Operation of inductive/capacitive loads on analog outputs

Analog outputs such as the EL40xx/41xx/EL47xx output terminals for standard signals (10 V/ 20 mA) or EL95xx adjustable power supply terminals are designed for ohmic loads. Since ideal ohmic resistive loads are difficult to achieve in practice, they can also drive inductive/capacitive loads within limits. Two effects must be taken into consideration when doing this:

• The current flowing through the inductor causes positive/negative voltage peaks depending on the rate of voltage rise.
• These must not exceed a level of 125 % of the max. nominal output voltage (unless otherwise specified in the respective device documentation).
  Example: The EL4102 terminal specification states a max. output voltage of 10 V
  → max. ±12.5 V voltage peaks are permitted against analog GND.
• Since the voltage/current source (i.e. the analog output) is controlled by the controller, the voltage/current rise can and must be deliberately selected to be slow enough to prevent unacceptably high voltages at the analog output.
• This must be checked by means of an oscilloscope during commissioning and an appropriate design must be selected to ensure that no overshoots occur at a later stage; if necessary, regular monitoring is required.
• A simulation is not sufficient.
• Example with EL4112 terminal: Due to big output jumps (10,000 digits/cycle), impermissibly high overvoltages occur at an ohmic-inductive load at approx. 100 mH/ 320 Ω:
  

→ by distributing the rise over several stages, for example 1000 digits/cycle, the overvoltages are significantly reduced:

• oscillations may occur if the overall setup (with inductive and capacitive loads) results in a system capable of oscillating;
• an oscilloscope must be used to check that the inevitable oscillation decays again quickly (within a few milliseconds):