# Can current flow both directions in 1 wire?

Thread Starter

#### bernardirafael

Joined Sep 21, 2018
4
Hello fellas,

I am doing some experiments in welding power sources. I will try to simplify it as much as I can in the image below.

https://prnt.sc/kx2txc

I consider first PS the left one, with higher current set, and the second PS the right one, with lower current set.

First question is: Can current flow both ways in the blue wire? If so, what are the consequences of it, if any?

I am measuring the current in my experiments and generally, the resulting value is the subtraction of both PS currents (so in the example given, it should be 20A). However, in the real experiment, I noticed that in some cases the current flows from the PS with higher current set through the PS with lower current. I recorded the current flow through the second PS freewheeling diode and its IGBT's gate command and as a result, the second PS IGBT stops switching (open circuit) and the current flows freely through the diode and cables, coming from the first PS.

My biggest question is: Why would the current flow through one PS (higher current) to the other (lower current) if this path should have a higher resistance (due to second PS internal components) rather than flowing through the blue wire (image), which I call commom wire? The existence of such common wire might be what causes it, or possibly one of the causes?

Thanks in advance!

Thread Starter

#### bernardirafael

Joined Sep 21, 2018
4
I can't edit the thread, I don't know why. I forgot to mention: disregard the resistance values. They are unknown.

#### crutschow

Joined Mar 14, 2008
31,521
Can current flow both ways in the blue wire?
Not at the same time, no.

Note that for 300A to flow through a 1k resistor, the voltage across the resistor would be 300,000V.
Why would the current flow through one PS (higher current) to the other (lower current) if this path should have a higher resistance (due to second PS internal components) rather than flowing through the blue wire (image), which I call commom wire?
The current flows from the voltages and resistances in the circuit according to Ohm's law.
Do you know and understand that law?

#### LesJones

Joined Jan 8, 2017
3,945
The current can't flow in two directions at the same time, The current through the blue wire will be the difference between the two current sources which will be 20 amps. I find it difficult to believe that you have built the above circuit as the 300 amp current source would have to provide 320000 volts and the 280 amp source would have to supply 300000 volts.

Les.

Thread Starter

#### bernardirafael

Joined Sep 21, 2018
4
The current can't flow in two directions at the same time, The current through the blue wire will be the difference between the two current sources which will be 20 amps. I find it difficult to believe that you have built the above circuit as the 300 amp current source would have to provide 320000 volts and the 280 amp source would have to supply 300000 volts.

Les.
Not at the same time, no.

Note that for 300A to flow through a 1k resistor, the voltage across the resistor would be 300,000V.
The current flows from the voltages and resistances in the circuit according to Ohm's law.
Do you know and understand that law?
As I mentioned in the bost below the main one, disregard the resistance values as they are unknown and I just made that schematics quickly to ease the understanting... but I forgot to remove the resistance values to avoid misunderstanding.
I do know and understand it. My main question is, as the current from PS1 is passing through the PS2 (in real life), the common cable might be one of the factors that causes it? Is it behaving like a "barrier" to PS2 provide current to the system?

#### LesJones

Joined Jan 8, 2017
3,945
If the blue wire is open circuit you have an impossible situation as both current sources can not provide their specified current as there is onle one loop. (So there is nowhere for the difference in currents to go.)

Les.

#### ebeowulf17

Joined Aug 12, 2014
3,307
Since we're talking about experimenting with real power supplies, not theoretical perfect current sources, it all comes down to the internal circuit design of each power supply. Of course, knowing the three external resistances would help. If they're not all equal, that changes the power sharing situation.

My very wild guess is something like this: if there are leakage paths that allow current from the 300A supply to pass through the 280A supply, then that current may interfere with the internal feedback loops it uses to control how much current it delivers. If current from the 300A supply makes it through the current sense resistor (or hall effect current sensor, etc) then the 280A supply won't be actively trying to supply more power to the circuit, so it's kind of bypassed.

Just wild speculation in that last paragraph, but the key point is that real power supplies are made up lots of real, non-ideal components, and they're designed with the expectation that they'll be used in certain predictable arrangements. If you try to make them fight each other, unexpected things will happen. The only way to understand those unintended consequences is if you get to know each circuit in great detail and analyze it accordingly. Analyzing them as ideal current sources is meaningless.

Thread Starter

#### bernardirafael

Joined Sep 21, 2018
4
Since we're talking about experimenting with real power supplies, not theoretical perfect current sources, it all comes down to the internal circuit design of each power supply. Of course, knowing the three external resistances would help. If they're not all equal, that changes the power sharing situation.

My very wild guess is something like this: if there are leakage paths that allow current from the 300A supply to pass through the 280A supply, then that current may interfere with the internal feedback loops it uses to control how much current it delivers. If current from the 300A supply makes it through the current sense resistor (or hall effect current sensor, etc) then the 280A supply won't be actively trying to supply more power to the circuit, so it's kind of bypassed.

Just wild speculation in that last paragraph, but the key point is that real power supplies are made up lots of real, non-ideal components, and they're designed with the expectation that they'll be used in certain predictable arrangements. If you try to make them fight each other, unexpected things will happen. The only way to understand those unintended consequences is if you get to know each circuit in great detail and analyze it accordingly. Analyzing them as ideal current sources is meaningless.
I can say that your speculation is right. The 300A passes through the second PS, as I recorded in the freewheel diode and the switching of its IGBT (which stops completely - open circuit but current keeps flowing). Here's a simple schematic of the power sources basic construction.

The probes next to the inductor are sensors hall that feed the electronics that controls the IGBT's gate. Under certain conditions the PS2's IGBT stops switching because PS1's current is flowing through the diode-inductor branch.

My biggest concern is if the existence of a common cable as seen in the first pic (first post), which allows current flow in 1 direction only, could help blocking PS2 and helping PS1 current to flow through it.

Is it non-sense tho?

#### MisterBill2

Joined Jan 23, 2018
14,233
I can say that your speculation is right. The 300A passes through the second PS, as I recorded in the freewheel diode and the switching of its IGBT (which stops completely - open circuit but current keeps flowing). Here's a simple schematic of the power sources basic construction.

View attachment 160316
The probes next to the inductor are sensors hall that feed the electronics that controls the IGBT's gate. Under certain conditions the PS2's IGBT stops switching because PS1's current is flowing through the diode-inductor branch.

My biggest concern is if the existence of a common cable as seen in the first pic (first post), which allows current flow in 1 direction only, could help blocking PS2 and helping PS1 current to flow through it.

Is it non-sense tho?
If the original circuit has the same polarities as the ones in this post then the situation will be messy, with 200 amps flowing through a short circuit. But if the polarities are the same, +=up, and the resistances not really 1000 ohms eacc, but something one would expect in a welding circuit, then 500 amps might flow through the blue wire. Current can only flow in one dorection at any one instant, that does not change. But what can be seen is a different thing completely.