I have a question. The company I work for makes PLC's. On the PLC are analog outputs. These outputs put out 0-20 ma. Most devices we control require 0-10V input signals. Therefore we typically install a 500 ohm resistor across out analog output terminals to convert the 4-20ma in 0-10VDC. Simple - right?
Apparantly that is true only if the connected device has a high input resistance (impedance). Most devices we control have input impedance of 50K ohms or higher. We can therefore connect several of those in parallel and still be able to sustain our 0-10V signal across the terminals.
However, when a device with a low input impedance (10K ohms or less for example) is connected, our PLC output (with the 500 ohm resistor across it) cannot sustain it's required voltage level to drive the device. It appears that the low impedance load acts like a short circuit and the PLC does not have enough capacity to source that low impedance load.
Can somebody explain this please. Why this happens, how this happens, what can be done, etc.
We have been using higher resistance resistors in lieu of the 500 ohm and that sometimes helps - depending on the situation.
Apparantly that is true only if the connected device has a high input resistance (impedance). Most devices we control have input impedance of 50K ohms or higher. We can therefore connect several of those in parallel and still be able to sustain our 0-10V signal across the terminals.
However, when a device with a low input impedance (10K ohms or less for example) is connected, our PLC output (with the 500 ohm resistor across it) cannot sustain it's required voltage level to drive the device. It appears that the low impedance load acts like a short circuit and the PLC does not have enough capacity to source that low impedance load.
Can somebody explain this please. Why this happens, how this happens, what can be done, etc.
We have been using higher resistance resistors in lieu of the 500 ohm and that sometimes helps - depending on the situation.