Basic question: How to regulate LED brightness with ridiculously LARGE resistor

[KarenColumbo]

20 or more items to drive an LED... 

(remarkably outrageous..)

  Nothing's ever easy with me, I know. But one of these days I'll know 3 or 4 ways to drive an LED with a 100k pot.
Μῆνιν ἄειδε θεὰ Πηληιάδεω Ἀχιλῆος
οὐλομένην, ἥ μυρί' Ἀχαιοῖς ἄλγε' ἔθηκε

[antonis]

Μῆνιν ἄειδε θεὰ Πηληιάδεω Ἀχιλῆος
οὐλομένην, ἥ μυρί' Ἀχαιοῖς ἄλγε' ἔθηκε

πολλάς δ' ἰφθίμους ψυχάς Ἄϊδι προΐαψεν
ἡρώων, αὐτούς δέ ἑλώρια τεῦχε κύνεσσιν
οἰωνοῖσί τε πᾶσι..

You can't always drive an LED the way you want..
(loose transliteration...)

[KarenColumbo]

Hehehe I actually learned ancient greek in school. I copied those two lines from Wikipedia, but I still can recite the first 8 verses (and probably write them almost correctly). Achilles' battle against SMD LEDs or so ...

[Rob Strand]

Here's the budget version for antonis's benefit,



It's possible to do a BJT version but with a high pot value like 100k the base current is going to mess things up,  perhaps even change the linearity of the control.

[KarenColumbo]

https://www.thetonegod.com/diy/rock
Found this in an old (2014) thread here on the forum

[KarenColumbo]

Here's the budget version for antonis's benefit,



It's possible to do a BJT version but with a high pot value like 100k the base current is going to mess things up,  perhaps even change the linearity of the control.

Will it endure 2 LEDs? Thay may have 100-120 mA together. If I read the Data Sheet correctly, a 2N7000 has a continous I_D of 200 mA. And the source resistor will "generate" a current, too. According to https://ohmslawcalculator.com/ohms-law-calculator there's 16 mA at R1, so it looks like we're getting close to that spec.

[Rob Strand]

ill it endure 2 LEDs? Thay may have 100-120 mA together. If I read the Data Sheet correctly, a 2N7000 has a continous ID of 200 mA. And the source resistor will "generate" a current, too. According to https://ohmslawcalculator.com/ohms-law-calculator there's 16 mA at R1, so it looks like we're getting close to that spec.

I was thinking 5mA not 120mA!   So is that 2xLEDs in parallel with 50mA to 60mA each?  That's pushing the LEDs no?

For the 2N7000 the datasheets are coming up with about 200mA max.  In that circuit I doubt it will do even that.  There's going to be about 9 - 1.7 -1 = 6.3V across the device.  That's 6.3 * 0.2 = 1.26W power dissipation and 312.5 (deg/W) * 1.26 = 400C rise in junction temp.   If you are thinking that looks hot, it is      Even at 100mA you still get 200C.   Whatever part and whatever circuit you use with a current source is going to need to dissipate the 1.26W (or whatever).   So you will need a small heatsink, 80C/W or preferably less.

If you can use a switching circuit that will reduce the power dissipation because the part doesn't have to sustain 6.3V across it but then you have to consider noise  and signal modulation issues.

[KarenColumbo]

So I'll swap in a beast of a MOSFET? Like this hulk here? https://asset.conrad.com/media10/add/160267/c1/-/en/000162798DS01/datenblatt-162798-infineon-technologies-irl540npbf-mosfet-1-n-kanal-140-w-to-220.pdf

[Fancy Lime]

So I'll swap in a beast of a MOSFET? Like this hulk here? https://asset.conrad.com/media10/add/160267/c1/-/en/000162798DS01/datenblatt-162798-infineon-technologies-irl540npbf-mosfet-1-n-kanal-140-w-to-220.pdf

No, you use less current. Most modern LEDs have the brightness range that is relevant for a good response in home-rolled vactrols somewhere in the 0.1-10mA range. That depends on the LED, the LDR and the way they are put together, of course. But I say, if you need more than 10mA through each LED to get to the minimum resistance of the LDR, use more efficient LEDs or more sensitive LDRs but don't try to turn a way pedal into a space heater. Remember, you almost never want to get near the maximum current that the LED can handle. On the one hand, it shortens it's lifespan, on the other hand, the region where LDR resistance and LED current correlate in a useful way is at the low end of the LED current range (just above where it turn on).

Andy

[PRR]

....2 LEDs? Thay may have 100-120 mA together. ....

Explain your thinking here.

The usual little LEDs are historically rated 20mA max. Back in 1985 we might need 15mA to be "bright". Today's better parts are BRIGHT with less than 5mA, even 1mA.
https://en.wikipedia.org/wiki/LED_circuit#Power_source_considerations
"The maximum current is shown on LED datasheets, for example 20 mA (0.020A) is common for many small LEDs. Many circuits operate LEDs at less than the recommended maximum current, to save power, to permit the use of a standard resistor value, or to reduce brightness."
https://commons.wikimedia.org/wiki/File:LEDs.jpg

[KarenColumbo]

....2 LEDs? Thay may have 100-120 mA together. ....

Explain your thinking here.

The usual little LEDs are historically rated 20mA max. Back in 1985 we might need 15mA to be "bright". Today's better parts are BRIGHT with less than 5mA, even 1mA.
https://en.wikipedia.org/wiki/LED_circuit#Power_source_considerations
"The maximum current is shown on LED datasheets, for example 20 mA (0.020A) is common for many small LEDs. Many circuits operate LEDs at less than the recommended maximum current, to save power, to permit the use of a standard resistor value, or to reduce brightness."
https://commons.wikimedia.org/wiki/File:LEDs.jpg

I seem to have picked some curious datasheet, can't remember how I got 50-60 mA per LED 

[Fancy Lime]

....2 LEDs? Thay may have 100-120 mA together. ....

Explain your thinking here.

The usual little LEDs are historically rated 20mA max. Back in 1985 we might need 15mA to be "bright". Today's better parts are BRIGHT with less than 5mA, even 1mA.
https://en.wikipedia.org/wiki/LED_circuit#Power_source_considerations
"The maximum current is shown on LED datasheets, for example 20 mA (0.020A) is common for many small LEDs. Many circuits operate LEDs at less than the recommended maximum current, to save power, to permit the use of a standard resistor value, or to reduce brightness."
https://commons.wikimedia.org/wiki/File:LEDs.jpg

Well, you *can* buy LEDs these days, whose current limit is reported in A, not mA. Not uncommon for lighting applications. I would not necessarily use them in a vactrol, though. OTOH, they generate several W of radiated energy, so you could use a thermistor instead of an LDR. Practical? No. But certainly unusual. There's extra mojo points for that, I'm sure. Or you can place the LED and LDR at opposite ends of a long tunnel and make the biggest vactrol ever.

Andy

[Fancy Lime]

....2 LEDs? Thay may have 100-120 mA together. ....

Explain your thinking here.

The usual little LEDs are historically rated 20mA max. Back in 1985 we might need 15mA to be "bright". Today's better parts are BRIGHT with less than 5mA, even 1mA.
https://en.wikipedia.org/wiki/LED_circuit#Power_source_considerations
"The maximum current is shown on LED datasheets, for example 20 mA (0.020A) is common for many small LEDs. Many circuits operate LEDs at less than the recommended maximum current, to save power, to permit the use of a standard resistor value, or to reduce brightness."
https://commons.wikimedia.org/wiki/File:LEDs.jpg

I seem to have picked some curious datasheet, can't remember how I got 50-60 mA per LED 

That's not a totally unusual value for the *maximum allowable* current through an LED before it blows up. But in actual practice, especially in a vactrol, you want to operate a modern, moderately bright LED *way* below it's maximum brightness (and brightness correlates more or less linearly with current).

Because LEDs and LDRs vary quite a bit, you may need to measure the correlation between resistance across the LDR and current through the LED to figure out where the right range of currents for your application is. Make sure to limit the maximum possible current during your test to no more than half of the maximum allowable value as per the datasheet. Or no more than 10 mA to be safe.

Andy

[FingerBlisters]

There's a pedal maker out there somewhere in france who has a trimpot inside for LED brightness to the owner can select their own level. I thought that was pretty neat.

[KarenColumbo]

Here's the budget version for antonis's benefit,



It's possible to do a BJT version but with a high pot value like 100k the base current is going to mess things up,  perhaps even change the linearity of the control.

This one works perfectly. Thanks again!