[AMBIKA] LCD Backlight problem (0k resistor)

Hi,
I assembled my first Ambika, i have no problem all works, but the backlight of LCD do not work.

I buy this (http://fr.mouser.com/ProductDetail/Optrex/C-51850NFJ-SLW-ADN/?qs=%2Fha2pyFaduhGbSh%2bwVDlK3l868dZYqWZKo%2FkSdcFDOSs1pocJ31mIQ%3D%3D) … (ref in BOM : 805-51850NFJSLWADN)

On first run, the backlight has worked, after 30s, the backlight has lightoff.

since the Backlight no longer works.

I have a doubt on 0K resistor (strap) … Should a specific value for this resistor?

Thanks

From the assembly instructions:

“”"

Note the bridge done with the leg of a resistor on the resistor labelled 0R. This is because the LCD module provided by Mutable Instruments does not need a current limiting resistor for its backlight. However, if your LCD module comes from Optrex (green on black, with a white frame), you need to solder a 100 ohm resistor there – otherwise the internal backlight LED of the module will be damaged.
"""

The backlight of the original display has a built in current limiting resistor but some, if not most, displays does not. To get the proper value for the external current limiting resistor we have to consider the the supply voltage, the forward voltage of the backlight led and the maximum current that the backlight can handle. The two latter can be found in the display datasheet and in the case of the Ambika the supply voltage is 5V. To calculate a safe value of the external current limiting resistor we use Ohm’s law U=R*I. U is the supply voltage minus the forward voltage of the backlight. I is the current bellow the maximum current you feel is safe, I use 80% of the value in the datasheet which would possibly cover some for over voltage and give a longer life for the backlight. This gives us R=(Us-Uf)/(Im*0.8). For example if Uf is 4.2V, Us is 5V and Im 60mA a safe R=(5-4.2)/(0.06*0.8)=0.8/0.048=17ohms. However the brightness of the backlight could make you choose a bigger R resulting in a less bright display.

Ok … I burned the backlighting.

Just a suggestion, put this paragraph on Bill of materials paragraph (Some friendly notes for the self-sourcing crowd)

After some test, the sound is really really good … Good job Pichenettes, i’m wait another voice card (For me, a Polyvoks and Anolog drum voice)

i have solder a 100 ohm resistor, now, the synth do not work correctly (no sound, noise …) … after release this resistor, the synth works …

Someone tested with Optrex LCD?

Since you’ve probably damaged the backlight it could be that the backlight now acts like a short, which is kinda strange when I think about it, and that your 5V can’t supply enough current and keep the voltage needed for the other parts at that supply to work. I use an Optrex black on white display and I know others that use other Optrex displays and they work fine.

ok, thanks :wink:

Check for any shorts (solder bridges, stray bits of metal) around the LCD and the 100 Ohm resistor. If there’s a short in the circuit for the backlight that would also explain why it went out.

@Lindeborg , … I also use black on white Optrex display. After putting it in a case just yesterday i now noticed the left side is darker then the right side. Almost a bit blueish. I think it came from that it becomes very hot on that side next to the regulators. Did you notice something similar ?

Hm, not yet. But mine has not been on for any long sessions since it came in the enclosure at approximately the same time as yours. I’ll keep an eye on it…

hi guys, sorry for the question, but i can’t find resistor labelled 0R, were i need to sold the 100 Ohm, has something to do whit the bridge on the sw?

thanks in advance!

Guess what sort of resistor has no resistance? the sort the looks like a piece of wire :wink:

If your display needs the 100 ohm resistor it needs to be soldered where it says 0R. If your display doesn’t need the 100 ohm resistor then you use a piece of wire as a bridge.

i can’t find where it says 0R :slight_smile: it’s my first ambika and I don’t have much experience

thx for the reply

Righthand side above the LCD, left to the trimmer, seen from above. Right side is there ->

ok! in my pcb is called R_LCD, :slight_smile: thx to all

Camouflage is essential….

I renamed it R_LCD because 0R might be misleading to the people who bought displays from other manufacturers than those that come with the kit.

Other (non-MI) kits have had jumpers and even a trimmer for the backlight, resulting in popped backlights. So if this causes people to pause and ask a question that is a good thing :slight_smile: