I have some 1 watt LED's that I want to flash on and off.I have a 317 current regulator supply set at 300 ma for 3 of them in series.Now, I know there are alot of professional designers on this group and was wondering if I couldn't get an opinion on my approach to this, as it makes sense to me.I know many decisions are made based on parts cost but either way I do this, the cost is about the same....the cost aspect is not what I'm concerned with.....just if I'm making "the right choice".I initially thought I'd use a 555 timer to have a constant flash rate on an optoisoltor LED and control on/off with the LED ground.....then use the opto output and a sink transistor to control the DC path.Cons: what if anything were to change with the transistor and present a change in the DC series circuit?....could I get an "on resistance" that would be low enough and STAY in a specific window? With 300 ma flowing through, some "potential" for disaster would be there.So....Why not this....which makes more sense to me....Control the AC to the transformer with the same setup....flash a triac driver opto LED with a 555, control on/off with the opto LED ground, use a triac to switch on and off the transformer primary?That's what I have breadboarded now...works fine.Don't you think it would be the proper way to go as far as a design decision?......it lets the cc power supply be stable and avoids risk?The only risk on doing it the AC way is an AC line surge but I'll use a 400 - 600v triac good for 1 amp or so.Thanks.

Maybe I missed something, but why did you need the 555 output to beoptically coupled to the LED driver transistor? Why not just connectthe ouput of the 555 to a transistor that can handle 300 mA? A 2N4401(NPN) will handle 600mA. Or find an appropiate FET to do the job. Itseems to me that controlling the AC side of the power supply makesthings more complicated than needed.HTH-Dave Pollum

Am I right?

T

Tom Bruhns 19 years ago

Hi Ed,

OK, yours will work, BUT you have to sink 300mA to do it! If you put the resistor in, then you only need to sink 1.25V/1k + the adj pin current, max 100uA. With the 1k in there, the LED string sees 1.25V plus Vce(sat), say 1.35V, but so what? Even red LEDs have negligible current when hot at 0.5 volts each. Much less power hungry to add the

1k in there. The 1k is nominal, of course; it could be 220 ohms, or 2.2k ohms, though with 2.2k, I'd start worrying about the voltage drop across the 2.2k when the LEDs light; that increases the drop across the 4 ohm resistor by about 0.1 volts. 1k seemed a reasonable compromise to get to low current in the "yank down control transistor." Revised version, as I had posted, below. Thanks for providing the framework to edit!

Cheers, Tom

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T

Tom Bruhns 19 years ago

Hi Ed,

OK, yours will work, BUT you have to sink 300mA to do it! If you put the resistor in, then you only need to sink 1.25V/1k + the adj pin current, max 100uA. With the 1k in there, the LED string sees 1.25V plus Vce(sat), say 1.35V, but so what? Even red LEDs have negligible current when hot at 0.5 volts each. Much less power hungry to add the

1k in there. The 1k is nominal, of course; it could be 220 ohms, or 2.2k ohms, though with 2.2k, I'd start worrying about the voltage drop across the 2.2k when the LEDs light; that increases the drop across the 4 ohm resistor by about 0.1 volts. 1k seemed a reasonable compromise to get to low current in the "yank down control transistor." Revised version, as I had posted, below. Thanks for providing the framework to edit!

Cheers, Tom

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T

Tom Bruhns 19 years ago

about 1.35mA when the LEDs are off.

Ooops. I failed to edit the top line in that diagram I posted earlier. The current is about 300mA when the LEDs light, but only about 1.35mA when they are off. The LED current is essentially zero, the current in the 1k resistor is 1.25V/1k, and the adj pin current is

100uA max.

Cheers, Tom

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J

jasen 19 years ago

triacs don't like transformers.

This seems easier to me.

+V -----[LM317]--[4R2]-+--->|-->|-->|--- 0V | | | LED X 3 ...... | | (300ma) . 555. | | .output---|

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jasen 19 years ago

+---[120R]---+ | |a

the resistor reduces the current through the transistor to ~11mA when the transistor is on, and when the transistor is off has essentially no effect.

Bye. Jasen

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E

ehsjr 19 years ago

Thanks, Tom. Neat. Picture, words, 1000 !

Yes the circuit I drew always pulls 300 mA. Just couldn't do the words/picture translation to get what you were saying.

Yes! Much better, and not just from power consumption, but heat production is lower with your circuit.

Ed

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ehsjr 19 years ago

Thanks!

Ed

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