Internet PLC Forum
General => Technical support => Topic started by: cch1955 on August 14, 2006, 09:27:44 AM
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I am using th analog output of the T100MD to control a pair of VFD'S. I noticed the comment that the outputs are "Unbuffered". I assume thismeans they have no protection for over current etc. The VFD'S have an input which is for 0-10VDC and has scaling so I can use the 0-5 output from the T100MD. My plan is to connect the reference ground from the VFD to the 0VDC connector by DAC outputs and the DAC #1 output to the VFD input. Do I need a current limiting resistor for this or other conditioning? The cabling is less than 5 fet and shielded.
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The DAC output on the PLC has a 2.2K ohm current limiting resistor in series to the output pin so you can connect it directly to your VFD analog input. I assume the VFD analog input is high impedance type which does not draw much current from the DAC. You can verify this by measuring the voltage before and after connecting to the VFD. If there is no significant drop in voltage then the analog input to the VFD is high impedance and does not load the DAC. Otherwise you will need a buffer amplifier to be connected between the DAC output and the VFD.
Some modern VFD may have RS232/RS485 port for you to control its operation. Consider using these communication port if your VFD has it as it will give you more precise control than possible with the analog input.