EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: hex4def6 on September 13, 2020, 10:36:40 pm
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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?
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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.)
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This seems a ready-made solution:
https://www.mouser.com/datasheet/2/395/TSCR400_402CX6H_A1905-1888726.pdf (https://www.mouser.com/datasheet/2/395/TSCR400_402CX6H_A1905-1888726.pdf)
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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.
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This seems a ready-made solution:
Also the LM334 (https://www.ti.com/product/LM334) as a general solution.
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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.
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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.
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https://www.ti.com/lit/ds/symlink/ref200.pdf?ts=1600630029710&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FREF200 (https://www.ti.com/lit/ds/symlink/ref200.pdf?ts=1600630029710&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FREF200)
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A 0.1mA CLD. ;)
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A 0.1mA CLD. ;)
0.1 or 0.2 or 0.3 or 0.4 :-DMM
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Here you go: