zener diode and more than adequate for digital circuits, analog bias supplies, and relay drivers.
Application Circuit and Capacitor Requirements
The minimum application circuit is simple: connect a capacitor from the input pin to ground, connect the IC with its ground pin to the circuit ground and its input pin to the filtered DC from the rectifier/filter capacitor, and connect a small capacitor from the output pin to ground. The output goes to the load.
The capacitors are not strictly optional. The input capacitor (0.33 µF ceramic) is only necessary if the regulator is physically far from the main filter capacitor. If the lead length from the filter capacitor to the regulator input pin exceeds about 7 cm (a few inches), the lead inductance can resonate with the regulator's internal capacitance and cause oscillation. The 0.33 µF damps this resonance. If your regulator sits directly on the same circuit board as the filter capacitor and the leads are short, you can omit the input capacitor.
The output capacitor (0.1 µF ceramic) is strongly recommended. It improves the transient response — when the load changes suddenly, the capacitor supplies the initial charge while the regulator catches up. It also improves stability and reduces high-frequency output impedance. Use a ceramic or film capacitor here, not an electrolytic alone, because an electrolytic has too much series inductance at high frequencies. If you add a larger electrolytic (say 10 µF) in parallel for improved transient response, keep the 0.1 µF ceramic as well.
Pin assignment for the 78xx in TO-220: Pin 1 = Input, Pin 2 = Ground (the metal tab of the TO-220 package is also connected to the ground pin and can be bolted to a heatsink without insulation), Pin 3 = Output. Always verify with the datasheet before wiring — some alternate-source parts reverse pin 1 and pin 3.
Input Voltage and Dropout Voltage
The regulator can only work if its input voltage exceeds its output voltage by at least the dropout voltage. For standard 78xx devices, the dropout voltage is typically 2.0–2.5 V. This means:
- A 7805 needs at least 7.0–7.5 V at its input to regulate to 5 V.
- A 7812 needs at least 14.0–14.5 V at its input to regulate to 12 V.
- A 7815 needs at least 17.0–17.5 V at its input to regulate to 15 V.
The input voltage also has a maximum — for 78xx devices it is typically 35–40 V (check the specific datasheet). Never apply a higher voltage than rated; the pass transistor will break down instantly.
The dropout voltage matters when designing from a transformer. A 12 V RMS transformer secondary produces a peak voltage of 12 × 1.414 = 16.97 V, which after rectification and filtering gives approximately 16.97 − 1.4 (bridge rectifier drop) − some ripple voltage. Under a 1A load with a 4,700 µF filter capacitor, the ripple might be 1.5 V peak-to-peak, so the minimum input to the regulator is about 15.5 V. This exceeds the 7812's 14.5 V minimum — the supply will regulate correctly.
If you use a transformer with too low a secondary voltage, the regulator periodically drops out of regulation at the bottom of each ripple cycle, producing a buzzing output (twice the line frequency, 120 Hz in the US). Always verify the minimum input voltage with a full load connected.
Package Types and Heat Sinking
The same 78xx regulator IC is available in several packages to suit different current requirements and mounting styles:
| Package | Max Current | Thermal Resistance (junction to case) | Notes |
|---|---|---|---|
| TO-92 (plastic small) | 0.1 A | High — limited by small size | Low-current applications only |
| TO-220 (standard tab) | 1.5 A (with heatsink) | 5°C/W junction to case | Most common for ham shack use |
| TO-3 (metal can) | 1.5 A | 4°C/W junction to case | Better thermal, older designs |
| D-PAK (SMD) | 1.5 A | Depends on PCB copper area | Surface mount, needs copper pour heatsink |
The critical calculation for heat sinking is the thermal resistance chain from junction to ambient air. The maximum junction temperature is 125°C (for continuous operation; the thermal shutdown triggers at 150°C). The power dissipated in the regulator is P = (Vin − Vout) × Iout. With a 7805, Vin = 9 V, Vout = 5 V, Iout = 1 A: P = 4 × 1 = 4 W. At 5°C/W junction-to-case and a heatsink resistance of 10°C/W (a small finned heatsink) in 25°C ambient air: Tjunction = 25 + 4 × (5 + 10) = 25 + 60 = 85°C. This is fine. Without any heatsink, the junction-to-ambient resistance for a TO-220 in free air is about 65°C/W: Tjunction = 25 + 4 × 65 = 285°C — far above the 150°C shutdown. The device would immediately trigger thermal protection and cycle on and off, delivering only intermittent output.
For a ham radio station supply delivering several amperes, a properly sized finned aluminum heatsink is essential. The heatsink area needed increases with the power dissipation. Use heat-transfer compound (thermal grease or a thermal pad) between the IC tab and the heatsink to minimize contact resistance.
The 79xx Negative Regulator Family
The 79xx series provides regulated negative supply voltages and is the complement to the 78xx family. Common members are the 7905 (−5 V), 7912 (−12 V), 7915 (−15 V), and 7924 (−24 V). They are used wherever a negative supply rail is needed — op-amp circuits, balanced mixers, AGC circuits, and many analog stages in transceivers require both positive and negative supplies.
A critical and frequently made mistake: the 79xx pinout is different from the 78xx pinout, even in the identical TO-220 package. On a 79xx in TO-220: Pin 1 = Ground, Pin 2 = Input (negative), Pin 3 = Output (negative). Many builders have destroyed 79xx regulators by assuming they share the 78xx pinout. Always check the datasheet. The confusion is compounded by some third-party pinout diagrams on the internet being incorrect. When in doubt, probe with a meter before applying power.
The 79xx requires negative input voltage at pin 2 — at least (Vz − 2.5) V below ground. For a 7912, the input must be at least −14.5 V for reliable regulation of −12 V. The dropout voltage specification is the same 2.0–2.5 V as the positive regulators.
Built-in Protection Features
One of the greatest advantages of 78xx/79xx IC regulators over discrete designs is the comprehensive built-in protection. Three independent protection mechanisms are active simultaneously:
Thermal shutdown: The IC monitors its junction temperature continuously. When the junction temperature reaches approximately 150°C, the internal thermal protection circuit cuts off the pass transistor, stopping all current flow. The IC cools down, and when the junction temperature falls back below about 140°C, the pass transistor turns back on. This means the regulator can survive a heatsink failure — it will cycle on and off, delivering intermittent output, rather than failing permanently. In practice, sustained cycling causes mechanical stress and eventually degrades the device, so a triggered thermal shutdown is a warning sign that something is wrong with the thermal design and must be fixed, not ignored.
Current limiting (fold-back): If the output current tries to exceed approximately 1.5–2 A, the internal current-limit circuit reduces the base drive to the pass transistor, preventing the current from rising further. The short-circuit current (with Vout shorted to ground) is typically around 0.75–1.2 A for 78xx devices. This means you can short the output accidentally without destroying the device — the regulator will simply limit the current and protect itself. However, if the combination of input voltage and short-circuit current exceeds the power the device can dissipate, thermal shutdown will engage as well.
Safe operating area (SOA) protection: The pass transistor is vulnerable to secondary breakdown — a failure mode where current concentrates in a small area of the silicon even at voltages and currents individually within ratings, if the combination is too extreme. The SOA protection circuit monitors both the collector-emitter voltage and the collector current simultaneously and reduces drive when their product approaches the danger zone. This is particularly important during start-up when the output capacitor is uncharged and the full Vin appears across the pass transistor while it is supplying high current.
The LM317 Adjustable Positive Regulator
The LM317 solves the problem of needing a supply voltage that is not in the standard 78xx series. Rather than having a fixed internal voltage reference connected to a fixed output pin, the LM317 holds a fixed 1.25 V between its output pin and its adjust pin. You set the output voltage by connecting two resistors that form a voltage divider from the output to ground. The adjust pin sits between these two resistors.
The output voltage formula is:
Vout = 1.25 × (1 + R2 / R1)
Where R1 connects from the output pin to the adjust pin, and R2 connects from the adjust pin to ground. The recommended value for R1 is 240 Ω.
The 1.25 V reference is called the reference voltage Vref. It appears between the output pin and the adjust pin regardless of what output voltage you have set. The internal circuit holds this voltage constant using a precision bandgap reference. Because the output pin is the high end of R1 and Vref = 1.25 V appears across R1, the current through R1 is always IR1 = 1.25 / 240 = 5.2 mA. This same 5.2 mA flows through R2, plus a small additional adjust pin current of about 50 µA. The voltage across R2 is (5.2 mA + 50 µA) × R2. The total output voltage is therefore Vref + VR2 = 1.25 + (5.25 mA) × R2, which simplifies to the formula above (the 50 µA adjustment pin current is usually negligible for practical values of R2).
Calculating R2 for a Given Output Voltage
Rearranging the formula: R2 = R1 × (Vout / 1.25 − 1) = 240 × (Vout / 1.25 − 1)
Target output: 13.8 V (the standard voltage for most ham transceivers)
R1 = 240 Ω (recommended standard value)
R2 = 240 × (13.8 / 1.25 − 1) = 240 × (11.04 − 1) = 240 × 10.04 = 2,410 Ω
Nearest standard value: 2.4 kΩ (gives Vout = 1.25 × (1 + 2400/240) = 1.25 × 11 = 13.75 V)
Alternatively, use a 2.5 kΩ trimmer potentiometer for R2 to allow precise adjustment to exactly 13.8 V after building.
R2 = 240 × (9 / 1.25 − 1) = 240 × (7.2 − 1) = 240 × 6.2 = 1,488 Ω
Nearest standard value: 1.5 kΩ
Actual Vout = 1.25 × (1 + 1500/240) = 1.25 × 7.25 = 9.06 V ✓
The LM317 operates over an adjustment range of approximately 1.25 V to 37 V, making it useful for virtually any regulated supply in the ham shack. Its current rating is 1.5 A, and with adequate heatsinking it will deliver the full current over the full voltage range. The same current-limiting, thermal shutdown, and SOA protection as the 78xx family are built in.
One practical detail: the output capacitor for the LM317 should be at least 10 µF electrolytic to ensure stability at all output voltages. Without it, the LM317 can oscillate at certain operating points. The input capacitor (0.1 µF ceramic) is recommended if the distance from the filter capacitor is more than a few centimeters.
LM317 application circuit: R1 = 240 Ω (always), R2 sets the output voltage. Use a trimmer for R2 to fine-tune the output to exactly the desired voltage.
View LargerLM317 Output Voltage Calculator
LM317 Output Voltage Calculator
Enter R1 and R2 to calculate the LM317 output voltage, or enter your target voltage to calculate the required R2 (with R1 = 240 Ω). Both modes are available below.
The LM337 Adjustable Negative Regulator
The LM337 is the negative supply counterpart to the LM317. It holds −1.25 V between its output pin and its adjust pin and can be set to any output voltage between −1.25 V and −37 V using the same two-resistor arrangement. The output voltage formula is:
The same recommendation applies: use R1 = 240 Ω and calculate R2 for the desired voltage. Pin assignments differ from the LM317 — check the datasheet. The LM337 provides the same thermal shutdown, current limiting, and SOA protection as the LM317, and its current capability is the same 1.5 A.
In a dual-supply design providing both +15 V and −15 V for op-amp circuits, you would use an LM317 for the positive rail and an LM337 for the negative rail, each fed from the corresponding half of a center-tapped transformer secondary through separate bridge rectifiers (or from a split supply).
Higher Current Variants: LM338 and LM350
When 1.5 A is not enough, pin-compatible higher-current versions of the LM317 are available. The LM350 is rated for 3 A and the LM338 is rated for 5 A. Both use the identical circuit — the same R1, R2, and capacitor values. The only changes required when upgrading from an LM317 to an LM338 are using a larger heatsink (because more power will be dissipated) and verifying that the input and output capacitors are rated for the higher current.
The LM338 at 5 A is the choice for a serious ham shack supply. Running 5 A into a modern transceiver at 13.8 V, the LM338 dissipates: P = (Vin − Vout) × I. With Vin = 17 V and Vout = 13.8 V: P = 3.2 × 5 = 16 W. This requires a substantial heatsink — a 2.5°C/W heatsink (medium sized finned aluminum) will keep the junction below 125°C in a 25°C ambient environment: Tj = 25 + 16 × (1 + 2.5) = 25 + 56 = 81°C. Perfectly safe.
Building a Regulated 13.8 V Ham Shack Supply
A 13.8 V regulated supply is the standard for modern amateur transceivers. Here is a complete design using an LM338 for 5 A continuous output.
Transformer: Secondary 18 V RMS, 200 VA (≈ 11 A RMS secondary rating; the peak current demand from the rectifier is much higher than average).
Rectifier: Bridge rectifier, 50 V PRV, 10 A rated (e.g. KBPC1010 bridge, or four 1N5408 diodes in a bridge arrangement).
Filter Capacitor: 4,700 µF, 35 V (one large electrolytic, or two 2,200 µF in parallel for better ripple handling). A higher capacitance like 10,000 µF gives less ripple and improves regulation.
Regulator IC: LM338T in TO-220 package (5 A rated, same pinout as LM317 — always verify).
R1: 240 Ω, 0.5 W (from LM338 output pin to adjust pin).
R2: 2.4 kΩ fixed + 500 Ω trimmer in series (from adjust pin to ground). This allows trimming to exactly 13.8 V.
Cin: 0.1 µF ceramic (from IC input pin to ground).
Cout: 47 µF electrolytic + 0.1 µF ceramic in parallel (from IC output to ground).
Protection diode D1: 1N4007 from output to input (protects the IC if Cout discharges back into the IC when the input is suddenly shorted). This is often omitted in simple builds but is best practice for any supply that will be left wired in the shack permanently.
Minimum input voltage check:
Peak from 18 V RMS secondary: 18 × 1.414 = 25.5 V
Less bridge rectifier drop: 25.5 − 1.4 = 24.1 V (no-load peak)
Ripple at 5 A with 4,700 µF: ΔV = I × T / C = 5 × (1/120) / 0.0047 ≈ 8.9 V peak-to-peak (worst case, full load, 60 Hz full-wave = 120 Hz ripple)
Minimum capacitor voltage: 24.1 − 8.9 = 15.2 V
LM338 dropout voltage: ~2.5 V, so minimum needed at input: 13.8 + 2.5 = 16.3 V
15.2 V is below 16.3 V — the 4,700 µF capacitor is marginal at full 5 A load. Increase to 10,000 µF for comfortable margin.
With 10,000 µF: ΔV = 5 × (1/120) / 0.010 ≈ 4.2 V, minimum = 24.1 − 4.2 = 19.9 V. Excellent margin.
This design, properly built with adequate heatsinking and safety enclosure, produces an extremely clean 13.8 V supply for HF, VHF, and UHF operation. The linear topology means virtually zero switching noise — the output noise is typically only a few hundred microvolts RMS, compared to millivolts in a poorly filtered switching supply.
Comparison: 78xx Fixed vs LM317 Adjustable
| Feature | 78xx Fixed | LM317 Adjustable |
|---|---|---|
| Output voltage | Fixed, set at manufacture | Adjustable 1.25–37 V |
| External components | 2 capacitors only | 2 capacitors + 2 resistors |
| Output accuracy | ±2–4% | ±1–2% (better bandgap reference) |
| Current limit | 1.5 A (TO-220) | 1.5 A (TO-220); LM338 = 5 A |
| Dropout voltage | 2.0–2.5 V | 2.0–3.0 V (varies with current) |
| Output noise | Moderate | Slightly lower (bandgap reference) |
| Protection | Thermal, current limit, SOA | Thermal, current limit, SOA |
| Flexibility | One voltage per device | Any voltage in range — one IC |
| Best for | Fixed voltage rails (5 V, 12 V) | Custom voltages, 13.8 V supplies |
Experiment: Verify the LM317 Formula with a Potentiometer
⚖ Experiment: LM317 Adjustable Output Verification
This experiment wires an LM317 with a potentiometer for R2 and verifies that the output voltage follows the formula Vout = 1.25 × (1 + R2/R1) as the pot is turned. You will directly observe how the output voltage changes continuously and confirm the mathematical relationship.
- LM317T or LM317L IC
- DC input supply: 12–18 V (a second regulated supply, or a 12 V wall adapter)
- 240 Ω resistor, 0.25 W (for R1)
- 5 kΩ linear potentiometer (for R2)
- 0.1 µF ceramic capacitor (input)
- 10 µF electrolytic capacitor (output, 25 V or higher)
- Digital multimeter
- Solderless breadboard and jumper wires
- Identify the LM317 pins: Output (pin 2 in TO-220), Adjust (pin 1 — the middle pin with the tab at the back), Input (pin 3). Verify with the datasheet — pin 1 is the Adjust, not ground!
- Connect the 0.1 µF capacitor from the input pin to ground, then apply your DC supply (12–18 V) to the input pin.
- Connect the 240 Ω resistor (R1) from the output pin (pin 2) to the potentiometer wiper.
- Connect the bottom of the potentiometer to ground (this is R2, set to zero initially).
- Connect the 10 µF capacitor from the output pin to ground (observe polarity — positive to output).
- Connect your multimeter (DC volts) from the output pin to ground. Turn the potentiometer fully counter-clockwise (R2 = 0). The output should read approximately 1.25 V.
- Slowly turn the potentiometer clockwise. Observe the output voltage rising. At the mid-point of a 5 kΩ pot (R2 ≈ 2,500 Ω): Vout = 1.25 × (1 + 2500/240) = 1.25 × 11.4 = 14.3 V. Note this value as you approach the mid-point.
- Turn the pot fully clockwise (R2 ≈ 5,000 Ω): Vout = 1.25 × (1 + 5000/240) = 1.25 × 21.8 = 27.3 V. Measure and record. (Do not exceed Vin minus dropout voltage — the regulator will clip.)
- Set R2 to get exactly 9 V by calculation and then verify with the meter.
The output voltage tracks the formula Vout = 1.25 × (1 + R2/R1) accurately across the full range of the potentiometer. At R2 = 0, the output is 1.25 V exactly. As R2 increases, the output rises proportionally. The measured values should agree with calculated values within 2–3%, which matches the LM317's specified output voltage accuracy. If the output does not change when you turn the pot, check that R1 is connected from the output pin (not the input pin) to the adjust pin, and that the pot wiper is correctly identified.
Frequently Asked Questions
Can I connect two LM317s in parallel to double the output current?
Not directly. Two LM317s connected in parallel without balancing resistors will not share current equally — whichever device has a slightly lower dropout voltage will try to supply all the current and may overheat while the other does almost nothing. The correct approach for higher current is to use the LM338 (5 A) or LM350 (3 A) directly, or to add a small ballast resistor (0.1–0.47 Ω) in each output lead before connecting them in parallel — this forces roughly equal current sharing. For serious current requirements (10 A or more), use a dedicated power IC or a properly designed pass transistor circuit.
Why does my LM317 regulator oscillate?
The most common cause is insufficient output capacitance. The LM317 requires at least 10 µF on its output to remain stable at all operating points. An electrolytic alone can sometimes be inadequate at high frequencies due to series inductance — always add a 0.1 µF ceramic in parallel. The second common cause is a large capacitor on the adjust pin (some designers add one for noise filtering) with no protection diode — if the input is shorted or drops rapidly, the adjust capacitor can back-bias and damage the IC. If you add a capacitor to the adjust pin, also add a 1N4148 diode from adjust to output to provide a discharge path.
What does dropout voltage mean in practical terms?
Dropout voltage is the minimum difference between the input and output voltages that the regulator requires to maintain regulation. Below this difference, the pass transistor is driven fully on but still cannot provide enough voltage headroom, and the output falls below the set voltage. In a ham supply with a 7812: if your transformer and capacitor provide only 13.5 V at the regulator input under full load, the 2.5 V dropout means the output can be at most 13.5 − 2.5 = 11 V — the regulator has dropped out of regulation. Low-dropout (LDO) regulators such as the LM2940 (1 V dropout) or LP2951 (150 mV dropout) are used where the supply headroom is tight, such as in battery-powered equipment running from a nearly discharged battery.
Can I use a 7805 to get 5 V from a 12 V car battery for mobile ham radio accessories?
Yes, and it is a common and reliable approach. The 7805 will be happy with the 12 V car battery input (well above the 7.5 V minimum). The power dissipated is (12 − 5) × I = 7 × I watts. At 500 mA this is 3.5 W — a small heatsink is needed. At 1 A it is 7 W — a larger heatsink. For currents above 1 A from a car supply, the 7805 wastes too much energy as heat and a switching regulator (buck converter) is more appropriate because it can convert 12 V to 5 V at 80–90% efficiency instead of the 42% efficiency of the linear approach.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.