Fault Capacity of -VREF

Alex Macintosh 3 months ago in Master modules / MC4x updated by Gustav Widén (System support) 3 months ago 5

In the IQAN MC4x/XC4x instruction book on page 67, it is recommended that sensors requiring a supply of more than 5V have their returns connected to -VREF instead of chassis ground. I understand the reason for doing this is to minimise sensor error, however, if the sensor supply is shorted to -VREF there will be a large fault current through the -VREF pin. What is the fault current handling capability of a -VREF pin? Furthermore, if this pin is not capable of handling high fault current for the time it takes for a typical thermal circuit breaker to trip, would powering the sensor from a digital output with current limiting be a preferred option?

In case there is a short across the sensor, the MC4x/XC4x traces on the -VREF on handle up to 10 A.

You could power the sensors from a Digital out HS (disable undercurrent detect), or supply the sensors via a maximum 10 A fuse. 


Hi Gustav,

Thanks for your reply. Could you please clarify for how long the -VREF can handle 10A? Reason I ask is that the fuse or circuit breaker will draw a lot more than its rated current for a short period of time before it trips. For example, a 3A circuit breaker will take up to 2 seconds to open at 600% overload (18A).

I would like to use a Digital out to power the sensors as these will trip faster than a circuit breaker, however most sensors (including the ones I am using) take more than the cycle time to start up and so I will get out of range errors on the voltage inputs until the sensor output stabilises. If there is a workaround for this, please let me know.

The 10A on the -VREF is continuous, but I don't have a calculation on over-current at hand. 

My previous reply with "maximum 10 A fuse" is probably a bit misleading. You should for sure stay below 10 A, not only to have margins on the -VREF, but also for the cable, as the max wire dimension that fits to the MX64 is 0.75 mm2. 

The combined current draw of the sensors should be quite low, so a 3 A fast automotive fuse might be a good choice.

To avoid over-current, I think a fast automotive fuse could be a better choice than a slow circuit breaker.

The alternative with a Digital out HS is also good. In IQANdesign, place the sensors in a function group enabled with a delay-on IDC to avoid seeing errors at startup. 


Thanks for the information and clarification. Also thanks for confirming how to prevent voltage input errors on start-up when powering the sensors from a digital output.