LED Driver

24V Constant-Current LED Driver

A compact 24V constant-current LED driver designed around the PT4115 switching LED driver IC, with an LM393-based automatic dimming control stage.

The project is designed as a practical, low-cost hardware solution for driving an LED load at approximately 3.5 W, while providing automatic switching between normal and dimming modes without requiring an external microcontroller.

Project status: Prototype / Development The PCB and circuit are currently being developed and require final bench validation before production use.

Features

• 24V DC input
• Constant-current LED driving
• PT4115-based switching regulator
• Approximately 3.5 W target LED output
• LM393-based automatic dimming control
• No external microcontroller required for the dimming decision
• Schottky freewheeling diode
• 68 µH power inductor
• Current-sense resistor for LED current regulation
• Input filtering capacitors
• Zener-based voltage protection for the control section
• Through-hole input/output connectors
• SMD power and control components
• Designed for a single-layer PCB
• KiCad 9 project

1. Project Overview

The purpose of this project is to develop a dedicated constant-current LED driver capable of operating from a 24V DC supply.

Unlike a simple resistor-based LED circuit, the PT4115 regulates the LED current through a switching converter. This allows the LED current to remain substantially more controlled as the LED's forward voltage changes.

The second part of the circuit uses an LM393 comparator to detect a predefined voltage condition and provide a control signal to the PT4115 DIM input.

The intended operating concept is:

                 +24V DC
                    │
                    ▼
          ┌──────────────────┐
          │ Input Filtering  │
          │ & Protection     │
          └────────┬─────────┘
                   │
                   ▼
          ┌──────────────────┐
          │     PT4115       │
          │ Constant Current │
          │      Driver      │
          └────────┬─────────┘
                   │
            Switching Node
                   │
          ┌────────┴─────────┐
          │                  │
       Schottky            68 µH
        Diode             Inductor
          │                  │
          └────────┬─────────┘
                   │

2. Operating Principle

Constant-current regulation

The PT4115 monitors the voltage across the current-sense resistor connected to its CSN pin.

The sense resistor determines the regulated LED current.

Conceptually:

LED current
     │
     ▼
Current-sense resistor
     │
     ▼
Voltage developed across Rs
     │
     ▼
PT4115 CSN
     │
     ▼
Switching duty cycle adjusted
     │
     ▼
LED current regulated

This makes the circuit a constant-current LED driver rather than a simple voltage source.

3. Dimming Control

The project uses an LM393 comparator to provide automatic dimming control.

The comparator monitors the selected voltage signal and compares it against a reference threshold.

When the monitored voltage crosses the defined threshold, the comparator changes its output state.

That output is connected to the DIM input of the PT4115.

The intended behavior is:

             Voltage monitored
                    │
                    ▼
               ┌─────────┐
Reference ───► │ LM393   │
               └────┬────┘
                    │
                    ▼
                 DIM
                    │
                    ▼
                PT4115
                    │
             LED brightness

This approach eliminates the need for a dedicated microcontroller for the basic dimming-mode decision.

4. Main Components

Component values may be revised during final validation.

5. PCB Design

The PCB is being designed in KiCad 10.

Board constraints

• Single-layer copper design
• F.Cu used for routing
• Ground implemented using copper where appropriate
• SMD resistors and small-signal components
• SMD power components where practical
• Through-hole bulk capacitor where required
• Through-hole inductor selected for mechanical and thermal considerations
• 1210 footprint for the current-sense resistor
• 0805 footprints for the smaller resistors/capacitors
• Compact placement while maintaining adequate spacing around heat-generating components

6. PCB Placement Philosophy

The placement is based on keeping high-current and high-frequency switching paths as short as possible.

The general arrangement is:

24V INPUT
   │
   ▼
INPUT CAPACITORS
   │
   ▼
PT4115 ─── D1 ─── L1 ─── LED OUTPUT
   │
   │
Current Sense
   │
   ▼
GND

        LM393
          │
          ▼
        DIM

The PT4115, Schottky diode, inductor and current-sense components are kept relatively close together because these components form the primary switching/current-regulation section.

The comparator section is kept separated from the high-current switching path as much as practical to reduce the possibility of switching noise affecting the threshold detection.

7. Protection and Reliability

The design includes basic protection and filtering components while keeping the circuit relatively simple.

Zener protection

A BZT52C4V7 is used in the comparator/reference section.

The selected device provides a convenient 4.7V Zener reference/protection element and has a readily available KiCad footprint.

The Zener is not intended to replace proper input protection for all possible supply transients.

Schottky diode

The SS14 is used as the freewheeling diode in the PT4115 switching stage.

A Schottky diode is preferred because of its relatively low forward voltage and fast switching behavior.

This helps reduce conduction losses in the switching path.

8. Thermal Considerations

The PT4115 and other components in the switching path can generate heat during operation.

For this reason:

• The PT4115 should have sufficient copper around its thermal pads/pins according to the package requirements.
• High-current traces should be kept short.
• The switching components should be positioned close together.
• The current-sense resistor should not be unnecessarily placed next to heat-generating components.
• Sensitive comparator components should be kept away from the main switching node.
• The PCB should provide adequate copper area for heat spreading.

Thermal performance will be verified during prototype testing.

9. Design Target

The current design target is approximately:

The exact LED current depends on the final current-sense resistor and the actual PT4115 sense threshold.

10. Repository Structure

The repository is intended to contain:

24V-Constant-Current-LED-Driver/
│
├── README.md
│
├── KiCad/
│   ├── schematic.kicad_sch
│   ├── pcb.kicad_pcb
│   └── project.kicad_pro
│
├── Gerbers/
│   ├── F_Cu.gbr
│   ├── B_Cu.gbr
│   ├── F_Mask.gbr
│   ├── B_Mask.gbr
│   ├── F_Silkscreen.gbr
│   ├── Edge_Cuts.gbr
│   └── Drill_Files/
│
├── Documentation/
│   ├── schematic/
│   ├── pcb/
│   └── calculations/
│
├── BOM/

11. Manufacturing

The PCB is intended to be manufactured as a single-layer PCB.

The repository will include the required Gerber and drill files once the design has passed the final design-rule and electrical checks.

Before manufacturing:

• Run KiCad DRC
• Verify all footprints
• Verify connector orientation
• Verify diode polarity
• Verify capacitor polarity
• Verify PT4115 pin mapping
• Verify LM393 comparator connections
• Verify current-sense resistor value
• Verify DIM logic
• Verify copper clearances
• Verify board outline
• Verify Gerber output

12. Testing Plan

The first prototype should be tested progressively rather than connecting the LED load immediately.

Stage 1 — Visual inspection

Check:

• Component orientation
• Solder bridges
• PCB traces
• Connector polarity
• Diode polarity
• Capacitor polarity
• IC orientation

Stage 2 — Power-off electrical checks

Measure:

• Resistance between +24V and GND
• Continuity of GND
• Continuity of the main current path
• Absence of unintended shorts

Stage 3 — Controlled power-up

Use a current-limited bench supply.

Initially verify:

• Input current
• PT4115 supply voltage
• LM393 supply voltage
• Comparator output
• DIM voltage

Stage 4 — LED load

Connect the intended LED load and verify:

• LED current
• LED voltage
• LED power
• PT4115 temperature
• Inductor temperature
• Diode temperature
• Sense resistor temperature

Stage 5 — Dimming test

Apply the intended control voltage and verify that the LM393 changes state at the expected threshold.

Then verify that the PT4115 DIM input responds correctly.

13. Current Development Status

Current status: Prototype / PCB development

The schematic and PCB are being developed iteratively with emphasis on:

• Correct electrical connections
• Component availability
• PCB manufacturability
• Single-layer routing
• Thermal management
• Current regulation
• Automatic dimming
• Low-cost implementation

Further protection features such as dedicated input reverse-polarity protection and TVS protection can be evaluated in a later revision.

14. Future Improvements

Possible improvements for future revisions include:

• Dedicated reverse-polarity protection
• TVS transient protection
• Improved input filtering
• Optimized switching-node layout
• Improved thermal management
• More robust dimming interface
• Alternative comparator/control topology
• Efficiency measurements
• Thermal characterization
• Multiple LED power variants
• Production-oriented PCB revision

15. Disclaimer

This project is an electronics development/prototype project.

The design should be electrically and thermally validated before being connected to a permanent installation or used in a safety-critical application.

Component substitutions should be checked for electrical ratings, package compatibility, thermal performance and PCB footprint compatibility.

Author

Rohan

Electronics / Hardware Development

Built using KiCad 9.

License

Suggested license: CERN-OHL-S-2.0

This project is intended to remain open-source while allowing others to study, modify and manufacture derivative hardware designs.

Documentation Note

Technical documentation was prepared by the project author with AI-assisted writing and editing.