Introduction

This article details the design and construction of a 12-volt LED driver circuit, enabling the use of inexpensive 230-volt LED lamps with a 12-volt DC power source, such as those found in boats or campervans. While connecting standard 12-volt incandescent lamps to an onboard battery is possible, their low energy efficiency can quickly drain the battery. LED lamps offer better efficiency, but dedicated 12-volt LED light fittings can be costly. This project aims to provide a compact and versatile solution by allowing the use of readily available 230-volt LED lamps.

By Louter van der Kolk (The Netherlands)

The project focuses on modifying the Ikea Jansjö LED wall/clamp spotlight, a popular and affordable option (€/£10). This spotlight is designed for 230-volt AC power outlets, necessitating a conversion for 12-volt DC operation.

Key Features:

• Powered from a 12-V battery
• Suitable for efficient LED lamps
• Configurable output current up to 350 mA
• Various dimming levels via the on/off switch

The Challenge: Adapting for 12-V DC

The primary challenge is that the Ikea Jansjö spotlight is intended for a 230-volt AC power source via an external adapter, not a 12-volt DC battery. Simply using an inverter to convert 12-V DC to 230-V AC, and then using the spotlight's original adapter to convert 230-V AC back to DC, is an inefficient and overly complex approach.

The most effective solution is a dedicated LED driver that directly converts the 12-V battery voltage into the correct current for the LED. The Ikea spotlight is ideal because its LED is directly accessible for modification.

Before designing the driver, the nominal operating current for the LED was measured using a standard DC ammeter, found to be approximately 165 mA. The driver circuit must supply this specific current.

The Circuit Design

Circuit Diagram Description (Figure 1): The circuit diagram illustrates a compact design centered around a single integrated circuit (IC1), identified as the ZLED7x30. It includes essential passive components: a diode (D1), an inductor (L1), three capacitors (C1, C2, C3), and two resistors (R1, R2). The IC features input pins for power (VIN), current sensing (ISENSE), a switching node (LX), dimming control (DIM1, DIM2), and ground (GND), along with outputs for the LED (LED+ and LED-).

The chosen component for this driver is the ZLED7x30 from IDT (Integrated Device Technology). This IC functions as a continuous-mode inductive step-down (buck) converter, designed to efficiently reduce voltage and regulate current. Its advantages include a compact footprint, an integrated dimming function controllable via the power switch, and the ability to set the output current accurately using a single external resistor.

A minor drawback is that the ZLED7x30 is supplied in an SMD (Surface Mount Device) package, which requires careful soldering, though it is manageable for hobbyists.

Available Versions: Table 1 lists different variants of the ZLED7x30 IC, distinguished by their maximum supported output current. The functionality and pin configuration remain consistent across these versions.

For this project, the ZLED7730, with its 350 mA maximum output current, is suitable for the required 165 mA. Users can select other versions from the table if higher currents are needed.

The circuit operates as a standard buck converter. The inductor (L1) and capacitor (C3) form an energy buffer to ensure a continuous, stable current supply to the LED. Capacitor C1 provides smoothing, particularly useful if the circuit were powered via a rectifier, though its value is less critical when powered directly from a battery. Capacitor C2 is placed close to the IC to mitigate ground bounce effects during the IC's switching cycles.

The recommended inductor value (L1) ranges from 30 µH to 220 µH. The circuit uses a 100 µH inductor, but experimentation may be needed for optimal stability with different output currents. Care should be taken to ensure the chosen inductor fits on the PCB.

Configuring the Output Current

A key feature of the ZLED7x30 is that the output current is precisely set by a single external resistor, referred to as Rsense. The relationship is defined by the formula:

Iout = 0.1V / Rs

To achieve the target current of 165 mA, two 1.2 Ω resistors (R1 and R2) are connected in parallel, resulting in an effective Rsense of 0.6 Ω. This configuration yields an output current of approximately 166 mA.

Caution: When experimenting with different resistor values, ensure the output current does not exceed the maximum rating for the selected ZLED7x30 variant to prevent activation of the IC's internal thermal protection.

Configuring the Dimming Mode

The ZLED7x30 IC also offers flexible dimming capabilities, controlled by two solder bridges, labeled DIM1 and DIM2, on the PCB. The presence or absence of these solder bridges determines one of four distinct dimming modes, as detailed in Table 2.

Switching between these dimming modes is straightforward: a quick on/off cycle of the 12-V power supply, typically controlled by the existing inline switch on the power cable, selects the next dimming setting.

Construction

PCB Design (Figure 2): The project utilizes a single-sided printed circuit board (PCB) designed for the driver circuit. The prototype board was milled rather than etched. All components are surface-mount devices (SMD).

SMD Soldering Advice: While SMD components can seem daunting, they are manageable for hobbyists. This project serves as an excellent introduction to SMD soldering. No specialized equipment like a reflow oven is required. Essential tools include thin solder, a soldering iron with a fine, tapering tip, solder wick (desoldering braid), and a magnifying reading lamp for precision. After soldering, a thorough check for any shorts using a magnifying glass is recommended.

Assembly: The recommended assembly order is to mount the passive components first, followed by the IC. The final result should resemble the assembled board shown in Figure 3.

Enclosure and Mounting (Figure 4): The author's prototype was housed in a small Hammond enclosure (type 1551MBK). This enclosure was then attached to the clamp of the Ikea spotlight using hook & loop tape or a bolt. The prototype board used by the author differs slightly from the one available from the Elektor store.

The original inline switch in the 12-V power cord can be used for operation. Alternatively, a small slide switch can be added to the enclosure.

Understanding Incandescent Lamps vs. LEDs

Incandescent Lamps: Conventional incandescent lamps, often found in older lighting, can be connected directly to a 230-V AC line. Their filament's resistance naturally limits the current flow. This resistance also increases as the filament heats up (positive temperature coefficient), stabilizing the light output once equilibrium is reached.

LEDs: LEDs operate differently. As semiconductor diodes, they have a specific forward voltage drop. Crucially, the current through an LED is not inherently limited. In simple applications, a series resistor is used to limit current, but its value must be recalculated if the supply voltage changes. Connecting an LED directly to a voltage source without current limiting, such as connecting the Ikea spotlight directly to a 12-V battery, would result in an immediate surge of current, likely destroying the LED.

Dedicated LED drivers, like the ZLED7x30, act as current sources. They efficiently supply the precise current required by the LED, automatically maintaining the correct voltage across it. This approach is far more efficient than using resistors, as it minimizes energy wasted as heat.

Web Links

• [1] IDT ZLED7x30 Datasheet: www.idt.com/document/dst/zled7x30-datasheet
• [2] Wikipedia - Ground Bounce: en.wikipedia.org/wiki/Ground_bounce

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