Pepperl+Fuchs Blog

Calculating Voltage Drop on AS-Interface

Posted by Tim Cicerchi on Thu, Apr 11, 2013

...or any long cable run

Voltage drop is something to think about before installing any industrial network that uses power and communication on the same two conductors.  These networks include AS-Interface, PROFIBUS PA, and FOUNDATION fieldbus.  For that matter, voltage drop should be considered anytime a device is located at the end of a long cable run.

Example 1: simple setup using a standard inductive sensor

A simple sensor example could be a 12 V battery powering up an inductive sensor.  If the sensor has a combined operating and load current total of 50 mA and an operating voltage of 10 V ... 30 V, how long can the cable be, from the battery to the sensor, before the sensor won’t operate anymore?  A standard sensor cable is .34 mm² and, per IEC 60228, the resistance is < .057 Ω/m.  
To calculate the maximum conductor length for a certain voltage drop, use the following formula: 

Calculate the maximum conductor length701 m is the complete current path out the brown conductor and back along the blue conductor.  To get the cable length, divide that in half.

Formula to determine cable lengthThis simple example shows that if the cable is longer than 350.5 m, then the voltage will fall below 10 V and the sensor will no longer operate properly.  If you have to travel a distance longer than that, you can do one of two things:

  1. Increase the gauge of the cable.  The increased gauge reduces the resistance per meter of the cable so that it takes a longer cable run to reach the same voltage drop. 
  2. Increase the supply voltage.  If you increase the supply voltage to, let’s say, 24 V instead of 12 V, then the allowed voltage drop increases from 2 V to 14 V.  This change alone increases the length by a factor of 7.

The main point of this example is to demonstrate voltage drop.  Long sensor cable runs like this may run into other issues.  They can be susceptible to picking up noise that can lead to nuisance triggers. Long cable runs can also be susceptible to damage if not protected properly.

Example 2: AS-Interface

AS-Interface is an industrial network where up to 62 nodes can be placed on the same cable.  The power supply starting voltage is typically 30.5 V.  AS-Interface modules have operating voltages of 26.5 V, with some as low as 18 V depending on the model. If the voltage drops are too high then the modules just won’t operate.  On AS-Interface, 8 A power supplies are available and that much current on the flat cable over long distances will generate a significant voltage drop.  If 8 A of AS-Interface modules were placed at the end of a 100 m cable run, the voltage drop would be just under 22 V.  Not a single module would run.  This situation would never happen in real life, but shows that it shouldn’t be treated lightly.

The wire gauge on AS-Interface yellow cable is 1.5 mm².  An AS-Interface network normally contains a yellow cable for module/input power and communication and a black auxiliary power cable.  A 2.5 mm² auxiliary power cable is also available.  This even larger diameter cable is good for 12 A and should be used if voltage drop or high current is a concern on the auxiliary power cable.

To save you from doing the math, we have come up with a voltage drop calculator. In this program, you can enter all of the operating current values of the sensors and pick the AS-Interface modules from a drop-down box.  The current consumption of each node and the AS-Interface cable length is used to calculate the voltage drop at each device. This power supply and network checking utility checks:

  • Voltage drop
  • AS-Interface revision compatibility
  • Current consumption on AS-Interface
  • Current consumption on auxiliary
  • Address availability

A video is available that shows you a sample network layout and how to use the AS-Interface power calculator.

What you see in the video is a rather large difference between measured and documented current consumptions of the devices.  This difference is completely normal.  Many factors are taken into consideration when the worst case current consumptions are put in the datasheets.  Current consumption will increase due to age of the device, changes in temperature, and raw material parts variations.

I will do the example, as seen in the video, here so you can see what is necessary to figure out the voltage drop.

Current consumption

Values needed to determine voltage dropAS-Interface module operating current – This is the current it takes to run the AS-Interface module even when no sensors are attached.

Load current – This is the current consumed by the AS-Interface module when the sensor is activated.  The internal load of the AS-Interface module is about 3 kΩ for each input.  This load leads to the current consumption of 9 mA for each sensor attached to the module.  We always assume that all inputs are on.  This is a worst-case scenario.  

Sensor operating current – This is the current consumed by the sensor even if there is no target in front of it.  It is often called the no-load supply current in the datasheet.

Once the total current has been added up, the voltage drop can be calculated using Ohm’s law.  Remember, in this example the AS-Interface cable is 90 m long.  This means the internal conductor length that carries the current is double that, or 180 m.  The resistance of AS-Interface cable that is 1.5 mm² is .0137 Ω/m

Voltage drop can be calculated using Ohm’s lawIf the starting voltage is 30.3 V, then the ending voltage worst case will be 30.3 V – 1. 6V = 28.7 V.  The 28.7 V is above the lowest operating voltage requirement of any node on the network.  So, we can safely say that this network will not have a problem due to voltage drop.

Voltage drop should be considered when putting a network together.  Take a common-sense approach when doing your machine layout.  Simple things like putting the power supply in the middle of the network or closest to the highest current consumers will probably reduce or eliminate most of your voltage drop problems.  If you think the voltage is too low at the end of your existing network, use a meter and take some voltage measurements.  If the voltage is too low, consider adding a repeater and another power supply.

If you're interested in learning more about AS-Interface, tune into TechTalk, our weekly podcast covering AS-Interface tips and topics.

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