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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?

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  • \$\begingroup\$ Most of the RS485 ICs will have short circuit, open circuit protection built in. Which IC is being used? \$\endgroup\$
    – User323693
    Commented Aug 6, 2015 at 5:16
  • \$\begingroup\$ @Umar I am looking at couple of parts which are SP485E and AD1485. They have about 200 to 250mA Short circuit protection included. However, my power budget won't allow the IC to consume that much current. My budget allows only about 100 mA. Will 100mA suffice to drive RS485 Line? \$\endgroup\$
    – Steve
    Commented Aug 7, 2015 at 7:08
  • \$\begingroup\$ RS485 input impedance is 12 kohms. If the circuit is connected to only one device al the time, then the bus currents will be less than a mA excluding IC operating current. 100 mA is more than sufficient. But, due to bus fault (short), if the current consumption increases to 100 mA can affect other modules on the board, current has to be limited. There are a few accurate current limited switches. \$\endgroup\$
    – User323693
    Commented Aug 7, 2015 at 10:22

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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.

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  • \$\begingroup\$ Won't PTC device come back to normal once the overcurrent drawing condition is removed? \$\endgroup\$
    – Steve
    Commented Aug 7, 2015 at 7:14
  • \$\begingroup\$ You're right - that's how they work. Once the overcurrent condition is cleared, the PTC will reset. It's a little bit less clear (for me) how that winds up working with active circuitry though (which you'd have if it was in-line with VDD). PTCs placed in series with the differential outputs would definitely self-reset, but I would also expect some performance loss through them. \$\endgroup\$
    – Nicholas Clark
    Commented Aug 10, 2015 at 17:13

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