Transformer requirement for LED lights

[mike31154]

Since the voltage vs current relationship for a diode or LED is nonlinear, the fact that your LED runs brighter with the 13.5 volt vs 12 volt supply makes sense. What doesn't make sense is that the "dimmer doesn't seem to have any effect". Not sure I'm fully understanding that part of your statement.

Depending on the load connected to a power supply, especially a smaller one loaded to near max, it will drop to a lower voltage level. That is, if you were to measure the voltage output of your 13.5 volt supply while it is under close to maximum load, I suspect you would read less than the rated 13.5 volts.

Here is a graphic of the voltage vs current curve of a LED or diode.



As you can see as the voltage nears the rated/maximum forward voltage of about 3 volts for the diode, the current rises sharply. This is why using a power supply of the correct voltage is so important, or you risk cooking your diode. However, something as simple as resistor in a circuit running a LED can keep the current at a safe level by dropping the voltage available to run the LED to its rated value. This is not efficient since the resistor wastes power & creates heat in doing so. This is why PWM (pulse width modulation) is commonly used to dim and control the current to LEDs, it is more efficient. That doesn't mean you can't also include components such as resistors and zener diodes in conjunction with PWM circuitry to help sense & control both voltage and current in designing your hardware. Often tradeoffs are made in efficiency vs simplicity of design in most of these components. This is why I was suggesting that it may be possible to use a 24 volt supply feeding your dimmer to safely run your 12 volt LEDs. Again, that may not be an experiment you wish to carry out. If I'm wrong, you could cook your lights.