I know that some resistors cause pull-up or pull-down or work as a voltage divider and/or current divider.
But sometimes I cannot understand their function in some simple circuits:
Or take this zener diode voltage regulator, The 40Ω resistor, ohm's law says that V = I x R, increasing the resistance lowers the current. But why is that 40Ω resistor needed at all? Why not let it flow straight through (~0Ω)?
Or lastly this RS232 level shifter, I understand the 10kΩ resistors. But why is there a 4.7kΩ resistor between RX(Device) and TX(Device)? (Should they even be connected?)