The max load resistance for an AO CHARM is 750 ohm. If I measure resistance across the terminals of a DVC positioner, I will get a meg or tens of megs of resistance. The resistance will slowly decrease, perhaps to hundreds of kilo ohms as I keep the meter leads on the terminals. My multimeter has a 9V battery, whereas a CHARM has a 24V power supply. For that reason, perhaps the resistance drops below 750 ohm when a CHARM is connected to the valve. I suppose it must be the case; otherwise, we wouldn't be able to drive any of our valves. This behavior is making it difficult for me to troubleshoot bad signals. Can anyone explain the circuitry at the DVC terminals, why I'm seeing this behavior and how a CHARM could possibly drive a valve with such high resistance? Are there methods I can employ to troubleshoot the AO connection and determine whether or not a DVC positioner must be replaced?
Generally speaking, smart positioners like DVC have semiconductor components like diodes and transistors in series and zeners in parallel with the loop so cannot be measured as a resistance with a multimeter. The way to engineer the current loop is to make sure that you have a budget of 11 V for the DVC. An AO CHARM has a 15 V output voltage swing ("compliance voltage") so you have a balance of 4 V for series loop resistance like the cable. Remember you don't need a 250 ohm resistor for AO. Some of this is explained in the essay found here:
In reply to Jonas Berge: