LED Drivers

Part of Lighting

How LED driver circuits supply constant current to LEDs, and how to build or adapt drivers for LED lighting systems.

Why This Matters

An LED is not a simple resistive load — it is a diode with a characteristic that makes it extremely sensitive to supply voltage. A small voltage above the threshold causes current to rise exponentially. Without a controlled current supply, a group of LEDs will either be grossly underpowered (dim and inefficient) or immediately destroyed as current races beyond safe limits.

An LED driver is the circuit that solves this: it converts whatever supply is available (battery, rectified AC, higher-voltage DC) into the controlled constant current that LEDs need. Understanding LED drivers allows a rebuilding civilization to make full use of salvaged LEDs and LED arrays from existing infrastructure, design new LED lighting systems from available components, and repair failed drivers by identifying the failed component.

This is practical electronics knowledge that sits between simply screwing in a bulb and understanding how electronic power supplies work. The principles apply beyond LED lighting to battery charging, motor speed control, and other applications where controlled current is needed.

Why LEDs Need Constant Current

An LED (light emitting diode) conducts current only above its forward voltage threshold: approximately 2.0–2.4 V for red LEDs, 2.8–3.6 V for white/blue LEDs. Below threshold, almost no current flows. Above threshold, current rises steeply — a 0.1 V increase in voltage may multiply current by 5–10×.

The relationship between LED forward voltage and current is governed by the Shockley diode equation. The practical implication: LED brightness is proportional to current (not voltage), and LED rated current (typically 20–350 mA for individual LEDs, 350 mA–1 A for high-power LEDs) must be maintained without significant deviation.

Too much current: overheating, permanent degradation of the phosphor and junction, rapid failure. A 350 mA LED running at 500 mA will be damaged within hours or minutes.

Too little current: low brightness, which wastes the LED’s potential.

Therefore: a precise, stable current source is needed. The LED driver provides this.

Linear Constant-Current Drivers

The simplest LED driver is a linear current regulator: a transistor or integrated circuit that maintains a constant current through the LED string regardless of supply voltage variations, by dissipating the excess voltage as heat.

Simple resistor-limited circuit: the most rudimentary approach is a series resistor. For a 3.3 V LED at 20 mA from a 12 V supply: R = (12 − 3.3) / 0.020 = 435 ohms. This is not truly constant current (current changes as supply voltage varies or as LED forward voltage changes with temperature) but is acceptable for non-critical applications where supply is stable.

Linear regulator IC: chips like the LM317 configured as a constant current source provide better regulation. Connect the LM317 with an adjustment resistor: I_out = 1.25 / R_adj. For 350 mA: R_adj = 1.25 / 0.350 = 3.57 ohms (use 3.6 ohm standard value). This circuit regulates current to within 1–2% across a range of supply voltages (V_supply must be at least 3 V above LED forward voltage plus adjustment voltage).

Efficiency of linear drivers: the efficiency equals V_LED / V_supply. For a 3.3 V LED from a 12 V supply: efficiency = 3.3/12 = 27.5%. The remaining 72.5% is wasted as heat in the regulator. This makes linear drivers unsuitable for battery-powered applications where efficiency matters. For mains-powered lamps, the efficiency loss is less significant.

Heat management: the LM317 or pass transistor must dissipate the excess power. At 350 mA and 8.7 V across the regulator: P = 0.350 × 8.7 = 3 W. The LM317 must be mounted on a heatsink with thermal resistance adequate to keep junction temperature below 125°C. For 3 W at a total thermal resistance of 30°C/W (IC + heatsink), temperature rise = 90°C, so junction reaches 90 + 25 (ambient) = 115°C — acceptable but marginal. Use a better heatsink.

Switching (Buck) Constant-Current Drivers

For efficient LED driving, switching regulators (buck converters configured for constant current output) are the standard approach. A buck converter uses a transistor switch (MOSFET), an inductor, and a capacitor to step down voltage with high efficiency (85–95%).

Operating principle: the MOSFET switches at high frequency (20–500 kHz). When on, current flows from supply through inductor to LEDs. When off, the inductor’s energy continues driving current through a freewheeling diode. The duty cycle (on-time / cycle-time) determines the average voltage and current to the LEDs. A feedback circuit measures LED current and adjusts duty cycle to maintain the target current.

Required components for a simple buck LED driver: one N-channel power MOSFET (rated for supply voltage and peak current), one Schottky diode (rated for output current, low forward voltage), one inductor (10–100 µH, rated for output current), filter capacitors, a current sense resistor, and a PWM controller IC (such as a 555 timer in astable mode with feedback, or a dedicated buck controller IC like the MP2307 or LM3409).

For a rebuilding civilization with access to basic electronics: a simple 555-based PWM controller driving a MOSFET and inductor can be assembled from readily available salvaged components. The design requires measurement of output current (across a sense resistor) and feedback to the 555 timer’s control voltage input. Several circuits are publicly documented and have been built by hobbyists with salvaged components.

Simple 12V LED Driver for Rebuilding Context

The most practical LED driver for a rebuilding civilization uses salvaged LED bulbs from existing infrastructure. A standard LED bulb (E27 base, rated 230 VAC input) contains an integral driver circuit and LED array — these can be salvaged as complete units.

For operation from a 12 V battery: harvest LED arrays from 12 V LED strips (flexible PCB with SMD LEDs, very common in pre-collapse buildings for under-cabinet and decorative lighting). These strips are designed for 12 V DC directly. Connect through a simple switch. No additional driver needed.

For high-power LEDs from battery: use a simple linear driver (LM317 configured for constant current) for efficiency-non-critical applications, or a simple buck converter from salvaged components for better efficiency.

LED array wiring: connect LEDs in series to increase voltage drop across the string, reducing the current that must be regulated and improving efficiency at higher supply voltages. A string of 3 white LEDs (3 × 3.3 V = 9.9 V forward voltage) on a 12 V supply needs only 2.1 V dropped in the driver, giving efficiency of 9.9/12 = 82.5% for a linear driver. Much better than a single LED at 3.3 V with 8.7 V wasted in the driver.

Dimming and Control

LED brightness can be controlled by adjusting current (analog dimming) or by pulse-width modulation (switching the LED on and off at high frequency with variable duty cycle). PWM dimming is preferred because LED color temperature and efficiency remain constant at any duty cycle; reducing current slightly changes color temperature (LEDs run cooler and shift spectrally).

PWM dimming at or above 100 Hz is invisible to most people. Below 60 Hz, the flicker is perceptible and unpleasant. Video cameras and some people are sensitive to PWM frequencies up to 1,000 Hz or more. For applications involving cameras (recording, photography) or photosensitive individuals, use analog current control (no PWM) or PWM above 1,000 Hz.

Dimming from a microcontroller: a simple 555 timer or microcontroller PWM output pin can drive the gate of a MOSFET in series with an LED string, providing PWM dimming with good frequency. This enables variable lighting control (bright for work, dim for evenings) without the heat generation of a linear dimmer.

Thermal derating: LED junction temperature rises at higher current and higher ambient temperature. Most LEDs should be derated to 70–80% of rated current if the heatsink temperature exceeds 60°C. Design heatsinking to maintain LED temperatures in the efficient operating range.