# Understanding replaced leds brightness behaviour (keyboard backlight)

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.

• How much current are you pushing? If you are way over the limit, increasing the current will dim the LED. – winny Jan 19 at 12:26
• Maybe you have a short in your soldering. Check it carefully for shorts and opens and make sure the new leds are exactly where the old ones were. – Spehro Pefhany Jan 19 at 12:54
• @SpehroPefhany I'm confident that this is not the case, components density is low, it should be easy to spot shorts, and besides that, the three leds I've replaced are far away from each others and they all behaves exactly the same – flagg19 Jan 19 at 13:27
• re-measure the LED voltage with it set to maximum brightness, use a diode in series with the voltmeter and probe both directions - trhe signal could be PWM and maybe you have the LEDs connected backwards – Jasen Jan 19 at 22:17
• You have not provided pictures or a schematic. Do the LEDs have series resistors? If so, metering voltage over them will reveal LED current. When you increase the brightness and the LEDs dim, do they become hot? Power down the circuit and carefully touch the LED immediately after the dimming behaviour is seen. When overdriven, LED efficiency decreases, increasing heating, which decreases "resistance", increasing overcurrent and the cycle repeats, only broken by some form of current limitation. Your current limitation was designed for a different LED and could be holding the new LED in OC. – K H Jan 20 at 0:22