[sdiy] voltage and LEDs

[Needham, Alan]

OOPS!
Just noticed I said 100k from VReg to +12v -- should have been to 0v

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Needham, Alan
Sent: 16 February 2010 10:29
To: Oren Leavitt; db
Cc: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] voltage and LEDs

Please bear with me on this ...
If an LM317 were used as a constant current source, the output of the
regulator goes via a series resistor to the LED.
The VReg leg of the regulator connects to the junction of the resistor
and the LED
So far, exactly as per the standard circuit; but...

If a resistor (maybe 100k) connected between the VReg pin and the 12v
supply it would have no effect on the circuit, passing too small a
current to affect the biasing; UNTIL...

If the connection between the VReg pin and the resistor/LED junction
were now broken, the regulator would switch off (almost) and the LED
would extinguish. This is a low current path that could be controlled
with a small signal FET. Let the LM317 do all the heavy work!


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