# Two ammeters from two different PSU to one load?

#### Laszlo Szabo 1

Joined Feb 27, 2017
13
I am looking for the answer/solution to this problem. I have a solar system, only a 24V system for my 3D printers. Also, it is a UPS with 2 diodes.
Works well for a long time now, but I wanted to make an upgrade to be able to view the current flow. I add two ammeters but they are measuring wrongly.
I know - as far as I learnt it - 2 ammeters in parallel is not a good choice. So I removed the wires now, only left the shunts as the system is online right now and my printers are working
Anyway, how can I do what I want? Apply the two ammeters?
I am looking forward to any reply. Thank you.

#### crutschow

Joined Mar 14, 2008
30,804
You can measure the sum of the two currents at the output of the two diodes, if that's what you want.
they are measuring wrongl
But two ammeters should not be a problem.
How are they measuring "wrong"?

#### BobTPH

Joined Jun 5, 2013
5,774
If the two voltages differ by more than about 0.7V, all of the current will come from the higher voltage supply. Is that what you expect?

#### Laszlo Szabo 1

Joined Feb 27, 2017
13
You can measure the sum of the two currents at the output of the two diodes, if that's what you want.

But two ammeters should not be a problem.
How are they measuring "wrong"?
Well, I am not so much interested in the currents on the output. Also, the ammeter shunts need to be put in the GND line, but I know that it is not a problem to add to the common GND line. However, I am not interested in measuring the sum of the currents.
I am interested in measuring the current flow in both lines before the diodes, to check the consumption of each Power unit. (So in other words, my printer is operating from the solar or from the PSU? at the time when I check. )
Measuring wrong:
The second ammeter is measuring current but the input has now current as I read the Solar Charger Controller and it shows 0.0amps. But when the solar had more voltage on the output than PSU current flow looked OK, as the second shunt was measuring almost all the current and the first one just a couple of milliamps. So it looked OK to me. The solar system supplied more energy...
As soon as the sun went down, the voltage dropped, the diodes worked accordingly and the current started to flow from the PSU slowly, then totally of course. But, as I mentioned the second ammeter still "thought" the current is still flowing there.

#### crutschow

Joined Mar 14, 2008
30,804
If the PSU ground and the Solar / Battery ground are separate and only connected together as shown at the GND node on the right, then each ammeter should indicate the correct current.

#### Laszlo Szabo 1

Joined Feb 27, 2017
13
If the two voltages differ by more than about 0.7V, all of the current will come from the higher voltage supply. Is that what you expect?
I do not expect it, this works for a half year now My printers never stopped when the 240V was gone.
Well, the diodes were in each printer so far, but yesterday I made a central unit for all the printers, and now only the 24V is supplied to them, they do not need to be connected to the wall anymore.
However, the ammeters are not working the way as I supposed it.

#### Laszlo Szabo 1

Joined Feb 27, 2017
13
If the PSU ground and the Solar / Battery ground are separate and only connected together as shown at the GND node on the right, then each ammeter should indicate the correct current.
That's what I thought. But unfortunately, this is not the case.
Maybe I am just tired today but I couldn't figure out what is wrong here.
It is not so complicated, and I am happy as you confirmed I was right when I built the circuit. But still unhappy as I do not know what is wrong here.

#### crutschow

Joined Mar 14, 2008
30,804
Sounds like a sneak current path somewhere.

#### Laszlo Szabo 1

Joined Feb 27, 2017
13
Sounds like a sneak current path somewhere.
Was thinking about that too, but where from? It is only possible now through the ammeter itself. I guess. I even supplied them with totally separate power for operation. (a small charger plug 5V from the main. ) The first time, I powered the ammeters from the battery directly but I thought that is the problem... somehow... so I add this separate PSU just for the test... but still the same.

#### BobTPH

Joined Jun 5, 2013
5,774
How did you connect both ammeters to a 5V power supply without connecting their grounds together?

#### Jon Chandler

Joined Jun 12, 2008
390
The measurement side of those meters is not isolated from the power supply side – I believe the ground connection is common. Unless each is operated from its own isolated supply, you'll get screwy results.

#### drjohsmith

Joined Dec 13, 2021
504
The current shunt of the amp meter should be floating,
there should be two terminals for the "load"
the meter might use the positive side of the load input as its power,
but the meters should be in series with the diodes,

#### crutschow

Joined Mar 14, 2008
30,804
Unless each is operated from its own isolated supply, you'll get screwy results.
Yes.
Using a single supply means the two ammeter grounds are connected together.
You need two separate isolated supplies for the ammeters.

#### bassbindevil

Joined Jan 23, 2014
563
You should be able to run each V/A meter from its associated power source, though it might be smart to use a series resistor to reduce the power dissipated in the voltage regulators in the meters.

Joined Jan 15, 2015
6,979
The measurement side of those meters is not isolated from the power supply side – I believe the ground connection is common. Unless each is operated from its own isolated supply, you'll get screwy results.
That would be correct.

Part of the problem is Power Supply BLK - and BLK Common are common to each other. Even wired as above I am not sure it will work. There are plenty of ways to do what you want to do but they are a bit more complicated and require Isolation. As crutschow pointed out separate power for each module.

Ron

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#### Laszlo Szabo 1

Joined Feb 27, 2017
13
How did you connect both ammeters to a 5V power supply without connecting their grounds together?
These ammeters need to be connected to a totally separate power supply, and only their GND are together. But GND and Vin are separate from the whole system. This is just for powering them... I use these digital ammeters for years, and never ever had a problem. Even, when I did not supply the power from a separate source. They working fine. Now, I have a different problem as you see... I never used them in this "parallel way" ...

#### Laszlo Szabo 1

Joined Feb 27, 2017
13
The measurement side of those meters is not isolated from the power supply side – I believe the ground connection is common. Unless each is operated from its own isolated supply, you'll get screwy results.
They are operating from a totally separated PSU.

#### BobTPH

Joined Jun 5, 2013
5,774
And you are sure the shunt is electrically isolated from the supply powering the ammeter module? Do you see the problem if it is not?

#### Laszlo Szabo 1

Joined Feb 27, 2017
13
The current shunt of the amp meter should be floating,
there should be two terminals for the "load"
the meter might use the positive side of the load input as its power,
but the meters should be in series with the diodes,

This is a reply to all. Thank you, by the way, I really appreciate all the help!
https://www.amazon.de/gp/product/B09TP2MNS1/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

And also I want to share a schematic of my project. I drew it exactly as it is now. I hope that helps to fix my problem.
Ps. Basically the system works ... right now I am using it. The PSU has a Watt mater in the socket and I can see when it is operating and roughly how much energy it uses. Also, the Solar Charger Controller has a built-in Ammeter and I know again roughly how much current coming from there.
Against all these, I want these two separate ammeters to work well.

I use 4 diodes 10A / each, even though 2 printers never get more than 4-5amps at the top. Two diodes are now 20Amp and I even use a fan to cool them. As you can see in my previous pictures. (without cooling in the printers - before this setup- they worked fine, never had problems.)

Thank you.