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What is the best excitation voltage for Interface load cells?

All Interface load cells use eight full bridge strain gauges with each leg normally being rated at 350ohms.

The preferred excitation voltage for Interface load cells is 10VDC as this guarantees the closest match to the performance achieved by Interface during calibration.  The reason for this is that the gauge factor is affected by temperature.  Heat dissipation in the gauges is coupled to the flexure through a thin layer of epoxy glue; the gauges are kept at close to the ambient temperature of the flexure. However, the higher the power dissipation in the gauges, the larger the deviation between the temperature of the gauge and that of the flexure.  For example, at 10VDC, a 350 ohm bridge dissipates 286m/w. A doubling of the voltage to 20VDC quadruples the dissipation to 1143m/w. This is a large a amount of power to have in small gauges and can cause a substantial increase in the temperature gradient between gauges and flexure.  Conversely, dropping the voltage to 5VDC decreases the dissipation to 71m/w, not a significant drop from 286m/w.

The excitation voltage has an impact on sensitivity. For example, operating at 20VDC would decrease sensitivity by approximately 0.07% from the original Interface calibration, where as operating the same load cell at 5VDC would increase sensitivity by less than 0.02%. This characteristic makes it possible to operate load cells at 5 or even 2.5VDC in order to save power. 

Some, portable data loggers will automatically switch the excitation for short period of time in order to conserve power. If the duty cycle is only 5% with 5VDC excitation, the heating effect is a tiny 3.6m/w. This could cause an increase in sensitivity of up to 0.023% from the original Interface calibration.   

Variations in excitation voltage can cause small shifts in zero balance and creep. This effect is most noticeable when excitation voltage is first turned on. The solution for this effect is to allow the load cell to stabilize by operating it with 10VDC excitation for the time required for the gauge temperature to reach equilibrium, but this can take up to 30 minutes.

  

Last updated: 11 Jul 2012

Keywords:
load, cell, excitation voltage, interface

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