Getting to Dead Zero with 0-20mA Signal

This is not specifically an arduino question but a general electronics question.

I am currently 0-20mA signal generated by an energy meter to trigger functions of a second controller but I am having a problem getting all the way to 0mA. For some reason, my second controller seems to read a very small amount of current even when the energy meter isn't outputting any signal. I don't know if this could be induced current over the lines of what. I do have a relatively long run between the two controllers.

Is there any way to add a small resistor to pull this value all the way to zero?

I am using 0-20mA instead of 4-20mA because I need the second controller to react in the event that the first controller fails. The second controller doesn't respond to a 0mA signal when it is set to read a 4-20mA signal.

A circuit diagram would be helpful


4-20 is used because anything less than 4 can be used as an error condition. You should be able to read valid values in the 4-20 range and report "not connected" when it drops below 4.

It's analog - you can't expect it to ever equal zero but more time and expense can get it closer to zero.

How do you read the current? How you convert the current to voltage?

Where does it say it has to be 0 ?
I've never heard of a 0-2 mA circuit. As already mentioned, it should be 4-20 mA, and 4 is "0".

0-20ma is just as valid as 4-20. Just because some of you "experts" don't know about it, that doesn't change the facts such that you get ignore the op's question.

If you cannot get to zero current typically you'd want to decrease your load resistance, not increase it. If that doesn't help, you might be dealing with a poorly designed source.

If you cannot get to zero current typically you'd want to decrease your load resistance, not increase it. If that doesn't help, you might be dealing with a poorly designed source.

It's an energy meter. How do you propose that the OP "decrease the load source" ?
Wouldn't it make more sense to treat the lowest value as "0" instead of chasing ghosts ?

Since 4 mA is equal to 0% output, it is incredibly simple to detect a fault in the system.

4-20 mA Current Loop Basics

Spmething like the enclosed should work…


20ma.pdf (16.1 KB)

Spmething like the enclosed should work...

That's an excellent method to generate a 0-20 mA current loop.

The only problem is that the current loop source is already provided by the energy meter and all the OP is doing is reading it, so he needs the exact oposite of that circuit , which I think you could derive by simply rearranging the components.

I think the problem is that the OP thinks the signal is a 0-20mA but I think energy meters use 4-20mA, not 0-20mA.

OK… try this approach


(edit) ps actually might be better if R3=R4 = 100.

20masrc.pdf (17.6 KB)

OK.. try this approach

That looks like the ticket...

There is a small error due to the Hfe of Q2 - but much less than the tolerance of the resistors...


Could replace the 2N2222 with a 2N7000 fet.
Then R6 can also go.

Fair enough.. But the basic idea is OK.


Yep, +1. First time I have seen a current to current/voltage converter like that.

It's an old trick covered in the famous natsemi linear databook - my copy is tattered and must be 20+ years old - a high-side current measurement circuit. Just tweaked a bit.

LT make such devices today.

I didn't invent it. Wish I did.


Old dogs like us (and many others here) are valuable to the younger generation, because of all those circuits are stored in the back of their minds. I'm also sure you COULD have invented this.


I spent years before the days of opamps and digits with plain semiconductors- and even valves. We made things that worked. Some of the old tricks are still useful.



I still remember that the price of a single glass-encapsulated germanium transistor was higher than a day's wages. You had to make the most of the very few parts you could afford.
I still think at component level.

Perhaps I should re-arrange that circuit to be entirely discrete - could do it with a diff-pair...

I'm sure that's how early 4-20mA systems were done. They've been around for a long time.

just for fun