Input Module Question

I’m sure this is a silly question but I’ve got to ask it anyway. I’ve got a current transformer (CT) that produces a 0-20 mA AC signal proportional to the measured load of 0 - 20 Amps AC. What Opto 22 input module should I use to capture that signal? I can’t use one of the 0 - 20 mA DC input modules can I?


Hi jbeauchamp,

Welcome to the forums!

Thats a pretty unusual CT you got there…
Normally CT’s are 0-5 amp or .333vAC.
Does your CT go into a transducer that converts its output to 0-20MA?

None the less, if its 0-20mA DC, then you can use any of the 0-20mA modules.
Do you need the voltage as well? (To work out power).


Yes, it is different. It’s a 1000:1 ratio using a 400 Ohm load across the terminals. So if I hook up a 12 AMP heater on my circuit that generates a 4.8 VAC drop across the resistors or 12 milliamps of current. All in AC. So is there an OPTO modules to read that AC current as an input, or do I have to run it through a rectifier?


Yeah. Ok. That is different…

We don’t have any AC current modules.
(BTW, quickest way I know find out what modules we do and don’t have have to check out our online configurator…)

Do you already have some Opto installed at the CT location?
Cant change the CT for something more standard?

Depending on those answers… I just am a little uncomfortable with the whole rectifier thing… You will lose your low end reading (the diodes will drop out around ~1.2v) and Im not sure what the linearity will be like… Not to mention if the input ripple (yeah, you could put a cap across it, but that will cause lag in the reading to some extent)…

Anyone else got any thoughts on this one?


I can’t believe that you would suggest that I get another one! This one was CHEAP! :wink:

Oh well, that’s what you get when you shop price alone. The good thing is that this is just in the testing phase and I don’t have any hardware purchased, nor am I committed to any one design.

I think I’ll try to find something a bit more standard…

BTW this is for current monitoring in a residential power setting and yes I know you have OptoEMU available… ;))

I’m reinventing the wheel, because I can! I’ve written an Optomux driver for Linux that runs on the Raspberry Pi and talks to a bunch of legacy hardware I have laying around (been messing with Opto since '85 - the first driver I wrote was assembly language running on a 68020 on VME bus). As you know, the B1 and B2 related digital and analog hardware is bomb proof and runs forever.



Too cool!

Ok, well, now we know what your up to… I say go for it… You don’t have much to lose…Just be very careful you don’t get zapped!
Use your multimeter to check for the voltage after the rectifier.
Perhaps even try just a single diode so you keep the drop down to .6 rather than 1.2? (In which case you will need a cap, but since you are just measuring stuff at home and keeping the cost down… a bit of non-linearity can be managed).
Chuck that into a DC voltage module and you will get something.
I was thinking that you were in a production / business application and bending the rules is not always a good idea there, but as you said, the CT was cheap, you are in a situation where accuracy to the nth decimal place is not critical, you will learn a bunch, so can afford to experiment a little.

Regarding the Pi, I have two of them and cant say much about what we are using them for, but enough to say its Opto related.
I’m a big Linux fan, but cant code for peanuts (and I love peanuts!), so that limits me to just hacking about with other peoples code snippet’s. (Perfect example can be found here; )

As you say, the old B1, B2 stuff is bomb proof. Put a bunch of that stuff in the hospital when I started there in the very late 80’s.
Its still there, still working to this day.

Lets know how you get on.


For about $75, you can buy a transducer that will take your AC signal and output it in Opto-friendly DC.

Probably better off just buying a different CT, but maybe this saves you a few dollars if rectifying on your own doesn’t work out.

There are CTs that are 4-20 or 0-20, but they invariably have a IC that converts 0-5A to 4-20mA.

If you’re serious, then the following link will get you a IC that can be connected to serial 485 Uart and will connect to Opto via the SSI module. It has the voltage inputs for power, and is capable of being interfaced to a resistive shunt which you can make (cheaply) yourself. The chip is $2 for one off and uart chips are in the pennies.

hmmm did it work in well way if I buy it in75$ please tell me you are sure that it work proply and I change My ac in Dc easily.reply me