📋 Overview
The LM317T Step-Down Linear Regulator Module is a versatile voltage regulation board that converts a DC input of 5–35V, or an AC input of 5–22V, to a continuously adjustable DC output of approximately 1–34V. Unlike many basic LM317T modules, this version includes a built-in rectifier bridge, allowing it to accept either DC or AC voltage input — making it especially useful for repurposing AC wall transformers and other AC power sources.
The output voltage is set using the onboard multi-turn potentiometer, which provides fine, precise control over the output. A large 1000µF input filter capacitor helps reduce output ripple, noise, and interference — cleaning up the "hash" noise commonly produced by switching wall warts and laptop power supplies. A PCB-mounted LED lights up when input power is applied, giving you a quick visual confirmation that the module is receiving voltage.
This module is a great choice when you need a clean, stable, adjustable DC voltage from a surplus power supply, wall adapter, or AC transformer. For example, a 19V laptop power supply can be used to create a clean, regulated, adjustable DC source from a few volts up to about 16V.
⚠️ Important: This is a step-down linear regulator. The input voltage must always be at least ~2.5V greater than the desired output voltage. The module cannot boost (step up) voltage. Do not exceed 35V DC or 22V AC input.
⭐ Key Features
- Dual Input Capability — Accepts both DC (5–35V) and AC (5–22V) input thanks to the onboard rectifier bridge
- Wide Adjustable Output Range — Output voltage continuously adjustable from approximately 1V to 34V DC using the onboard multi-turn potentiometer
- Low-Noise Output — Large 1000µF input filter capacitor reduces ripple, noise, and interference from switching power supplies
- Power Indicator LED — PCB-mounted LED illuminates when input voltage is applied
- Precision Adjustment — Multi-turn potentiometer allows fine-tuning of output voltage with excellent repeatability
- Built-in Protection — The LM317T IC includes internal current limiting, thermal overload protection, and safe operating area compensation
- Screw Terminal Connections — Easy-to-use screw terminals for input and output wiring — no soldering required
- Compact Design — Small form factor with integrated heatsink for the LM317T regulator
📊 Specifications
| Parameter | Value |
|---|---|
| Regulator IC | LM317T (TO-220 package) |
| Input Voltage (DC) | 5V – 35V |
| Input Voltage (AC) | 5V – 22V |
| Output Voltage | 1V – 34V DC (continuously adjustable) |
| Max Output Current (Vin − Vout < 10V) | 600mA |
| Max Output Current (Vin − Vout > 10V) | 400mA (derate to prevent overheating) |
| Minimum Dropout Voltage | ~2.5V (input must exceed output by at least this amount) |
| Line Regulation | 0.01%/V (typical, per LM317 datasheet) |
| Load Regulation | 0.1% (typical, per LM317 datasheet) |
| Input Filter Capacitor | 1000µF |
| Voltage Adjustment | Onboard multi-turn potentiometer |
| Input Rectifier | Built-in diode bridge (enables AC input) |
| Input/Output Connectors | Screw terminals |
| Power Indicator | PCB-mounted LED |
| Built-in Protections | Current limiting, thermal overload, safe operating area |
| Operating Temperature | 0°C to 100°C |
| Dimensions | Approx. 50 × 25 × 25 mm (2.0 x 1.0 x 1.0 inches) L × W × H (28 mm / 1.1 inches width including heatsink) |
| Weight | 24 grams |
🔌 Wiring / Connections
The module has two pairs of screw terminals — one pair for input and one pair for output. The terminals are labeled on the PCB.
DC Input Wiring
| Module Terminal | Connection |
|---|---|
| IN+ (Input Positive) | Positive (+) lead from your DC power source |
| IN− (Input Negative) | Negative (−) / Ground lead from your DC power source |
AC Input Wiring
| Module Terminal | Connection |
|---|---|
| IN+ and IN− | Connect one AC lead to each input terminal. Polarity does not matter for AC — the onboard rectifier bridge handles it. |
Output Wiring
| Module Terminal | Connection |
|---|---|
| OUT+ (Output Positive) | Positive (+) input of your load / device |
| OUT− (Output Negative) | Negative (−) / Ground of your load / device |
⚠️ Important: When using a DC input, observe correct polarity on the input terminals. Reversing DC polarity may damage the module. When using an AC input, polarity does not matter — connect one AC wire to each input terminal.
🔧 Setup / Configuration
- Connect your power source to the input screw terminals (DC or AC, as described above).
- Do NOT connect your load yet. It's best to set the output voltage before connecting your device to avoid accidentally applying too much voltage.
- Apply power. The onboard LED should illuminate, confirming the module is receiving input voltage.
- Measure the output voltage across the output terminals using a multimeter set to DC volts.
- Adjust the output voltage by turning the multi-turn potentiometer with a small flathead screwdriver. Turn slowly — multi-turn pots require several full rotations to sweep the full voltage range.
- Once the desired output voltage is set, remove power, connect your load to the output terminals, then re-apply power.
- Re-check the output voltage under load. The voltage may drop slightly under load — readjust the potentiometer if needed.
💡 Tip: The multi-turn potentiometer provides very fine adjustment. If you don't see the voltage changing, keep turning — it may take 10–25 full turns to go from minimum to maximum output. This is normal and is what gives you such precise control.
🔋 Power and Heat Management
Understanding the Dropout Voltage
The LM317T is a linear regulator, which means it reduces voltage by dissipating the excess energy as heat. The output voltage must always be at least ~2.5V lower than the input voltage. This minimum difference is called the dropout voltage.
For example:
- A 12V DC input can produce a maximum output of approximately 9–9.5V
- A 19V DC input (laptop power supply) can produce a maximum output of approximately 16–16.5V
- A 24V DC input can produce a maximum output of approximately 21–21.5V
Heat Dissipation and Current Derating
Because the LM317T is a linear regulator, the difference between input and output voltage is dissipated as heat. The power dissipated as heat is calculated as:
Heat (Watts) = (Vin − Vout) × Output Current
For example, if your input is 19V and your output is 5V at 400mA, the module dissipates (19 − 5) × 0.4 = 5.6 watts as heat. That's a lot of heat for a small module!
- When the input-to-output voltage difference is less than 10V, the maximum continuous output current is 600mA.
- When the voltage difference is more than 10V, keep the output current below 400mA to prevent overheating.
- At higher ambient temperatures or higher output currents, active cooling (such as a small fan providing forced airflow over the heatsink) may be required.
💡 Tip: If the heatsink feels too hot to touch comfortably (above ~60°C / 140°F), you're pushing the module close to its thermal limits. Consider reducing the load current, using a lower input voltage to reduce the voltage differential, or adding a small fan for active cooling. As a rule of thumb, try to keep the voltage difference × current below about 4–5 watts for reliable long-term operation without forced air cooling.
Using AC Input — Voltage Considerations
When using an AC power source, the effective DC voltage at the regulator input will be higher than the AC RMS voltage printed on the transformer. This is because the rectifier bridge and filter capacitor convert AC to DC at approximately the peak voltage:
DC Voltage ≈ (AC RMS Voltage × 1.414) − 1.4V
(The 1.4V accounts for the voltage drop across the two rectifier diodes in the bridge.)
For example:
- A 9VAC transformer produces approximately (9 × 1.414) − 1.4 ≈ 11.3V DC at the regulator input
- A 12VAC transformer produces approximately (12 × 1.414) − 1.4 ≈ 15.6V DC at the regulator input
- An 18VAC transformer produces approximately (18 × 1.414) − 1.4 ≈ 24.0V DC at the regulator input
⚠️ Important: The maximum AC input is 22VAC, which produces approximately (22 × 1.414) − 1.4 ≈ 29.7V DC at the regulator. This stays within the LM317T's safe operating range. Do not exceed 22VAC input.
🎯 Applications
- Surplus Power Supply Conversion — Convert laptop chargers, computer power supplies, and wall adapters into adjustable bench power sources
- AC Transformer Repurposing — Use AC wall transformers (5–22VAC) to produce clean, adjustable DC output without needing a separate rectifier circuit
- Noise Filtering — Clean up noisy, hash-filled output from switching wall warts for use with sensitive audio or radio equipment
- LED Powering — Provide a precise, adjustable voltage for LED strips and individual LEDs
- Prototyping and Breadboarding — Supply adjustable voltage to breadboard projects and prototype circuits
- Sensor and Microcontroller Power — Power sensors, Arduino, ESP32, and other microcontrollers at specific voltages
- Battery Charger Voltage Source — Provide a set voltage for simple battery charging circuits (with appropriate current limiting)
- Educational Projects — Learn about voltage regulation, linear regulators, and power supply design
💡 Tips
- Always set the output voltage before connecting your load. This prevents accidentally applying too much voltage to sensitive components.
- Use a multimeter to verify the output voltage — don't rely on the potentiometer position alone.
- Keep the input-to-output voltage difference as small as practical. A smaller difference means less heat and more efficient operation. If you need 5V output, a 7.5–9V input is much better than a 24V input.
- The 1000µF input capacitor does a great job of filtering noise from switching power supplies, but for ultra-sensitive analog circuits (audio, radio), you may want to add an additional small capacitor (10–100µF) directly at the output terminals.
- Secure your wiring. Make sure screw terminal connections are tight to prevent intermittent contact, which can cause voltage spikes or dropouts.
- Don't confuse this with a switching regulator. Linear regulators like the LM317T are less efficient than switching (buck) converters, but they produce a much cleaner output with virtually no switching noise — which is why they're preferred for audio, radio, and precision analog circuits.
- If you need more than 600mA, consider using a switching buck converter module instead. The LM317T is best suited for lower-current applications where clean, low-noise output is more important than efficiency.
- The LM317T has built-in thermal shutdown. If the IC overheats, it will shut down automatically to protect itself. If your module keeps shutting off, you're exceeding its thermal capacity — reduce the load or improve cooling.
- For AC input, use only isolated transformers — never connect mains AC (120V/240V) directly to this module. The AC input is designed for low-voltage AC from a step-down transformer, not mains voltage.
🛠️ Troubleshooting
| Symptom | Possible Cause | Solution |
|---|---|---|
| LED does not light up | No input power, input voltage too low, or reversed DC polarity | Verify your power source is connected and providing at least 5V DC or 5V AC. Check DC polarity — positive to IN+, negative to IN−. |
| Output voltage is zero or very low | Potentiometer turned to minimum | Turn the potentiometer clockwise slowly. It may take many full turns (10–25) to increase the output across the full range. |
| Output voltage won't go high enough | Input voltage too low (dropout voltage limitation) | The output can only reach about 2.5V less than the input. Use a higher input voltage to achieve a higher output. |
| Module is very hot | Large input-to-output voltage difference and/or high current draw | Reduce the load current, use a lower input voltage to decrease the differential, or add active cooling (small fan over the heatsink). |
| Output voltage drops under load | Load current exceeds module capacity, or loose wiring | Reduce load current to within rated limits (600mA max, 400mA if differential >10V). Check that all screw terminal connections are tight. |
| Output is noisy or has ripple | Poor input power quality or inadequate output filtering | Add a small capacitor (10–100µF electrolytic) across the output terminals. Ensure input connections are secure and the power source is adequate. |
| Module shuts down intermittently | Thermal protection activating due to overheating | The LM317T has built-in thermal shutdown. Reduce load current, improve ventilation, add a heatsink fan, or reduce the input-to-output voltage difference. |
| Output voltage drifts or is unstable | Loose potentiometer or vibration | Ensure the module is mounted securely. If the potentiometer is worn, the module may need replacement. |
| Smoke or burning smell | Reversed DC polarity, input voltage exceeded, or short circuit on output | Disconnect power immediately. Check all wiring for correct polarity and shorts. Verify input voltage is within rated limits. The module may be damaged and need replacement. |
⚠️ Important Notes
- This is a linear regulator, not a switching converter. It is less efficient than switching (buck) converters, especially when there is a large difference between input and output voltage. The excess voltage is dissipated as heat.
- Do not exceed 35V DC or 22V AC input. Exceeding these limits may damage the LM317T regulator and other components on the module.
- Do not connect mains AC voltage (120V/240V) to this module. The AC input is designed for low-voltage AC from an isolated step-down transformer only. Connecting mains voltage will destroy the module and create a serious safety hazard.
- Observe DC input polarity. When using a DC power source, connect positive to IN+ and negative to IN−. Reversed polarity may damage the module. (AC input has no polarity requirement.)
- Do not exceed the rated output current. Drawing more than 600mA (or 400mA at high voltage differentials) may cause overheating and potential damage.
- The output voltage is always lower than the input voltage. This module cannot boost (step up) voltage. If you need a higher output than input, you need a boost converter instead.
- Allow adequate ventilation around the module, especially at higher current loads. Do not enclose the module in a sealed space without airflow.
- This module is not short-circuit proof for extended periods. While the LM317T has internal current limiting, a sustained short circuit will cause excessive heat buildup. Always use appropriate fusing or current protection in your circuit.
🏪 Where to Buy the LM317T Module
This module is available at Envistia Mall.
Buy the LM317T DC/AC Step-Down Linear Regulator Module →
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📚 Additional Resources
- LM317 Datasheet (Texas Instruments) — Complete technical specifications, application circuits, and design guidelines for the LM317 regulator IC
Sold and supported by Envistia Mall. Ships from the USA. The manufacturer and Envistia LLC (dba Envistia Mall) are not responsible for any damages or losses resulting from the use of this product. Always follow proper electrical safety practices when working with electronic components. Specifications are based on manufacturer data and are subject to change without notice.