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Am I right?

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

Reply to
Tom Bruhns
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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

Reply to
Tom Bruhns

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

Reply to
Tom Bruhns

triacs don't like transformers.

This seems easier to me.

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

+---[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

Reply to
jasen

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

Reply to
ehsjr

Thanks!

Ed

Reply to
ehsjr

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