I have an application where I need to monitor if 25 AC fans are operating correctly. I was hoping to monitor the current, so that I can do it remotely. I was thinking of a current monitoring device that reports over I2C, SPI, etc. but I couldn't find anything. The other alternative is some sort of current to voltage converter with an ADC module. But now it is getting quite expensive.
I could just use a shunt resistor (to handle getting the current to voltage), but I haven't worked with AC current before, so I don't exactly know how to handle getting it to the DC side.
Does anyone have any suggestions? Does anyone know of something that just goes from AC Current to an addressable serial line, etc.?
One way you may be able to do it is if you can rig up a photo detector (or some other inexpensive sensor) that can detect the rotation of the blades. That way you don't have to get involved with the AC current. A mega board could sequence through each of 25 pins in turn and use the pulse rate to determine fan speed
Another way that does involve you with the AC current is to put a small resistor in line with the AC and then put the LED side of an opto isolator across the resistor. Make the resistor such a size that it drops enough to power the LED and put a series current limiting resistor in the LED line.
Then just monitor the output of the opto using a pin or as you have a lot of them a few shift registers.
Of course this will only detect a fan that fails open circuit but then most do. Those that don't soon burn out and when the flames die down it's open.
Alright, I see where you are going with this. I am not sure I follow the hook up though. Excellent idea though. Simply use the optoisolator as a current indicator, not a current to voltage converter.
I had thought of that, but with 25 fans within a fairly tight area, I was thinking that it might give me some trouble. Either saying that a given fan was working due to another fan moving the microswitch, or blocking the airflow. Probably not a problem, but it was enough to make me think to measure the current.
