Author Topic: Burned LCD reverse capacitor?  (Read 1987 times)

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Offline @rt

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Burned LCD reverse capacitor?
« on: May 10, 2016, 11:40:00 am »
Hi Guys,
I just connected a character LCD directly to an interface that was running a similar one fine for some time.
It was just changed for a change of colour to white on black rather than the ordinary black on green ones.

The LCD ran ok for just under 14 seconds and burned with smoke, and didn’t work again!
It was a bit dark, but I didn’t have time to try adjusting contrast, but definitely readable.
I’m wondering what the reason would be for the negative supply line to connect to the positive end of this capacitor (C2).
The other side of the cap is not common with the positive. I’ve checked the datasheet, and this LCD is definitely ground on pin 1.
It has to be because it worked for 13 seconds.
I have a spare of these LCDs, but afraid to connect it or the same thing might happen.
Cheers, Brek.

 

Offline JPortici

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Re: Burned LCD reverse capacitor?
« Reply #1 on: May 10, 2016, 01:21:15 pm »
Stupid question: Are you driving it with 5V Logic? Does it whitstand 5V Logic (or whatever voltages you give it)?
 

Online wraper

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Re: Burned LCD reverse capacitor?
« Reply #2 on: May 10, 2016, 01:35:16 pm »
Negative voltage supply provided by internal charge pump.
 

Offline Jeroen3

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Re: Burned LCD reverse capacitor?
« Reply #3 on: May 10, 2016, 01:37:07 pm »
Those yellow capacitors are tantalum capacitors. These have a black indicator for the positive side.

Example: http://datasheets.avx.com/TAJ.pdf
« Last Edit: May 10, 2016, 01:40:42 pm by Jeroen3 »
 

Offline @rt

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Re: Burned LCD reverse capacitor?
« Reply #4 on: May 10, 2016, 01:50:17 pm »
Ok, thanks for the replies. Understood about the markings on the caps :)
I don’t have an explanation other than a dud module then.
It’s a 5 Volt module, and the rest of the circuit has other output and still 5 Volts, and chugging away fine without the LCD.
The transistor turning the backlight on can still light an LED, and the LCD backlight still works well.
Given that every other connection had to be correct for the LCD to work, I don’t have anything else lol.


Edit,,, yeah that’s what I said, the negative supply line to the module is directly connected to the positive side of C2.
« Last Edit: May 10, 2016, 02:53:43 pm by @rt »
 

Offline @rt

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Re: Burned LCD reverse capacitor?
« Reply #5 on: May 10, 2016, 04:30:37 pm »
The second one is a dud as well lol. It’s like a 15 or so degree viewing angle.
I got better looking fancy character LCDs for cheaper ten years ago.

 

Online wraper

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Re: Burned LCD reverse capacitor?
« Reply #6 on: May 11, 2016, 02:54:07 pm »
Edit,,, yeah that’s what I said, the negative supply line to the module is directly connected to the positive side of C2.
Do you understand difference between GND and negative supply?. LCDs need negative supply which is "more negative" than GND. As current recuired is low, usually there is charge pump circuit which provides it.
 

Offline @rt

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Re: Burned LCD reverse capacitor?
« Reply #7 on: May 12, 2016, 02:15:27 am »
Yes, but I shouldn’t have to worry about it except where a contrast pin requires a voltage below zero.
Are you saying that’s why the capacitor is connected like that? I understand there’s probably not an error in both the silkscreen,
and placement, but still wondering why. Maybe that’s what the eight legged chip is for.

I have substituted this one anyway. It’s backlighting was not uniform, and it does require a voltage below zero for it’s
contrast pin if you want to be able to view it straight on which is not what's happening in the photo, I’m looking down on it slightly.


 


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