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The current in the two LEDs looks pretty constant for supply voltages between about 8 and 30 volts.
- posted
8 years ago
some constant current LEDs
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- posted
8 years ago
Why bother with the TL431? The LEDs are close enough to being voltage references that a ring-of-two arrangement, with an NPN transistor more or less replacing the TL431, would work just as well, and be cheaper.
-- Bill Sloman, Sydney
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- posted
8 years ago
The band gap of the TL431 should provide better temperature stability. IIRC the standard ring of two current source is not particularly stable over temperature, as the current is set by the vbe of the bottom transistor.
It may not be worth an extra 5 cents...
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- Vote on answer
- posted
8 years ago
The current in each leg is set by the difference between the relevant Vbe and the voltage drop across the relevant LED. Both decrease by about 2mV/K, so the current is remarkably stable with temperature.
The Vbes and the forward voltage of the LEDs aren't particularly closely toleranced, so the actual current through each leg is a bit uncertain, but it is stable with temperature.
The TL431 is more closely toleranced, but since you don't care all that much about the precise current levels, why bother?
-- Bill Sloman, Sydney