100uA LED current source ideas - Page 1
Forum section: electronics / projects

author: hex4def6
author_rank: Newbie
timestamp: September 13, 2020, 10:36:40 pm
content: 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?

author: exmadscientist
author_rank: Frequent Contributor
timestamp: September 13, 2020, 11:24:20 pm
content: 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.)

author: bdunham7
author_rank: Super Contributor
timestamp: September 13, 2020, 11:46:10 pm
content: This seems a ready-made solution:https://www.mouser.com/datasheet/2/395/TSCR400_402CX6H_A1905-1888726.pdf

author: S. Petrukhin
author_rank: Super Contributor
timestamp: September 14, 2020, 12:00:33 am
content: 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.

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: September 20, 2020, 05:31:48 am
content: Quote from: bdunham7 on September 13, 2020, 11:46:10 pmAlso theLM334as a general solution.

author: Someone
author_rank: Super Contributor
timestamp: September 20, 2020, 07:33:11 am
content: Quote from: hex4def6 on September 13, 2020, 10:36:40 pmCheck your data sheets, very few LEDs are binned to that level of consistency.

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2020, 08:27:13 am
content: 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.

author: daqq
author_rank: Super Contributor
timestamp: September 20, 2020, 07:27:27 pm
content: https://www.ti.com/lit/ds/symlink/ref200.pdf?ts=1600630029710&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FREF200

author: madires
author_rank: Super Contributor
timestamp: September 20, 2020, 07:40:09 pm
content: A 0.1mA CLD.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: September 20, 2020, 07:47:57 pm
content: Quote from: madires on September 20, 2020, 07:40:09 pm0.1 or 0.2 or 0.3 or 0.4

author: Benta
author_rank: Super Contributor
timestamp: September 20, 2020, 09:43:56 pm
content: Here you go: