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LED circuit contains caps, diodes, resistors, what's going on with this?

Photomultiplier

Nov 22, 2022
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Nov 22, 2022
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Hello,
I purchased a DMM and I'm satisfied with it minus it's backlight not staying on and the LED on the front being even dimmer than the backlight. To fix this, I thought I'd open it up and change out the resistor. "It's probably too high a resistance," I thought. I don't/didn't think I'd damage the circuit by changing the resistor's value. Almost all SMD transistors and MOSFETs are able to handle 20mA (there's literally only one transistor on mouser which cannot; 2SC5095-R, and 3 SOT-23-3 MOSFETs).
Well, I opened it up and I have absolutely no idea what's going on. I expected a resistor and a transistor to switch it on. What I got was anything but.
It's a TA801C DMM. It's a 2 layer board but I can't see much of the front without removing the LCD, which I'm unwilling to do because I'm uncertain that I could reconnect it properly. I've highlighted 2 photos using red and blue. My apologies for the quality of the photos. The Iphone appears to have tried to focus on the wires on one, and the other, well, the modern camera sensor should be big enough, but you can't clearly see the 300 on the SMD resistors surface.
The LED's VCC pin goes to one resistor, 30ohms, then through 2 diodes with a capacitor in the middle (why?) then through another resistor, which is also 30ohms, before returning to the positive battery terminal.
The LED's negative terminal goes to a transistor. I have had a problem tracing the leads, I'll try harder if you'd like. One of the leads goes to another diode, that's white (is it special somehow? ), in the opposite direction to a capacitor, and to another transistor before disappearing underneath of the buzzer.

So, what's going on with this LED circuit? Would it be advisable to just change out the resistors, or do I need to do something more serious with it?

Thanks!

PS: To the guy who points out that an LED is a diode, so my subject line is redundant and confusing.:p
 

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kellys_eye

Jun 25, 2010
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The LED's VCC pin goes to one resistor, 30ohms, then through 2 diodes with a capacitor in the middle (why?) then through another resistor, which is also 30ohms, before returning to the positive battery terminal.
I very much doubt the crystal body connects to Vcc and there's no way an LED would work with BOTH ENDS connected to Vcc...... neither would a capaitor be part of the arrangement as they block DC. Clearly your tracing is incorrect.

What is this LED? As far as I understand it is a 'built-in torch'........ a gimmick at best and a waste of time any other way.
 

Delta Prime

Jul 29, 2020
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Hello there :)U1 TASI 5101-0002 is either SD 7501 or SD7502 controller if your DMM came with a peak hold and surge measurement you have a SD 7502 and you have to do your homework and find that one. This is why data sheets are so important they give you all the answers!
This is just a single page of the entire data sheet and if you do find one you're going to have to translate it. Now you can hit the like button thumbs up. ;)
Screenshot_20230508_050831.jpg
 

Photomultiplier

Nov 22, 2022
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Nov 22, 2022
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I very much doubt the crystal body connects to Vcc and there's no way an LED would work with BOTH ENDS connected to Vcc...... neither would a capaitor be part of the arrangement as they block DC. Clearly your tracing is incorrect.
I wrote it out in my post, but the blue line is pointing to the other side of the resistor. I used blue to differentiate it to red, not to indicate polarity.
In any case, the resistance is practically zero between these points, so, unless my other DMM is triggering something, they are connected.
 

Photomultiplier

Nov 22, 2022
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Nov 22, 2022
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As Kellys_eye would apparently like a fully marked up image (I was trying to avoid covering over the traces and vias), I've attached an image below fully marked up. I traced things a bit farther this time, but it doesn't really help matters.
EDIT: Unlike last time, this time the colors are red for VCC and blue for ground.
 

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Photomultiplier

Nov 22, 2022
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Nov 22, 2022
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Yeah, both datasheets use example circuits... I'll have to figure out what they're doing to some extent in my design.
 
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