Author Topic: 100uA LED current source ideas  (Read 587 times)

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Offline hex4def6

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100uA LED current source ideas
« on: September 13, 2020, 10:36:40 pm »
Hi all,

I'm trying to keep an LED lit to a fair amount of accruacy at ~100uA. Right now, I'm just using a 5V rail, LED, and resistor. However, I'm concerned that there will be device to device brightness bariation doing this, due to variable Vf of the LEDs.

Does anyone have a good idea for a cheap / low part count solution? I'm mulling using a mosfet current mirror / Widlar current source, but I'm concerned that mosfet mismatch is going to be a problem doing this.

I don't need super accurate -- 10% variation is fine.

Suggestions?
 

Offline exmadscientist

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Re: 100uA LED current source ideas
« Reply #1 on: September 13, 2020, 11:24:20 pm »
MOSFET (or better yet, BJT) current mirrors are great if you have a reference current available, but not so helpful if you don't. To get that reference current, the easiest way is usually to use a voltage reference. And since you don't need any great accuracy, you might as well use a cheap "adjustable" reference. A TL431 and a couple of resistors should be able to do exactly what you need. Just set up the LED in series with a sense resistor, and configure the '431 to regulate the IR drop across the sense resistor to your desired current. (You might find that you prefer one of the variants or alternatives with 1.24V feedback voltages.)
 

Offline bdunham7

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Re: 100uA LED current source ideas
« Reply #2 on: September 13, 2020, 11:46:10 pm »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 

Offline S. Petrukhin

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Re: 100uA LED current source ideas
« Reply #3 on: September 14, 2020, 12:00:33 am »
I think the difference in Vf for LEDs of the same type is insignificant and will not greatly affect the brightness. However, if you use LEDs from different manufacturers and different types of the same manufacturer, the brightness will be very different even at constant current.
And sorry for my English.
 

Offline I wanted a rude username

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Re: 100uA LED current source ideas
« Reply #4 on: September 20, 2020, 05:31:48 am »
This seems a ready-made solution:

Also the LM334 as a general solution.
 

Offline Someone

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Re: 100uA LED current source ideas
« Reply #5 on: September 20, 2020, 07:33:11 am »
I'm concerned that there will be device to device brightness bariation doing this, due to variable Vf of the LEDs.
...
I don't need super accurate -- 10% variation is fine.
Check your data sheets, very few LEDs are binned to that level of consistency.
 

Online Zero999

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Re: 100uA LED current source ideas
« Reply #6 on: September 20, 2020, 08:27:13 am »
Even if you regulate the current perfectly, variations between LEDs will cause them to be slightly different brightnesses. Power = VI, so an LED with a higher forward voltage, might be slightly brighter, or have a shorter wavelength, thus be a different colour, than one with a lower forward voltage.
 

Online daqq

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Believe it or not, pointy haired people do exist!
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Offline madires

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Re: 100uA LED current source ideas
« Reply #8 on: September 20, 2020, 07:40:09 pm »
A 0.1mA CLD. ;)
 

Offline S. Petrukhin

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Re: 100uA LED current source ideas
« Reply #9 on: September 20, 2020, 07:47:57 pm »
A 0.1mA CLD. ;)

0.1 or  0.2 or 0.3 or 0.4  :-DMM
And sorry for my English.
 

Offline Benta

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Re: 100uA LED current source ideas
« Reply #10 on: September 20, 2020, 09:43:56 pm »
Here you go:

 


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