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woody 16th March 2007 02:59 PM

Need a soft start current source
 
I have a group of HIGH power leds that need about 24v to
run. I would like to turn them on slowley going from off to
500ma over a period of an hour. Any realy simple circuit
ideas ?

sherelec 16th March 2007 05:34 PM

Woody,

The 1 hour progressive turn-on time for your LED driver seems to make inexpensive analog solutions impractical. So, a 5V microP with a DAC output, driving a GND referenced opamp, driving a NPN or N mosfet current source would seem to be simple and cheap.

A LM324 class opamp (cheap, rugged) has common-mode range down to GND so its reference input could be directly driven from the DAC out. The sense resistor on the Em (Source) to GND would be scaled for 500mA at max DAC output voltage. The LM324 could be powered from your LED bulk supply. The programming could be changed to accomodate various turn-on/off strategies, and unused inputs could be programmed for external sensors for additional contol options.

You could use a large variety of cheap PIC processors, or Basic programmed derivatives, should you wish to avoid the obtuse and frustrating initialization for the bare PIC's. My favorite derivative is the PICAXE from the UK (Revolution?). A suitable microP would cost less than $10. One reseller in the USA is phanderson.com. He is a prof. at Towson State in MD and has written a lot of microP applications material on the web in support of his classes using microP's. If you DIY for the programming support hardware; you can get away for less than $10 (assuming you have a PC available). Manuals/Forum/PC Software free on the web from the UK.

Just a thought!
VSR

gmphadte 21st March 2007 11:05 AM

All u need is CD4541, CD4017,Cd4093,LM317 and few transistors.
Make an oscillator from CD4541 of 1 pulse in 7 minutes, connect a CD4017 and use the outputs to control LM317. For details see on the applications of LM317 on national site. Use 4093 to inhibit the counting.
Edit: Use 4093 for power on reset also

Gajanan Phadte

peranders 21st March 2007 11:55 AM

Re: Need a soft start current source
 
Quote:

Originally posted by woody
I have a group of HIGH power leds that need about 24v to
run. I would like to turn them on slowley going from off to
500ma over a period of an hour. Any realy simple circuit
ideas ?

Just curious: What is your application?

gmphadte 22nd March 2007 04:38 AM

I suppose it could be increasing the life of the LEDs
Woody only knows

Gajanan Phadte


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