Using 4-20ma sensors with IoTaWatt

Support
#1

I want to use the IoTaWatt to monitor a solar array on the DC side. I think the best (safest) way to do this would be to buy some industrial amp/voltage meters which output 4-20ma signals, and then feed that to the IoTaWatt.

I’ve looked through the blog posts and the support forum, and while it is mentioned in e.g. temperature monitoring, it’s never addressed directly.

Can someone explain what the setup needs to be to wire up a 3.5mm jack with 4-20ma signals, and how it should be configured in the Setup page?

Backstory: I’m looking at a solar power setup with a hybrid inverter/battery management system. The batteries are charged internally to the inverter, so the AC output is not necessarily representative of the solar input at any particular time. I want to monitor solar coming in, AC coming out, AC consumption in my house, and grid export, all at the same time.

#2

It might be possible but I’ve never tried it. IoTaWatt measures 0-50mA at each input by measuring the voltage drop across a 20 ohm resistor. If the 4-20mA sensor can deliver to a 20 ohm load, you should be OK, with some caveats if you have multiple sensors with common ground it won’t work.

What is the DC current that would be represented by the 4-20mA signal? You will need to configure the input as a generic CT with turns specification that reflects that ratio. For instance if 20mA represents 40 Amps, you would use a turns ratio of 40 / .016 = 2500. Then you would create an output that subtracts 10 Amps from the result:

.004A x 2500 - 10 = 0 A
.012A x 2500 - 10 = 20A
.020A x 2500 - 10 = 40A

Same basic idea for a sensor that senses anything. Turns ratio of maximum scale / .016 and subtract .004 x turns ratio from the reading to normalize from 4-20mA to 0-16mA.

#3

I was looking at something like this:

I don’t know much about 4-20ma signals, except that they’re a standard measurement signal.

From what you’re saying, I need to identify meters which produce isolated outputs at a minimum.

Which of the 3.5mm connector bands do I need to hook the inputs to?

Edit: I think this gives the answer:

https://learn.openenergymonitor.org/electricity-monitoring/ct-sensors/extending-ct-cable

It’s the tip and the sleeve that carry the signal.

#4

It’s not uncommon that solar arrays can get up to 500vdc per string

#5

great, i will consider to test after Orthodox Easter.
I will use adapters with insulated outputs.

#6

Yep, hence I’m trying to find suitable isolating meters.

I’ll size the meters to the strings I’ve got - 600vdc is a standard meter size, and my system is two strings of 270v each.