Tiny LEDs - I forgot all the physics from school and would love advice

(Jack Silby) #1


So I was looking at tiny LEDs recently for a project that would require quite tiny lights about 3cm or so large. I found some really cheap ones https://tinyurl.com/y3sj5h4t in china. I notice even these small led lamps require 12 volts.

I understand 12 volts is approx 8 regular aa batteries (1.5 each). There is such a thing as a 12 volt aa looking battery / a smaller looking battery - though these seem uncommon and I would like to stick to a more common size if possible.

Is there a better way of doing all this? Are my assumptions even correct?

I am trying to acheive small light + long lasting (30hr+)+easy to get batteries+not having to load alot of them into the battery pack to power the light - say over 3.

Many thanks!

(Andrea Campanella) #2

Yeah no, that LEDs are ment to be use in 12V lights , not for battery operated projects , you need normal LEDs and a led driver…

(Rich Maynard) #3

(Jack Silby) #4

Hey thanks for your replies. Throwies look like good fun and got me thinking! Suggestions have LED me (insert laughter here) to here https://www.mouser.co.uk/ProductDetail/Inolux/INL-3AW30?qs=qSfuJ%2Bfl%2Fd6qMaZAE7x9qw%3D%3D and here https://www.mouser.co.uk/ProductDetail/JKL-Components/7207?qs=sGAEpiMZZMucm%2F%2BFOY0TQtr%2FchrLORC09jXegDEKhG0%3D

What I have learnt: To still want to use AA batteries at 1.5 volts I would need at least 2 for 3 volts. due to physics which I do not understand extreme end spectrum light frequences such as green and red and blue can be produced at lower voltages, but not white. At 3 volts I can get the second linked LED - thought the light it produces seems like it would be a too pitiful amount of lumens - even for a tiny model light. The first link shows a 3.1V light - this I understand would be too high a barrier of volts except that the 1.5 volts in an aa battery is nominal and they usually come when ‘fresh’ with about 1.65V. I dont know about voltage drop off rates over time but maybe this is a way forward.

My current plan (unless anyone can tell me why this may not be a great idea) is to get a battery holder with switch and just connect all this together to test - https://www.ebay.co.uk/itm/AA-AAA-9V-PP3-Battery-Holder-Connector-Enclosed-or-Open-with-Switch/281526522654?hash=item418c49d31e:m:mEJysrPszTERc5fcpbXpJtg

I have not bought any drivers - it was really hard to find one for my voltage levels - maybe everything is more or less ok with this small amount of volt disparacy? :face_with_raised_eyebrow:

(Andrew Chisholm) #5

If you just connect LEDs to a battery, it’s very likely you’ll burn out the LEDs. Once the battery voltage exceeds the ‘forward voltage’ of the LED, the current can become very large, exceeding the LEDs’ ability to dissipate heat.

LED controllers are great, but require a bit of wiring up etc. The lowest tech solution is a resistor in series with the LED(s). If you know the battery voltage, the LED forward voltage and the desired current, then the minimum resistance you need is
( V(battery) - V(forward) ) / desired current

A higher series resistance is safer. LEDs often specify the max current they can handle, usually written as Imax. I would use a lower value, eg if Imax is 100mA, you might use 60mA ie 0.06 in the above formula.

Apologies if that’s all obvious!

(Jack Silby) #6

Nothing is obvious to me! My first time! Ok so taking my LEDs that are rated 3.1V with a forward current of 20mA (which I think is the same as lmax?) and your maths.

New Battery Example

(3.3V - 3.1V) / 0.2A = 1 ohms?

Old/Nominal Battery Example

(3V-3.1V) 0.2A = a minus number so no resistor required

Is this right? With this in mind - should I both with a 1 ohm resistor?

(Andrew Chisholm) #7

Well, yes and no. 20mA is 0.02A, not 0.2A. So you’d need 10ohms, not 1.

Things is, you’re relying on the apparent voltage of the batteries and the forward voltage too heavily. Forward voltage is pretty rough and ready. You’ll get a massive dip in brightness as the batteries start to fade.

Could you bear to use 3 batteries instead of 2? If so, you’ll get much more consistent light output over the life/charge of the batteries.

Re flat battery: personally, I would never connect an led to a battery without some kind of current control - you can destroy an led instantly.

(Andrew Chisholm) #8

With three batteries, the calculation would be

(4.95-3.1) / 0.02 = 92.5ohms. I’d use 120 ohms or more to be on the safe side … The higher the resistance, the longer your battery life.

Good luck :smiley:

(Rich Maynard) #9

The reason the throwies work without any obvious resistor or current limiter is that the internal resistance of the battery limits the current

(Andrea Campanella) #10

True, the next step up from limiting resistors are led drivers btw.

(Howard Batchen) #11

There are plenty of dirt cheap led driver boards mostly made for torches with white LED’s, Many run from a single AA or a single Li-ion cell. They are constant current boost converters / regulators and use chips designed for the purpose.