Electronics > Repair
BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed)
inse:
Have a close look at the comparators (or LM311) datasheet which shows the internal structure.
The output is not push-pull like an OPAMP, it‘s open collector.
Also in your schematic, every(*) output has a pull-up resistor, so if you parallel them, either one can pull that node down.
It is of the same use as an OR gate in a logic circuit.
max.wwwang:
--- Quote from: inse on December 25, 2024, 05:38:41 am ---Have a close look at the comparators (or LM311) datasheet which shows the internal structure.
The output is not push-pull like an OPAMP, it‘s open collector.
Also in your schematic, every(*) output has a pull-up resistor, so if you parallel them, either one can pull that node down.
It is of the same use as an OR gate in a logic circuit.
--- End quote ---
Good point. Must be some sort of logic gate equivalent.
It's LM339, and yes open collector according to its equivalent circuit. This however should mean binding two outputs is equivalent to an AND gate, not OR gate. See attached. Please correct me if I'm wrong.
inse:
One OR the other transistor pulls down the output.
It’s NOR, to be precisely
max.wwwang:
--- Quote from: inse on December 25, 2024, 08:50:37 am ---One OR the other transistor pulls down the output.
It’s NOR, to be precisely
--- End quote ---
I don't get that. In my eyes, any output of LO pulls down to LO. Output is HI only if all (if more than two tied together) is HI.
Daughterboard schematic revised. It starts making some sense (though still a long way to go).
Question now is that why U1A (op-amp) does not have feedback loop. How does that work, will any tiny difference between its inputs blow the transistor on its output? (I need to verify if the schematic is correct).
inse:
If we break it down to the output stage as in your equivalent circuit, it’s definitely NOR.
As long as we agree that 0=0V and 1=Vcc (or at least >0V).
The truth table is shown for example here:
https://www.build-electronic-circuits.com/nor-gate/
The Transistor cannot be blown as the base current is limited by the emitter resistor (1k) and whatever circuit is behind it.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version