I've followed this tutorial to replace some broken leds on my keyboard. The leds I used are:
Emitted Color : White Size (mm) : 3mm Lens Color : Water Clear Color Temperature (K): 6500 ~ 7000 Forward Voltage (V) : 3.0 ~ 3.4 Forward Current (mA): 20 Reverse Current (uA) : <= 30 Luminous Intensity Typ Iv (mcd) : 6000(Typical) ~ 8000(Max) Viewing Angle : 20 Degree Pin Length: 28.5mm and 26.5mm
They should be ok according to that tutorial (I haven't been able to find the exact specs of the ones that came with the keyboard).
Polarity was not specified on the board so I used a voltmeter and made sure the voltage was reported as +3.xx volts. Anyway, I've solder them and it worked, the type of white is a bit different but close enough.
Problem is, backlight can be turned off or made brighter/dimmer, I usually keep it at mid brightness (and at mid brightness all looked fine).
- If I lower the brightness, replaced leds brightness do not change
- If I raise the brightness, replaced leds get dimmer!
- If I turn of backlight, replaced leds stay on
I plan to keep it at mid brightness 99% of the time so that's not really a problem but I'd like to know why it behaves that way. If I got the polarity wrong they should just stay always off right? Brightness variation is done by changing the current right? Maybe the replaced leds have a lower minimum current? This could explain why they don't get dimmer as the others and don't turn off, but then why do they get dimmer when I rise the brightness setting? Could they be receiving too much current and get dimmer as result of being in the process of breaking? If that's so, why do they regain normal brightness when I set it to medium again?
Thanks, as you might have noticed I know very few of electronics.
EDIT: I've measured volts/amps with all brightness settings:
v.o = volts across an original led v.r = volts across a replaced led ma.r = milliamps through a replaced leds brightness v.o v.r ma.r max 1.3 0.8 0.01 mid 3.3 1.8 0.15 min 4.2 2.3 0.54 off 4.6 2.4 0.59
At this point I wanted to know the current through an original led so I desoldered one, and discovered that I indeed got the polarity wrong as @Jasen suggested in the comments. I've re-soldered everything and 2 of the 3 replaced leds behaved as intended, the remaining one stayed off. So I did not have 3 broken leds, but 2 broken leds (that worked when replaced correctly) and a broken "slot", and guess what "slot" I used to figure out polarity at my first try? The broken one.
Now, I would be happy with just one broken led, but I must have done something wrong during the last session of tests/soldering because now the keyboard does not work any more, every key repeat itself lot of times. Damn me the moment I thought I could not live with 3 keys not lighting up.
Fun fact, the keyboard worked with the 3 replaced leds with inverted polarity.
To give this question an answer, the correct guess was that of @Jasen, it would have worked if I tried it first with any slot but the broken one. If you want to write a small answer I'll accept it, otherwise I'll answer my own question.