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super simple and cheap pwm driver for led ?

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What's a 317 minimum drop-out voltage, with Iout= 350 mA?

Plus, how much would the battery terminal voltage drop to because of its internal resistance ?
Why not look on the datasheet? There are maximum guaranteed worst-case ratings and graphs showing typical spec's.
 
Why not look on the datasheet? There are maximum guaranteed worst-case ratings and graphs showing typical spec's.

Yes, that's the very point that I'm making. I ask because would there be any overhead available for a 317 to regulate with? (i.e. after the battery drops to 1 V per cell)

Beware though, many an engineer has found reality on the breadboard different to data sheet interpretation believed as gospel at the time. I've had engineers try to tell me the datasheet is wrong, after they'd replaced a "faulty" IC! Until pointed out that datasheet data is often percieved out of context. The manufacturer often shows performance in the best posisble light, i.e. how low their 'drop out' is, it looks impressive until you read the small print down the bottom of page says tested at only 1 mA.

I have the feeling an LDO would be better than a 317.

LEDs do not work in parallel unless their voltages are matched. Their voltages are very different
Voltages matched to what? (I think you mean 'currents need to be matched' don't you?, the voltage being a function of colour, current and LED chip temperature) Dramatic proof, connect a red LED in parallel across a lit green LED, the green LED goes out completely.

In practice, when it's only low currents same-colour LEDs 'kinda work OK' connected in parallel. But when the LED's are run hot, as one warms up it takes a bigger and bigger share of current, because its diode drop voltage falls as LED chip warms up, I think that's what you were saying?

Anyways, LEDs for illumination tend to be better connected in series for this reason.
 
But a battery voltage drops as it discharges which will dim the LEDs.

LED current would drop as the battery discharges, yes, but light output probably un-noticeable unless side-by-side with another unit with a fresh battery. I gather the end use is only a one-time event only meant to last one deployment, it's not a consumer product with a guarantee etc. If an amount of dimming is tolerable, a simple battery and ballast resistor could be considered, instead of 317's, PWMs etc.

Choosing a power source with a flat voltage discharge curve makes LED current regulation easier to begin with, i.e. an NiMH cell only drops during discharge by 0.2 V per cell over most of its cycle. If the voltage drop over an LED ballast resistor was chosen 0.4 V per cell (I believe the present 317 circuit already costs total of 3 V, and below that it dims the LED), then total dimming over life would be something like 50% rough guess which may be acceptable. If this isn't, then to improve it the choice could be buy more Ampere-hours to flatten out the discharge curve, or to add a PWM. Or replace the 317 (3V min.) with an LDO instead (1.3V min.)

My 0.02c conclusion: I think adding batteries might be cheaper and easier (and lower overall carbon footprint) if this is only to be used once or twice. Plus there's the advantage of long and graceful dimming as battery is discharged. This is slightly compensated for because as the current drops the luxeon LED chip cools and becomes more efficient. With PWM, expect 'sudden death' battery failures, since there's little or no dimming preceding it.
 
My red Fairchild LEDs are spec'd with a forward voltage of 1.5V to 2.4V at 20mA.
If a 1.5V one is paralleled with a 2.4V one then the 1.5V one will have double the current and the 2.4V one will not light.
So of course their forward voltages must be matched if in parallel.
 
My red Fairchild LEDs are spec'd with a forward voltage of 1.5V to 2.4V at 20mA.
If a 1.5V one is paralleled with a 2.4V one then the 1.5V one will have double the current and the 2.4V one will not light.
So of course their forward voltages must be matched if in parallel.


Have you actually tried this with LEDs from the same wafer batch? What was the result?

"1.5 to 2.4" is the max deviation possible between different wafer lots. This includes an amount of extra padding the manufacturer adds on to cover themselves. You'll find in practice that a small amount of ballast resistance in each diode will make them current share better, as you do with emitter resistors in paralleled BJT transistors.
 
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Have you actually tried this with LEDs from the same wafer batch? What was the result?
I can't buy LEDs from the same wafer batch unless maybe I buy tens of thousands of them. The ones I buy might not even be made in the same year. So their forward voltages do not match.
 
Chances are, two consequetive LEDs taken from the same multi-pack will be of very large number that originated from the same wafer. Almost certainly they will be of the same year.

So anyway.... out of curiosity, how well did your two LEDs current-share when you tried connecting them in parallel?
 
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I never connect LEDs in parallel like some cheap Chinese flashlights do.
Their brightest LEDs burn out in a few hours. The surviving LEDs have a fairly high forward voltage and are a good match.
 
Isn't the title of this thread "super cheap LED driver" circuit ?

So have you tried connecting LEDs in parallel? How badly did they current-share ?
 
LEDs do not share current unless their forward voltages are matched.
 
LEDs do not share current unless their forward voltages are matched.

In text books and as taught, 'yes', but in the real world, this happens:

**broken link removed**

These two LED's are in PARALLEL, with NO series resistors. The inherent slope resistance of the LEDs tends to encourage them to share current. Overall current is limited by internal resistance of the battery.

They'll share according to alignment coincidence of their respective V-I curves. These two (chosen at random from the pack they were supplied in) are likely off the same wafer.

If a bunch of paralleled LEDs have very short leads and are thermally coupled to eachother, there'll be less tendency of hot-spots. I believe that's how cheap multi-LED torches are internally arranged, otherwise they'd have to have a resistor in every LED chain.
 
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A half-alive 9V alkaline battery can provide up to 400mA to blow up your LEDs without current-limiting resistors.
I have never seen a pink LED.
 

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I have, Rapid electronics sell them
 
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