This post is a follow up question of this question. I apologize if this seems to be double posting, but I cant comment (not enough reputation), inappropriate to ask question as answer and cant find the email of Trevor.

So here is the question. What does it mean in the post? (shown in Figure 2 below)

The issue with mixing grounds occcurs when you pass the returning high current through the control sides low current circuits as shown below.

From what I understand, Figure 2 is the same as Figure 1 in terms of circuit connection which will be shown in the following steps.

Figure 3 splits out the parallel ground connection for easier understanding and manipulation of circuit which is done on Figure 4. In Figure 4, I shifted the 5V to the right side and the lamp + mosfet to the left side. And that Figure 4 is the same as Figure 1.

I did observed that the current flow is a little different. But how can one be sure that current from 60V will not flow through the Arduino if all the common is connected together and furthermore is in parallel? From my understanding, same circuit with different positioning = same current flow. Which means if Figure 2 returns high current through the control sides, the same applies to Figure 1.

Circuit diagram All these are based on my understanding. Please do critique my understanding if it is wrong (with image and current flow if possible ><).

Once again I apologize if this seems to be a double post.

EDIT: I read this post regarding schematic and decided to draw it again. They are basically the same as the previous image. Schematic


I would say that Figure 1 and 2 are not the same. They differ in the following way:

In Figure 2, the current through the emitter of the transistor has to return to the 60V source. The return path is shared by the 5V return path. That means that there will be a potential difference between the ground of the 5V source, and the ground of the Arduino board.

In Figure 1 the 5V source and the Arduino board is virtually on the same potential, but there could be a potential difference between the Arduino boards's ground and the emitter of the transistor. The current from the emitter to the ground of the 5V source will be very small, thus the potential difference should be small.

The Figure 1 design is better than Figure 2.

Figures 3 and 4 are the same as Figure 1, just drawn differently.

  • \$\begingroup\$ Ground of 60V source, ground of 5V source, ground of Arduino and ground of emitter is in parallel for Figure 1 and 2, right? If it is right, does that mean even in parallel connection, current can flow differently? To my understanding, the ground portion of Figure 2 and Figure 3 is the same, which is parallel connection \$\endgroup\$ – iHateUni Apr 23 '17 at 16:27

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