A linear power supply unit drops excess voltage as heat to achieve sub-millivolt output ripple, making it mandatory for precision ADCs, RF synthesizers, and high-end audio preamplifiers. While switching regulators dominate modern consumer electronics with 85-95% efficiency, a well-designed linear topology remains the undisputed king of low-noise power delivery. If you are building a 24-bit audio DAC or powering a sensitive load cell amplifier, the 40-60% efficiency penalty of a linear design is a worthwhile trade for an ultra-clean DC rail.
This guide walks through the design of a bench-style, adjustable 1.25V to 15V linear power supply unit capable of delivering 2A continuous current. We will cover topology selection, exact component values, thermal derating math, and the input protection required to keep the magic smoke inside the silicon.
Linear vs. Switching: Topology Comparison and Noise Floors
Before cutting copper, you must justify the thermal overhead. The decision between a linear power supply unit and a switched-mode power supply (SMPS) hinges entirely on your load’s sensitivity to high-frequency switching noise and electromagnetic interference (EMI). Switching regulators operate in the 100kHz to 2MHz range, generating harmonic spikes that can alias into audio bands or disrupt precision sensor readings.
| Parameter | Linear (e.g., LM317 + Pass Transistor) | Switching Buck (e.g., LM2596 / TPS5430) |
|---|---|---|
| Efficiency (120VAC to 12VDC) | 35% - 55% (High heat dissipation) | 85% - 92% (Minimal heat) |
| Output Noise / Ripple | < 1mV RMS (Sub-µV with LDOs like LT3045) | 15mV - 50mV p-p (Switching node ringing) |
| Transient Response | Excellent (No inductor delay, limited by loop bandwidth) | Good (Limited by inductor slew rate and compensation) |
| EMI / RFI Generation | Negligible (No high-frequency switching nodes) | High (Requires strict PCB layout and shielding) |
| BOM Cost & Size | $35-$50 (Heavy iron transformer, large heatsink) | $5-$12 (Small ferrite inductor, tiny SMD IC) |
When to choose linear: Choose a linear topology when your load includes 16-bit+ ADCs, RF VCOs, or audio op-amps where power supply rejection ratio (PSRR) degrades at high frequencies. When to choose switching: Choose an SMPS for digital logic, microcontrollers, motors, or battery charging where a few tens of millivolts of ripple won't cause functional failures.
Designing a 15V/2A Linear Power Supply Unit: Schematic and BOM
For a robust 2A output, a standalone LM317 is insufficient (it maxes out around 1.5A and would require an impractical heatsink due to its TO-220 package limits). Instead, we use the classic LM317 driving an external NPN Darlington pass transistor. The Darlington configuration provides high current gain, keeping the drive current requirements well within the LM317’s capabilities.
We use a Texas Instruments LM317 as the error amplifier and voltage reference, paired with an ON Semiconductor TIP142 Darlington transistor. The TIP142 is chosen over a standard 2N3055 because its wider Safe Operating Area (SOA) resists secondary breakdown under high voltage/high current conditions.
| Ref Des | Component | Value / Part Number | Purpose |
|---|---|---|---|
| T1 | Transformer | Hammond 162 Series 24VAC, 3A | Step-down and galvanic isolation |
| BR1 | Bridge Rectifier | KBPC5010 (50A, 1000V) | AC to full-wave DC conversion |
| C1 | Filter Capacitor | 4700µF, 50V Electrolytic | Bulk energy storage, ripple smoothing |
| C2 | Bypass Capacitor | 0.1µF, 50V MLCC/Film | High-frequency noise shunting |
| U1 | Voltage Regulator | LM317T (TO-220) | Voltage reference and error amp |
| Q1 | Pass Transistor | TIP142 NPN Darlington (TO-247) | Main current handling element |
| R1 | Sense Resistor | 6.8Ω, 2W Metal Film | Turns on Q1 when I > 205mA |
| R2, R3 | Feedback Divider | 240Ω (R2), 5kΩ Pot (R3) | Sets output voltage (1.25V - 15V) |
The Sense Resistor Math
The 6.8Ω sense resistor (R1) across the base-emitter junction of the TIP142 is critical. The Darlington pair has a base-emitter turn-on voltage ($V_{be}$) of approximately 1.4V. Using Ohm’s law ($I = V/R$), the LM317 will supply current directly to the load until the voltage drop across R1 reaches 1.4V.
$$I_{turn-on} = \frac{1.4V}{6.8\Omega} \approx 205mA$$
Below 205mA, the LM317 handles the load alone, providing ultra-fast transient response. Above 205mA, Q1 turns on and shunts the heavy current, protecting the LM317 from thermal overload.
Thermal Derating and Heatsink Math (The Reality Check)
The Achilles heel of any linear power supply unit is thermal management. You cannot guess heatsink sizes; you must calculate them based on worst-case dropout voltage.
Calculating Peak Input Voltage
A 24VAC transformer outputs 24V RMS. After the bridge rectifier, the peak DC voltage on C1 is:
$$V_{peak} = (24V \times \sqrt{2}) - 1.4V_{bridge\_drop} \approx 32.5V$$
Under a 2A load, transformer regulation will cause this to sag to roughly 30V DC, but we design for the 32.5V worst-case (light load/high mains voltage).
Worst-Case Power Dissipation
The pass transistor dissipates the most heat when the output voltage is set to its minimum while delivering maximum current. If you set the output to 3V at 2A:
$$V_{drop} = 32.5V - 3V = 29.5V$$ $$P_{diss} = 29.5V \times 2A = 59W$$
Dissipating 59W in a single TO-247 package pushes the TIP142 close to its Safe Operating Area (SOA) limits due to secondary breakdown. If your application requires sustained 3V/2A operation, you must parallel two TIP142 transistors with 0.22Ω emitter ballast resistors, or step down the transformer to 12VAC and use a buck pre-regulator (like an LM2596) to drop the bulk voltage before the linear stage. For standard 12V/2A operation, $P_{diss}$ drops to a manageable 41W.
Sizing the Heatsink
To keep the TIP142 junction temperature ($T_j$) below 110°C (well under the 150°C absolute max) in a 25°C ambient room:
$$R_{\theta JA} = \frac{T_j - T_{ambient}}{P_{diss}} = \frac{110 - 25}{41W} = 2.07 ^\circ C/W$$
Subtracting the junction-to-case ($R_{\theta JC} \approx 1.5 ^\circ C/W$) and case-to-sink thermal resistance (using thermal paste, $\approx 0.5 ^\circ C/W$), the heatsink itself must have a thermal resistance of 0.07 °C/W or lower. This requires a massive extruded aluminum profile (like a Wakefield 641K) with at least 200 CFM of forced air cooling. If you rely on passive convection, you must derate the maximum continuous current to 1A.
Input Protection, Inrush, and Ripple Expectations
A bare transformer and bridge rectifier connected to the mains is a liability. Inrush current, voltage transients, and short circuits require dedicated protection stages in a reliable linear power supply unit.
Inrush Current Limiting
When power is applied, the discharged 4700µF capacitor looks like a dead short. The inrush current can exceed 50A, tripping your 15A wall breaker or blowing the secondary fuse. Place an NTC thermistor (such as the Ametherm CL-90, 5Ω cold resistance) in series with the transformer primary. It limits inrush to a safe ~17A peak, then heats up and drops to <0.5Ω during steady-state operation.
Fusing and Transient Suppression
- Primary Side: 0.5A slow-blow fuse (handles transformer magnetizing inrush).
- Secondary Side: 4A fast-blow fuse (protects the bridge rectifier and wiring from sustained overloads).
- TVS Diode: Place a bidirectional TVS diode (e.g., 1.5KE36CA) across the secondary winding to clamp inductive kickback and mains-borne surges before they punch through the bridge rectifier.
Ripple and Noise Expectations
With a 4700µF capacitor and a 2A load, the input ripple voltage ($V_{ripple}$) before the regulator is approximately:
$$V_{ripple} = \frac{I_{load}}{f \times C} = \frac{2A}{120Hz \times 0.0047F} \approx 3.5V_{p-p}$$
The LM317 boasts a ripple rejection ratio of roughly 65dB at 120Hz. This means the output ripple will be attenuated by a factor of $10^{(65/20)} \approx 1778$.
$$V_{out\_ripple} = \frac{3.5V}{1778} \approx 1.9mV_{p-p}$$
For 95% of bench and audio applications, <2mV ripple is exceptionally clean. However, if you are powering a 24-bit DAC or an RF local oscillator where even microvolts of noise cause phase jitter, swap the LM317/TIP142 stage for an Analog Devices LT3045. The LT3045 is a modern ultra-low noise LDO that achieves 0.8µV RMS noise and 76dB PSRR at 1MHz, though it is limited to 500mA and requires meticulous PCB layout to realize its datasheet performance.
Building a linear power supply unit is an exercise in thermal and analog discipline. By respecting the SOA limits of your pass transistors, calculating exact heatsink requirements, and implementing proper inrush protection, you will yield a power source that outperforms commercial switching supplies in the metrics that matter most for precision electronics.
