Forward voltage drop of LEDs in a low voltage circuit - Page 1
Forum section: electronics / projects

author: leonhart88
author_rank: Regular Contributor
timestamp: October 06, 2015, 07:07:53 pm
content: Hey Guys,Hoping to get some advice on a decision that I'm going to need to make. I'm designing a handheld device that will have a membrane button assembly with integrated tactile domes, etc. in it. I was initially thinking of using integrated LEDs into the membrane as well. The LEDs that the supplier can use are shown here:http://www.nicomatic.com/data/files/8f53295a73878494e9bc8dd6c3c7104f/22102014120936-nicomatic-metal-domes--components-products-datasheet.pdfI plan to look around to see I can find any others, but at the moment, these are the only LEDs that will fit into a membrane assembly for this specific manufacturer. I've been fairly happy with the pricing and support from this company so ideally I'd like to stick with them.The concern I have is that our entire circuit is running at a regulated 2V (max 3.6V from batteries). This 2V is getting fairly close to the forward voltage drop of the LEDs (in fact, the max forward votlage drop of some of those are higher than 2V). Also, the light output for these LEDs is fairly small considering a 20mA current. Battery life is a concern in this regards.My question is...assuming I cannot find other LEDs and that I still plan to go with this supplier. Should I:a) Just try it with 2V and hope for the best?b) Simply put the LEDs on the PCB instead and live with the fact that the viewing angle of the LEDs may not be as nice (or use light pipes)?c) Tie the LEDs directly to the battery output - this means that as the battery life fades, the brightness of the lights will changed) Look for some LED drivers - this means extra componentsThanks for the advice!

author: Gyro
author_rank: Super Contributor
timestamp: October 06, 2015, 07:16:05 pm
content: Those LEDs are effectively point sources (no lens). I would think you would get usable light output at significantly less than 20mA.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 06, 2015, 07:43:46 pm
content: You could drive LEDs with drop resistors off the battery voltage instead of limiting yourself to 2V.Alternately, you could use a CUK/SEPIC/pump converter to power the 3.7-3.8V blue/white LED variants.

author: JoeB83
author_rank: Regular Contributor
timestamp: October 06, 2015, 09:27:41 pm
content: You say you'll get 3.6V from the battery-is it a LiPo or similar? The reason I ask is damage to the cell will happen around 3V or so, (obviously) meaning you'll have a minimum of 3V. A Linear Technology LT1963 regulator has a maximum 340mV dropout (& much lower at low currents, ~100mV @ 100mA) you could set it up to regulate at 2.5-2.65V, which should work. I believe there's an LT1963 that's pre-programmed at 2.5V as well. It's tiny, too, I believe you can get it in SOIC-8 and SO-223.Forgot to mention its quiescent current is only 1mA, so it's good for battery use.Here's the data sheet:http://cds.linear.com/docs/en/datasheet/1963fc.pdf

author: leonhart88
author_rank: Regular Contributor
timestamp: October 06, 2015, 09:41:22 pm
content: Quote from: Gyro on October 06, 2015, 07:16:05 pmThis may be true, but I'd like to be 100% certain before committing to purchasing these membrane assemblies, as we would have to commit to a larger quantity in order to make it cost effective. I could possibly ask for some samples to test at 2V with, but I would still be concerned if the max forward voltage is 2.4V from batch to batch (unfortunately, they don't have any curves indicating what the forward voltage is for lower currents...perhaps I shall ask).Quote from: DanielS on October 06, 2015, 07:43:46 pmYes, I may need to do that and live with the fact that the brightness will change with the battery voltage. I will be mainly looking at the yellow and green/red LEDs, which have a max forward drop of 2.4V.Quote from: JoeB83 on October 06, 2015, 09:27:41 pmThese are disposable Li-SOCL2 batteries that are 3.67V. The circuit needs to be regulated to 2V in order to meet temperature range restrictions and other parameters (including a voltage drop for an intrinsic safety current limiting resistor). I can revisit these numbers again to see if I can bump up the regulated voltage a little, but I think I am mostly restricted here.

author: Someone
author_rank: Super Contributor
timestamp: October 06, 2015, 10:07:23 pm
content: Quote from: leonhart88 on October 06, 2015, 09:41:22 pmThe slope of the I/V curves is so high that you will not be able to rely on batch to batch (and quite possibly device to device) variation to be low enough to work with such a low voltage source. You'll need to look at a current controlled design with some from of voltage boosting, the energy can be limited by design but EMC might be a problem.

author: Fungus
author_rank: Super Contributor
timestamp: October 07, 2015, 02:45:45 am
content: Those LEDs don't seem to put out much light even at 20mA. I'd definitely get a few samples to play with before making any decisions.Quote from: leonhart88 on October 06, 2015, 09:41:22 pmHow far will you discharge he battery? 3.6V down to 3.0V?How many LEDs will be on the board? A constant current circuit for multiple LEDs could add up to a lot of components. Might need to go to an IC.

author: leonhart88
author_rank: Regular Contributor
timestamp: October 07, 2015, 03:58:36 am
content: Quote from: Someone on October 06, 2015, 10:07:23 pmAgreed, the lack of reliability is exactly why I had a second thought about this.Quote from: Fungus on October 07, 2015, 02:45:45 amYeah, they definitely do not output much light. I can't remember the number I calculated, but I believe I was planning to discharge the battery to only 2.3V or so and force the mcu into sleep mode if less than 2.3V. The entire circuit is regulated to 2V.There are 7 LED indicators (not all on at the same time).I'm starting to think more and more that the easiest and least risky solution is to forgoe the option of integrated LEDs on the membrane button and find some LEDs that will work just from my PCB. If I can find LEDs with a large enough light output, I'm sure they will work great as an indicator. The only issue here is since the LEDs are behind the enclosure face on the PCB, the viewing angle will be diminished. However, this is a handheld device and the viewing angle really isn't that important here.If viewing angle is an issue, I can always try to add light pipes onto the PCB...