The current consumption for the RS-485 IC is about 10 to 20 mA.
However, if the two ends are shorted, the transciever can draw excessive current.
So, I am trying to put PPTC device on it's VDD line to prevent it's excessive current from affecting the whole product.
The question is, what is the appropriate value for the current limiting?
Do i simply get the value by 5V divided by the termination resistor they use?
If so, what is the smallest termination resistor they will use? 75 Ohms?
Best Answer
As Umar says, most of those transceivers have built-in short circuit protection. So that'll save the transceiver from damage, but it won't necessarily prevent it from drawing more current than you want.
A PTC fuse is a great solution to this, as long as you are OK with its behavior - once it shuts off due to overcurrent protection, it will stay off until you power down the device. To use one, putting it in-line with the transceiver's VDD should work just fine.
I would size it by looking at the recommended maximum single-leg output current and then double it, and add on the device's quiescent current. Then find the PTC size that's the same or bigger.