# What's the current for a LED when Vcc < Vfwd?

I have a project that powers RGB LEDs with 3v coin battery (CR2032)
so Vcc = bat = 3.0v (drops to 2.7v when nearly dead), which is below the forward voltage of Green and Blue (3.2V).
(actually I’m powering them with AVR pins, where another 0.4v drop is applied, so it’s actually 2.3~2.6v)

for Red, I just put a 120 ohm to limit the current to 10mA.
I’m using a 1 ohm resistor to limit the current, but what I wanna know what’s the actual current applied to Green and Blue?

I wanna give Green and Blue ~10mA too, to get similar brightness as Red.

it’s a really small device, and LED brightness is not really that important, so I don’t want a LED driver IC.

Another coin cell would be impossible?

Having less than VF, being there on the knee like, puts you in a weird area.
It’s not where you want to be, you don’t “design” to take advantage of the knee region.

i’m making something like a earrings or pendant, put 2 coincell… would be a bit too heavy i guess…

Just see whether connecting the CR2032 directly to your blue or green LEDs will light them (and measure the current with a multimeter on the 200mA range). If it will, then it is not unreasonable to control it with a microcontroller because the switching component is a FET, not a transistor, and the “0.4V drop” you cite is actually a resistive drop specified at a higher current, thus proportionally lower at lower currents.

It has been pointed out to me that the “dead” voltage of a lithium coin cell is actually 3.1V, not 2.7V, so you have more leeway there anyway.

Finally, a 2032 would make for rather large earrings. If you can actually accommodate that, then two 2016 cells make up the same thickness and weight. Mind you, you would then actually have to drop the voltage (with two diodes) to supply the microcontroller VCC but have the LEDs powered from the full 6.5V. Since the LEDs do introduce a voltage drop, they cannot pull the pins above Vcc and it is perfectly safe to connect them in this way.

this is the design (copper fill removed for clearance).
the ATtiny13 is actually a ATtiny84, they share same pin def but there’s not SO-8 tiny84 in fritzing.

and yes, Green and Blue are lid down to 2.3v (just very dim), have to look very close or shut the lights.

for the 0.4v drop… i think driving a pin with 30~40mA would be considered “high current” for an AVR, if i’m not using any current limiting techniques the LEDs will draw as much current as it can.
just dunno how much…

I = V * some unknow factor (at least I dunno…)
V = I * some known factor (written in the datasheet)
solve for V, I, easy math.

stacking 2 2016… might be a solution, but it cuts capacity more than half (240mAh vs 90mAh), and the linear regulator (the 2 diodes acts as a linear regulator here, it don’t really “regulate” tho…) consumes power, too.

For around 3-4 bucks on ebay you can find LED earrings. ..

cjdelphi: For around 3-4 bucks on ebay you can find LED earrings. ..

no PWM, they just lit.

i actually have a idea to make one with cr1025 battery, just can't find the holder.

Depends, some of them come with a tightly fitted 5mm led, which cycles between colours, that probably uses pwm...

maybe i should just put a USB connector there and allow users to upload custom patterns so ppl think its superier.

hmm... don't bother, 1 ohm is drawing too much current from the battery and make to voltage drop to some point that avr will just reboot (even tho i'm running the core @ 1mhz...).

had to use 120 and 220 anyway.