LEDs must come on one-by-one (with good EMC)

[eem2am]

4017 mosfet

hello,

I am designing a SMPS which powers 8 300mA LEDs.

The spec is that a short while after the SMPS powers up, the 8 LEDs must light up..............BUT..........


Not all at once.

One must light up, then about 500ms later the next...and so on...until they are all lit up, and then it just stays like that, with all LEDs on.

So the LEDs must light up kind of "chaser" style.

-------------------------------------------------
Anyway,

So far, it all works , BUT................

I have the LEDs in series and 7 MOSFETS short out each LED.
-then one by one each FET is turned off to bring on the next LED etc etc.

here is the representative schematic.....

https://i29.tinypic.com/2mpa1ix.jpg

here are the waveforms which show each LED coming on
(timescales adjusted to speed simulation)

https://i26.tinypic.com/nye60y.jpg

The problem is...........................

When the next LED is about to be brought ON, all the LEDs (that are ON) go OFF for a millisecond or so, then come back on again.

...Though this flicker cannot be seen , the sudden OFF-going of LED current will create a bad EMC problem. .....is this correct ?

also, any ideas on how to get round this OFF-going of the LEDs greatly appreciated.

----
By the way, here is the .asc file for LTSpice

https://www.2shared.com/file/6729207/cf156782/_this_is_it_LED_niteride_170709.html

 

[bobcat1]

Hi

Use the digital way: use 4017 connected to 500 msec osc(555) and drive each led from the 4017 digital output (drive the each led mosfet) when the 4017 count 10 stop the OSC

You can do the same with a small microcontroller like pic16F84 but you need to know how to program it

All the best

Bobi

 

[betwixt]

I think the 4017 would turn each LED on and off again rather than just turn each one on after a time delay.

I'm guessing here, if you short out the LEDs to turn them off, are you not drawing more current from the regulator? Perhaps the flicker is the regulator re-compensating for the change in demand. A more power efficient way of sequencing them would be to sequentially turn them on rather than un-short them out.

Very easy to do with a micro or a shift register and the small step increases in current should not cause EMC problems. If you parallel switch the LEDs rather than series, you also reduce the required supply voltage and you could probably eliminate the regulator and associated components altogether.

Brian.