Photomultiplier
- Nov 22, 2022
- 18
- Joined
- Nov 22, 2022
- Messages
- 18
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.
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.