COUT open load error calculation

Justin Wagner 2 years ago in Master modules / MC4x updated 2 years ago 4

We have a machine that has sporadic nuisance open load errors on a function controlled by a unidirectional current output (COUT-G) on an MC43. Since this happens with some frequency, I suspect we are right up against the limit of the open load detection threshold. They test out well in assembly, but any increase in resistance occurring in the field might be setting it over the edge of the error threshold.

How can I calculate at what load resistance, commanded current, and supply voltage an open load error will occur?

This function is controlled with a Parker CCP024H 1/2" super coil with nominal resistance at room temperature of 30.30 ohms. Machine runs on 24V (around 28V when alternator charging). Maximum commanded current is set to 600mA.


If the problem is that you are just at the limit for open load detect due to under-current, you should be seeing the warning status Saturated on the COUT before the open load error. 

The Saturated warning is not logged, but you could measure it from the display or via IQANrun/IQANgo. 

For a quick calculation, assume that the coil resistance increase 50% when hot, then calculate the maximum achievable current from the law of Ohm. 

Assuming R=30*1,5 = 45 ohm and U = 24 V (coil operated also when engine not running? ), your theoretical max is 45/24=0.53 A

The limit for when saturated COUT on MC4x/XC4x triggers open load error is 70%. (value in Appendix B)

For a more detailed calculation, the voltage drop and max pulse with can also be considered. (values in Appendix A)

Thank you for the reply, Gustav.

I'm not sure if this particular instance had a saturated coil error, but the saturated coil error did occur previously in the shop, followed by an open load error when cranking the engine. I.e. voltage drops during cranking, and the output can no longer provide >70% of the commanded current, which triggers open load error and makes perfect sense.

It seems we are right at the edge of the operating limits of the COUT channel, given our supply voltage. We'll need to reevaluate if we can lower the maximum output to something below 600mA.

I am still curious about the "Maximum load" specification in Appendix A of the MC4X/XC4X instruction book (see below). Is this the maximum load resistance (and inductance) on the output? If so, I would expect them to be higher.


It is actually the other way around, a lower resistance gives a higher current, thus a higher load. 

So with 30 ohm at room temperature, you have plenty of margins to the maxim load limit. 

It does seem as if you have the opposite problem, that the commanded current is sometimes higher than what is physically achievable with this coil and supply voltage due to the law of Ohm.

At least when you were having the coil activated during cranking and got the saturated warning and open load error. 

I have seen this in many applications, it seems like a common mistake when designing manifolds with cartridge valves; to only look at e.g. the current vs pressure diagram, without taking achievable current of the coil during operation into account. 

Thank you, again, for the explanation. It does make sense, I was just thinking of the "load" differently and assuming the output driver had some power processing/manipulation that would adjust voltage/current as needed dependent on the load resistance, and not solely governed by Ohm's law between battery voltage and coil resistance.

This was on an existing design (luckily not a critical function) but it's definitely something to keep in mind in the future when looking at solenoid ratings.