** Schematic diagram corrected and updated **
In my previous post, I slapped together a quick LED lighting solution for my workbench… but it is truly a hack. What I really want to do is make a simple constant current driver, so the power LEDs can be used in other projects. One of those projects is an LED swimming pool light. It needs to be running at maximum brightness and low cost.
After much digging and testing, I found a simple circuit using a power FET, an OP Amp and 0.5 ohm resistor.
This simple circuit accepts a VCC up to 32v (limited by the Op-Amp). The 78L05 regulator provides a stable 5v reference and R1 is a potentiometer serving as a voltage divider, with the output on pin 2 serving as a reference voltage for the basic LM358 Op-Amp.
R2 is a 3W 0.5 ohm resistor (note the power rating is incorrect in the schematic!) serving as a current sense resistor. The higher the current flow through the resistor, the higher the voltage difference to ground. In this circuit, a 1A current will have a 0.5 volt differential across the resistor.
By adjusting R1, the LM358 will adjust its output and drive the FET gate. If the voltage at R2 is too low, it will increase output and allow more power to flow through the FET. If the voltage at R2 is too high against the reference voltage, the LM358 will decrease the output voltage and close the FET gate, limiting current.
I tried various circuits to set the reference voltage, including zeners, resistor networks, etc, but the only configuration I found to be stable independent of VCC was to use the 78L05 regulator as a reference. Once this circuit is calibrated for maximum current, it can be used at any voltage and current will be stable.
Below are a few photographs of the first prototype pool light using this current control circuit.
Unfortunately, things don’t always work right on the first try. After 30 mins under water, there is a noticable amount of water inside the protective enclosure. Still more testing to do.