Driving LEDs is an interesting topic. For what we do, the main issues are variable voltage (if it is mounted in rolling stock, polarity changes, voltage changes, it can be DC or DCC), and that is what my products are meant to deal with.
In higher power products, think flashlights, or driving our LEDs hard, you find out that there is another issue.
We all know that LEDs get hot. Every LED I’ve ever seen has what is called a negative temperature coefficient. Mean it’s internal resistance goes down has it gets hotter!
Think about that for a second. If we are driving an LED with a resistor and a constant voltage, as the resistance of the LED goes down, more current flows. More current flowing lead to the LED getting hotter, and the cycle continues. You get thermal runaway, and the LED fails. This is why if you are going to use an LED for full scale lighting, you must use some sort of circuit to limit current.
If just taken LED current limiting to the lower current levels we typically use in our models, and added a bit to add supply independence.
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