On 3/7/2018 1:44 PM, Dave, I can't do that wrote:
I have a 3d printer with a 12vdc-30A power supply (PS). The PS has a common +/- rails with three screw connectors for positive and three for negative. The printer has two sets (+/-) of wires coming from the PS to a junction block. Then there are two sets of wires (+/-) going to two screw terminals on the controller card.

I recently bought a clamp meter that does DC-Amps, pretty clever and seems to be very accurate when compared with an in-line Amp meter.

So here's the issue.
If I clamp around one wire from the PS when both the heated bed and nozzle are drawing current I get 11A on one wire and 9A on the other. If I clamp around both wires I get about 14A so I am confused.

Can I clamp around both wires for a total current reading?
If I clamp one, is it showing a true reading?


ASCII Art

0-vdc ----+-+ +-+----- 12vdc [ PS
| | | | [ measure each 11A and 9A
| | | | [ measure both 14A
+---+ +---+
| | | | [ Junction block
+---+ +---+
| | | | [ measure each 11A and 9A
| | | | [ measure both 14A
Hotbed --+ | | +--
Nozzle ----+ +----

Thoughts?

Thanks

What happens when you do the same experiment with the inline
DC amp meter?

What happens when you reverse the direction of one of the wires
so that the currents subtract?

Is it really DC current?
If the current is not really constant, the meter has an algorithm
that converts what it sees to a DC current value for display.
Average? RMS? other? The waveform of the AC components of the currents
will affect
the converted number that the meter calls DC depending on what
algorithm the meter implements and the shapes of the two waveforms.

You also have to worry about saturation in the meter core.
Your loads are likely pulse-width modulated and have a peak current
well in excess of the average...maybe...
If the peak current exceeds the range of the meter, the top end
will get compressed.

Put the biggest caps you can find on the load end of the printer
connections.
Measure the current in the wires between the caps and the PS.
If the caps are BIG, the current will look more like DC
and the readings should converge toward what you expect.

Doesn't look like it here, but watch out for paths that
circumvent your measurement. Ground here is not the same
as ground over there. It's possible to have current go in directions
that don't include the wire you're enclosing.