Reverse polarity protection for AD5412

Hi, 

i am successfully using an AD5412 in my design. The user can connect 3 wires:

- 24 V (industrial power supply)

- GND (ground from power supply)

- Output (4-20mA or 2-10V)

If the user connects the wires accidentally in a wrong way the AD5412 gets burned. I assume because of too high current for the interal clamping diodes.

So i added a reverse polarity schottky diode in series in each wire and everything is fine.

This solution has one disadvantage: The diode in the ground-path generates an unwanted offset voltage.

What is the recommended way to protect the chip for reverse polarity? Especially, how can i avoid to place a diode in the ground-path?

Thank you very much

  • 0
    •  Analog Employees 
    •  Super User 
    on Sep 15, 2020 9:47 AM

    Hi, 

    You can try adding your diodes from output and 24V to GND. Just make sure the diode can survive the max current probably from the industrial power supply when the reverse scenario occurs. And that it can survive long enough for the user to figure out that he made the wrong connection.

    You can even add an led + R in this case to alert the user. 

    This can also be done with an additional fuse in series with the output and 24V. just make sure that the fuse can survive the transients on the lines but not so high that it won't be "tripped" when reverse connection occurs. 

    One more way, i personally do this for my designs, is to use polarized connectors on the power in/out pins. it helps a lot to reduce human error. 

    These are all just basic reverse protection schemes. Btw, Is this for a loop-powered application? Maybe you can share your schematic and we can figure out a better way if needed. 

    Best regards,

    Ian

  • Hi,

    unfortunately there is not enough space left for a diode from 24V to GND which can handle > 2 Ampere @24V from industrial power supply. The design is used for a sensor. For this the fuse has to be resettable automatically. A keyed connector is not possible because the user has to connect single wires. The sensor should only be powered by the power line and not by the loop.

    The following schematic shows the design with a polarity diode only in the 24V line. If i insert a diode into the output and GND line everything is fine except that i want no "GND-offset" because of the diode in the GND path.

    Case1: Everything is fine

    Case2: Wrong connection by user: AD5412 gets destroyed because of current from GND to AD5412-output.

    Thank you

  • 0
    •  Analog Employees 
    •  Super User 
    on Sep 15, 2020 11:26 AM in reply to Orle

    Hi, 

    what current are you expecting from the source when case 2 happens? Is the board with AD5421 made already? if so with space constraints, it might really be hard to fit big components for protection.

    When you measure the output voltage with the ground offset, which ground are you using? the circuit gnd or gnd from industrial ps?

    Best regards,

    Ian

  • Hi, 

    the power supply is provided by the user, so maybe 5 ampere...the dimension of the board is fix, but schematic and layout can be modified. The following solution is working. But i think diode D1 with it´s voltage drop of about 300mV is not good for EMC and other not shown parts of the schematic like the sensing part. Also i have never seen a diode in the ground line...

    Do you think i can insert D1 into the ground path of the AD5412 only so that other parts of the design are not affected. For example between AVss, AVss1 and GND? Which GND pins of the AD5412 have to be protected by a series diode? All of them?

    Problem is that without D1 current can flow from user GND to Vout and/or Iout in case of reversed polarity.

    Thank you