Lm317 led current?

[dizwold]

Hi,

I'm building a PCB drill with a set of LED's, to point on the item being drilled. Apart from the drill side-of-things, could someone advise me accordingly with regards to my proposed circuit below.
I apologise in advance for R3 'i couldn't find the correct part in Eagle', but it is, a 1K Pot, using 1 input, and then the output from the slider. The LED's '8 of them', are 'i think', 2.8volts 20ma.
What i'm trying to accomplish is a variable 'dimmer', for the LED's, but to also have a feedback to keep the required current.

Any input is welcome.

Regards.

Dizwold.

 

[BobK]

Why not use the LM317 in it's constant current configuration? A 1K pot in series with a 62.5Ω resistor would allow control from 20mA down a 1.1mA.

http://users.telenet.be/davshomepage/current-source.htm

Bob

 

[cjdelphi]

would an el cheapo 1k pot handle the current? 0.020 x voltage (1.25v)

 

[KJ6EAD]

Finish the calculation. How many Watts will the pot be required to dissipate; and the fixed resistor? What's the power rating of the pot?

 

[cjdelphi]

I would not.. I'd build a regulator instead

 

[dizwold]

The pot is rated @ 0.20W. It's from TE;

http://www.te.com/catalog/bin/TE.Co...6368,23815&LG=1&PG=1&IDS=490932,490933,490934

I've tried BobK's idea of using the device a Current Source, but this circuit doesn't allow much control with the pot, it's more like an on/off switch with a little bit of variable light control.

The circuit @ the top of this thread seems to work well. I've had it operating for about 20 mins with no problems, and nothing seems to get to hot. It also gives me a lot more variable control of the LEDs?