I am looking for an easy way to solve a voltage dropping problem to drive a 120 mA LED from 24V. This LED and the voltage dropping device are potted in potting compound in a small case so I can't just use a voltage dropping resistor. A resistor would need to dissipate over 2.5W and I just can't do it in the package that I need it in.
What I want is a buck converter to drop the voltage without building up a bunch of heat. It needs to be small and have a parts count of no more than a few parts. The obvious solution is one of the off the shelf switching power supply chips or LED drivers. They are available in single chip packages and would do the job just fine.....except for one gotcha, I can only have ground and power going into the device and I need the LED to dim. This device is designed to work off of existing lighting circuits that are full bright at 24V and dim with a rheostat. All of the power supply and LED driver chips are designed to give a fixed output regardless of the input voltage. Some have dimmer inputs, but that won't help me because I can't add a third connection for the dimmer. Essentially I need something like an existing LED driver or power supply chip with the voltage regulation part of it removed. A PIC or oscillator circuit set up for PWM would do the job if it could keep working down to just a few volt input and could drive 120 mA. I could build a simple circuit with a 555 timer and a FET, but I need to keep it as small and low parts count as possible. This is for something that I will build a few hundred of by hand a year so a surface mount board with ten parts would be a real pain in the neck. Thanks in advance for any advice. Brian Kraut
