Thomas Research LED20W-40-C0500-D compatible potentiometers


My name is Matt McKenna with WE-EF Lighting USA. I have a question regarding the Thomas Research Products LED driver LED20W-40-C0500-D. Does anyone know what potentiometers will be compatible with this driver?

Any information you have is helpful.

This is a 20 watt driver so I look at any potentiometer that is 25 watts or more as good to use, just physical size may be an issue. part like
RJS35RE-ND not sure the resistance you are looking for ?

Hi m.mckenna,

Not 100% certain on this, but I believe that a potentiometer valued at about 50K would be reasonable. The maximum source current is only 2mA, so the power rating for the pot would be minimal (probably no more than 100mW).

My assumption is that the LED20W-40-C0500-D has an internal 10V supply connected via a pull-up resistor to the purple Dim(+) wire. Since the source current can be no more than 2mA and the minimum dimming voltage can be no more than 1V when the external potentiometer’s resistance is zero, the internal pull-up resistor must be at least 4.5K Ohms {(10V - 1V) / 2mA = 4.5K Ohms}.

Then, to ensure that the voltage is at least 9V for 100% brightness, the potentiometer must be at least 9x the value of the pull-up resistor, or 40.5K Ohms. Therefore, a 47K or 50K pot would seem a reasonable value. Going much higher than this would tend to make most of the control over a smaller range of the potentiometer’s full rotation angle, and going much lower would likely limit the maximum brightness of the driver.

Hi David,

Thank you very much for your response! My thoughts were that taking 10V/2mA to get 5K Ohms was all I needed to find the necessary resistance. I am still new to the industry, however, and was unaware of the typical steps taken to calculate this and other factors to consider.

This is all very helpful and I really appreciate it!