ExpertPower & Lighting
LM317 10 mA Constant-Current LED Driver
Configure the LM317 as a simple current source and calculate LED current from the resistor between OUT and ADJ.
Parts from the database
- LM317T × 1Regulator as current source
Other supplies
- regulated 9 V DC supplyCommon build supply
- 120 Ω resistor rated 0.25 W or moreCommon build supply
- LED with known current ratingCommon build supply
- multimeterCommon build supply
- 100 nF input capacitorCommon build supply
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Connections
- Verify LM317T pinout. Connect 9 V to IN and ground to return; connect 120 Ω from OUT to ADJ.
- Connect LED anode to ADJ and cathode to ground. Put 100 nF from IN to ground.
- Nominal current ≈1.25 V/120 Ω=10.4 mA. Input needs headroom for LED forward drop, reference voltage, and regulator dropout.
- Measure current in series before raising supply voltage.
Build steps
- Build with power off and check LED polarity.
- Use a current-limited 9 V source and measure current.
- Compare with 10.4 mA estimate and calculate resistor heat as I²R.
- Do not parallel LEDs without separate current balancing.
Software and firmware setup
This is a hardware-only circuit; no firmware is required.
Test your build
Current stays near the calculated value while regulator headroom is available and thermal limits are respected.
Troubleshooting
- LED dark: trace IN→OUT→resistor→ADJ→LED→ground.
- Low current: check supply headroom and resistor value.
- Hot regulator: reduce voltage drop/current and check thermal limits.
Safety and limitations
Low-current LED demo only, not a battery charger or general supply.
Manufacturer and platform documentation
Community Notes for This Project
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