LED Driver Circuit Diagram

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An LED driver regulates current through the LED — not voltage — because LED brightness and junction temperature are both direct functions of current, and small voltage variations produce large and damaging current swings.

An LED is not a resistive load. Its forward voltage versus current characteristic is exponential: a small increase in applied voltage produces a disproportionately large increase in current. For example, increasing forward voltage by 10% on a typical white LED can increase current by 200% or more. This is why powering an LED directly from a fixed voltage source through only a series resistor is unreliable — component tolerances, temperature variation (which shifts the LED's forward voltage), and supply voltage variation all produce significant changes in LED current and therefore brightness, colour point, and lifetime.

A purpose-designed LED driver solves this by regulating current, not voltage.

**Constant-current drivers**

A constant-current (CC) driver maintains a fixed output current regardless of the load voltage. LEDs connected in series share the same current; the driver adjusts its output voltage to maintain that current as the LED forward voltages vary with temperature and age. Constant-current drivers are specified by their output current (e.g., 350 mA, 700 mA, 1 A, 1.5 A) and an output voltage range (e.g., 25–42 V) that the driver can regulate across.

This is the correct topology for driving LED strings — each LED in the series string sees identical current, giving identical brightness from every LED regardless of manufacturing spread in forward voltage.

**Constant-voltage drivers**

A constant-voltage (CV) driver maintains a fixed output voltage (typically 12 V DC or 24 V DC) with a maximum rated current. This is not a current-regulated supply — if a single LED or LED strip is connected without its own current-limiting (whether a built-in resistor on the LED strip PCB or an external current limiter), the LED can over-drive destructively. LED strip lights sold for 12 V DC operation have resistors built onto the PCB for each group of LEDs, making them compatible with a 12 V constant-voltage driver.

**Topology options**

Common LED driver circuit topologies: - **Linear regulator with current sense resistor:** simplest design; the sense resistor sets the LED current via negative feedback. Low efficiency (wastes power as heat in the pass element); only suitable where the supply voltage slightly exceeds the LED forward voltage. - **Buck (step-down) switcher:** for supplies higher than the LED string voltage. Efficient (85–95%). Inductor, switch, diode, and current sense resistor; the control IC adjusts duty cycle to maintain constant inductor (LED) current. - **Boost (step-up) switcher:** for supplies lower than the LED string voltage. Less common in mains-powered applications but used in battery LED drivers. - **Flyback switcher:** provides isolation between mains and LED output; standard topology for mains-connected LED drivers where safety isolation is required.

**Dimming**

LEDs are dimmed by either: 1. **PWM dimming:** switching the LED current on and off at high frequency (typically 200 Hz–20 kHz). LED brightness is proportional to duty cycle. PWM preserves colour quality because current magnitude does not change. 2. **Analogue dimming:** reducing the LED current magnitude. Simple but shifts LED colour point slightly at low currents.

**Thermal management**

LED junction temperature directly affects lifetime and lumen depreciation. Heat must flow from the junction through the PCB or metal-core substrate to a heatsink. LED driver thermal derating — reducing maximum output current at high ambient temperature — is essential for reliable operation.

This is a generic educational reference. Mains-connected LED drivers must comply with IEC 61347 (lamp control gear), IEC 60598 (luminaires), and applicable local electrical installation codes.

How to wire led driver circuit diagram

Determine the LED string configuration Decide how many LEDs are connected in series (determines the required driver output voltage range) and in parallel strings (determines the total output current). For best uniformity, use series strings driven by a single constant-current source rather than multiple parallel LEDs from one current source.
Calculate total LED string forward voltage Sum the typical forward voltages of all LEDs in one series string. For example, ten white LEDs at 3.1 V typical = 31 V total. Select a constant-current driver with an output voltage range that covers the expected range, accounting for temperature variation (Vf decreases approximately 2–3 mV per degree Celsius rise).
Select the constant-current driver Choose a driver with an output current matching the LED's rated forward current. Verify input voltage range covers your supply. Check efficiency at operating point — a 90% efficient driver at 10 W dissipates 1 W; ensure the driver enclosure can handle this heat.
Design or select the power stage topology For mains supply: use a flyback or buck topology with safety isolation. For low-voltage DC input above LED string voltage: a buck (step-down) converter. Choose a control IC with adjustable current sense threshold to set output current via a sense resistor: Iout = Vref ÷ Rsense.
Set the output current with the sense resistor Calculate the sense resistor: Rsense = Vref ÷ Iout, where Vref is the IC's reference voltage at the current sense input (check the datasheet — typically 100–200 mV). Use a low-inductance, metal-film or metal-element resistor. Choose the next standard value below the calculated value to avoid exceeding rated LED current.
Add output filtering and LED connection Place a small output capacitor (typically 10–100 µF) across the LED string to reduce switching ripple current through the LEDs. Connect LEDs in series between the driver positive output and negative output. Ensure the LED PCB has adequate heatsinking.
Test at reduced voltage and current first Power up with a bench supply at reduced voltage and verify output current with a series ammeter or by measuring voltage across the sense resistor. Verify the LED string illuminates evenly. Gradually increase to rated conditions while monitoring LED temperature with a non-contact thermometer.

Specifications

Common constant-current output values	350 mA, 500 mA, 700 mA, 1 A, 1.5 A, 2.1 A (application dependent)
Typical forward voltage — white LED	2.8–3.5 V at rated current; temperature coefficient approximately −2 to −3 mV/°C
Constant-voltage driver output voltages (common)	12 V DC, 24 V DC
Typical switching frequency (buck driver)	100 kHz–2 MHz
Driver efficiency (switching topology)	85–95% typical at rated load
PWM dimming frequency (recommended minimum)	1 kHz for general lighting; 200 Hz absolute minimum
Safety isolation standard (mains-connected drivers)	IEC 61347-1 and IEC 61347-2-13
Output current accuracy (typical regulated driver)	±5% of set current across load voltage range

Safety warnings

Mains-connected LED drivers contain lethal voltages. The primary circuit is at mains potential even after power is removed if bulk capacitors have not discharged. Do not open a mains LED driver enclosure without verifying discharge with a voltmeter — allow at least 60 seconds after disconnecting from mains, then verify voltage is below 50 V before touching internal components.
LED drivers for mains use must provide safety isolation between primary (mains) and secondary (LED) circuits in accordance with IEC 61347 and applicable safety standards. Do not substitute non-isolated (non-SELV) drivers in applications where users or installers can contact the LED circuit.
High-power LEDs produce intense light that can cause permanent eye damage within milliseconds of direct viewing. Never look directly into a high-brightness LED when powered, even briefly. Use diffusers and do not bypass optical guarding.
LED heatsinks can reach temperatures of 60–90 °C in normal operation — sufficient to cause burns on sustained contact. Allow adequate cooling time before touching heatsinks after switching off. Design heatsink mounting to prevent accidental contact.

Tools needed

Digital multimeter (DC voltage and current; resistance)
Bench power supply with current limiting (for safe initial testing)
Series ammeter or precision current sense resistor and voltmeter (for output current verification)
Oscilloscope (for verifying switching waveform, ripple, and PWM signal)
Soldering iron and solder
Non-contact infrared thermometer (for LED and heatsink temperature monitoring)
LCR meter (for inductor and capacitor value verification)

Common mistakes

Using a constant-voltage 12 V power supply to drive LED strip that already has resistors, then connecting additional bare LEDs in parallel without individual current limiting, burning out the bare LEDs immediately.
Selecting an output capacitor with insufficient voltage rating for the LED string voltage — an electrolytic capacitor operated above its rating fails within hours and can vent or rupture.
Using an inductive sense resistor (wire-wound) for the LED current sense, introducing a voltage spike across the sense input of the IC that causes false triggering and erratic current regulation.
Sizing the MOSFET or pass transistor based on average current rather than peak inductor current, causing the device to operate in saturation and overheat.
Setting PWM dimming frequency below 200 Hz for lighting applications where people work under the LED — this frequency produces visible flicker detectable by most observers.
Omitting thermal derating — operating the LED driver at its maximum rated current at ambient temperatures above 25 °C without reducing current, which significantly shortens driver and LED lifetime.

Troubleshooting

LEDs glow dimly or inconsistently in a series string: Cause: One LED in the string has higher forward voltage than the driver's maximum output voltage range, causing the driver to lose regulation; or one LED in the string has failed open-circuit, breaking the series path Fix: Measure voltage across the LED string and compare to the driver's rated output voltage range. If the string voltage exceeds the driver's maximum, the driver is out of regulation — reduce the number of series LEDs or select a driver with a wider output voltage range. Identify any failed (open-circuit) LED by measuring voltage across each LED individually.
LED driver overheats and shuts down: Cause: Ambient temperature exceeds the driver's thermal derating point, insufficient airflow, or output current set above the de-rated maximum for operating temperature Fix: Measure the driver case temperature. If above the rated maximum, improve airflow, reduce ambient temperature, or derate the output current setting (increase the sense resistor value). Verify the output current is not higher than specified — check the sense resistor value.
LED flickering at mains-synchronous frequency (100 or 120 Hz): Cause: Insufficient bulk output capacitance to smooth the rectified mains current ripple from the driver's power factor correction or rectifier stage Fix: Increase output capacitance (within the driver's design limits) or replace the driver with one designed for lower output ripple. Verify the flicker rate corresponds to twice the mains frequency (100 Hz at 50 Hz mains; 120 Hz at 60 Hz mains) by observing the LED with a camera in slow-motion video mode.

Frequently asked questions

What is the difference between a constant-current and constant-voltage LED driver?

A constant-current driver regulates output current to a fixed value regardless of LED forward voltage variation. It is correct for LED strings without built-in current limiting. A constant-voltage driver regulates output to a fixed voltage (12 V or 24 V DC); LEDs powered from it must have their own current-limiting resistors or circuits, as provided on most commercial LED strip products.

Why do LEDs fail when powered directly from a voltage source without a driver?

An LED's forward voltage decreases as junction temperature rises. Lower forward voltage means more current flows from the fixed voltage source, which increases temperature further. This thermal runaway rapidly drives current far above the LED's rated value, destroying the junction within seconds to minutes. A current-regulating driver prevents this by reducing its output voltage as current tries to increase.

What causes LED driver flicker and how is it measured?

Flicker occurs when LED current varies periodically, either from insufficient filtering in a mains-rectified supply (100/120 Hz ripple) or from low-frequency PWM dimming below approximately 200 Hz. The IEEE PAR1789-2015 report recommends that flicker at or below 10% modulation depth at frequencies above 1250 Hz is unlikely to cause visual discomfort or health effects. Flicker is measured as a percentage modulation of luminous output amplitude.

How do I calculate the resistor value for a simple single-LED circuit?

R = (Vsupply − Vf) ÷ If, where Vf is the LED forward voltage (typically 1.8–3.5 V depending on colour) and If is the desired LED forward current (typically 10–20 mA for standard LEDs). The resistor power rating must exceed (Vsupply − Vf) × If by a margin of at least 2× for reliability.

What is the minimum PWM frequency for LED dimming to avoid visible flicker?

For general lighting applications where people may look at the light source or where shadows are cast by moving objects, a PWM frequency of at least 1 kHz is recommended. At or below 100–200 Hz, stroboscopic effects are visible to many people and can cause discomfort or headaches in sensitive individuals.

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