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Supply Voltage Overvoltage Protection

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#1 jthurbs

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Posted 01 October 2014 - 06:08 AM

Is there any overvoltage protection on the supply voltage to a UE9?  The user guide lists a range of 3.6V to 5.3V for the supply current.  I have a labjack that I know has had 12V plugged in the supply power jack.  It appears to be functioning but yesterday I noticed that it is having issues communicating over I2C using FIO3 as SDA and FIO2 as SCL.  What idea what liklihood is that the unit was damaged?


Thanks much

#2 LabJack Support

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Posted 01 October 2014 - 08:57 AM

The UE9 has zener diodes on the supply inputs (Vext & Vusb) that will clamp negative voltages and overvoltages, but since this is a supply input there is no series resistance to limit current.  The zeners are rates for 500mW, so if the 12V supply can't provide more than 500mW the zeners might provide continuous protection but if the supply can provide more than that it will likely blow out the zeners pretty quick and cause further damage to other parts of the device.


The most common damage would be to the supply input circuitry such that the device does not work at all.  Damage to just a couple DIO lines from a supply input overvoltage would be very unusual.  To check for supply damage use a DMM to resistance with nothing else connected to the UE9 (no power, no I/O, nothing but the DMM):




The resistance of each power rail to ground (with nothing connected to device), should be kiloohms or more (>50k is typical for most rails).  Rails to check versus GND:

VS (any screw terminal)
Vusb (pin 6 of U17)
Vext (pin 8 of U17)
3.3Vcontrol (pin 5 of U14)
3.3Vcomm (pin 5 of U15)
2.5Vcomm (pin 4 of U16, C145 is best place to measure)



To check out a particular I/O, use a 1k resistor, DMM, and the test panel in LJControlPanel to do some tests:


Jumper FIOx to GND with a 1k resistor.

    - As input FIOx should read low.

    - As input I read 0.025V on FIOx (versus GND).

    - As output-low I read 0.003V on FIOx.

    - As output-high I read 2.18V on FIOx.


Jumper FIOx to VS with a 1k resistor.

    - As input FIOx should read high.

    - As input I read ~VS on FIOx (versus GND).

    - As output-low I read 1.58V on FIOx.

    - As output-high I read 3.82V on FIOx.


The numbers for an EIO/CIO/MIO would be a little different because they have higher series resistance.

#3 jthurbs

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Posted 03 October 2014 - 08:09 AM

It looks like it all checks out.  Thanks for the help.

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