Floating Lamp Circuit Design: Solar, Battery and Light

A lamp that floats on a lake or a harbour basin has to generate its own power, store it, use it efficiently and survive years of immersion. The circuit design follows from those four requirements rather than from the lighting specification.

What the Product Is

The light source, the battery, the controller and the radio are sealed inside a buoyant body that rises and falls with the water level but always stays above it. Power arrives from a photovoltaic panel mounted on the shore or on a mast.

That arrangement separates the energy source from the load, which is unusual for a portable product. The cable between them is part of the electrical design, and it has to survive weather and movement.

The Energy Budget

The design starts with the load: the light output, the hours of operation and the efficiency of the driver. That gives the energy per day, which the panel has to supply with margin for the days when there is no sun.

The battery is sized from the autonomy requirement, and the autonomy requirement is a commercial decision as much as a technical one. A product that lasts five cloudy days costs more than one that lasts two, and the difference is visible to the customer.

Solar Input and Maximum Power Point Tracking

A photovoltaic panel has a voltage and current characteristic whose maximum power point moves with illumination and temperature. A charger that tracks that point extracts more energy than one that simply connects the panel to the battery.

The tracking is implemented in the charger controller, which adjusts its input voltage to hold the panel at the optimum. The inductor, the switch and the layout follow the same rules as any switching converter.

Battery and Protection

A lithium battery needs protection against overcharge, over discharge, overcurrent and temperature. The protection is either part of the charger or a separate circuit, and it must operate without the processor.

The battery also has to be charged within its temperature window. A pack that is frozen at minus twenty degrees cannot accept charge, and the charger has to detect that condition rather than damage the cells.

The LED Driver

The light source is driven by a constant current converter, because the output of a light emitting diode is set by its current rather than by its voltage. The driver has to work over the range of battery voltage and hold the current constant as the battery discharges.

The efficiency of the driver matters because every percent lost is energy the panel has to replace. A switching driver with a low loss inductor is the usual choice for a product that runs every night.

<img src="https://www.gopcba.com/wp-content/uploads/2025/09/金手指pcb.jpg" alt="MPPT charger and LED driver board from a floating lamp” />

Dimming and Light Output

Dimming is done by reducing the current or by pulse width modulation. Current reduction is quieter but shifts the colour of the light, while modulation holds the colour and can cause visible flicker if the frequency is too low.

The control signal comes from the processor or from a light sensor, and the profile is a design decision. A lamp that brightens gradually at dusk is more acceptable than one that switches on abruptly, and the difference costs nothing but firmware.

The Wireless Link

A long range mesh radio reports the status and accepts commands. Its consumption is significant compared with the light, so it is operated on a duty cycle and the reporting interval is part of the energy budget.

The antenna has to work close to water, which absorbs energy and detunes the structure. The matching network and the position of the antenna relative to the metal parts of the assembly decide the range that is actually achieved. The general principles are described under antenna design in a small product.

Sensors and Water Detection

The lamp may also measure the water level, the battery temperature and the current from the panel. Those measurements are used for control, for reporting and for protection.

A water detection electrode has to be driven and read carefully, because a direct current through water corrodes the electrode. An alternating drive with a series resistance is the usual arrangement.

Waterproof Packaging

The electronics are inside a sealed enclosure, and the pressure difference between the inside and the outside changes as the temperature changes. The seal has to accommodate that without admitting water, and the enclosure has to be tested after assembly.

Conformal coating provides a second barrier. The coating protection is not a substitute for a seal, but it turns a seal that weeps into a product that survives rather than one that fails.

Thermal and Sealing

The heat sources are the driver and the charger, and they are inside an insulated box. The enclosure has a metal base in most designs, which conducts the heat into the water it floats on.

That path only works if the board is mounted against the base with a thermal interface rather than an air gap. The mechanical drawing and the board layout have to agree on that contact.

Surge and Electromagnetic Compatibility

A product with a cable that runs to the shore is exposed to induced surges. A varistor at the entry and a series impedance before the converter are the first line, together with a layout that keeps the transient current away from the sensitive circuits.

The switching converter and the radio are the sources of emission, and the cable is the antenna. Filtering at the entry and a controlled return path are the measures, as for any product with an external cable.

Test and Production

Testing covers the charging behaviour under a solar simulator, the battery protection, the light output and the radio range. A leak test at pressure is part of the production procedure, because a seal that fails in the field cannot be repaired.

The battery is the component with the longest lead time and the strictest transport rules. Its specification and its source should be settled before the production run is scheduled.

Failure Modes to Design Against

The likeliest failures are a seal that weeps, a battery that is deep discharged during a long cloudy period and a radio whose range collapses when the water level rises. Each of them can be mitigated in the design.

A low voltage disconnect protects the battery, a generous seal and a coating protect the electronics, and an antenna placed high on the body protects the link. The current capacity of the charging and lighting paths should be checked against the worst case current before the layout is released.

Process Control and Verification

On a design of this kind, floating lamp is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

FAQ

Why track the maximum power point? Because the optimum voltage of a panel moves with light and temperature. Tracking extracts more energy from the same panel, which matters on cloudy days.

Can the battery be charged below freezing? No. Charging a lithium cell below its temperature limit damages it, so the charger has to detect the condition and wait.

How is the antenna protected? By placing it away from the water and from metal parts, and by sealing it with the rest of the electronics. Its matching is verified with the product assembled.