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Help with LED Driver Circuit

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PCChazter:

--- Quote from: Hero999 on July 19, 2018, 08:45:04 am ---The Hfe is not a well controlled parameter and varies widely, from device to device, even with the same part number and temperature: it increases, the hotter the device gets. Look at the data sheet for the BC817. For the BC817-25, with a collector current of 100mA, and voltage of 1V, it can vary from 160 to 400. The power dissipation is also far too high for the BC817. The data sheet says 350mW maximum. A red LED will drop about 2.2V and the current is 400mA, giving a 2.8V drop at 400mA across the transistor, which would be a power dissipation of 1.12W!
https://www.diodes.com/assets/Datasheets/ds11107.pdf

A much more sensible solution is to add an emitter resistor and drive the base at a constant voltage. The voltage across the emitter resistor will then be equal to VB = VBE and the current VE / RE. The Hfe is now unimportant and can be removed from the equation.

Of course the solutions discussed here are linear and therefore very inefficient, which is why the original poster decided to go for a switched mode regulator.



--- End quote ---

Right...1.12W is quite a bit for a transistor to dissipate, not sure where my head was with that one. I have used it in the past for low power (SMD) LEDs and haven't had a problem, which is why I thought of it, but you're absolutely right, it's inefficient, and would not suite OP's application. As well, the HFE being unreliable, but I had mentioned that as being something to consider whether or not it would be suitable. So thank you for showing the emitter resistor setup as an alternative.

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