(Answered/resolved - thanks!) Paths of conventional flow with two power sources.

***ANSWERED

It was really a combination of everybody’s input below that set the concepts in stone.

Thanks all.


Good Day,

I’ve attached an image that will clarify (and has text which poses) my questions. My questions are a bit obtuse without the referenced diagram in the image.

After reviewing the image, would somebody please provide your insight and expertise?

  • What’s forcing the flow to go to the transistor’s drain instead of looping back to the power bus via the diode’s anode. My understanding is that the 1-way path functionality of the diode is only stopping any back voltage from (+) on the power bus spiking back into the transistor. Current should be able to flow from the motor ground into the anode of the diode though, no? Is this path of least resistance in action?
  • How are both of these circuit’s completing their loop?

Thank you very much in advance for your time.


Chris

Can you post a schematic of your bits & pieces instead of wires & black boxes?

I hate piling on here... but this kind of question is exactly why we use schematics and don't use the method you used to "analyze" or explain circuits with each other.

I also have a slight concern about your use of floating ground. I feel you should be saying "common" ground.

Your question about the intrinsic diode in the MOSFET is easily answered. The potential at the ANODE is lower than the CATHODE (Drain side) and therefore, current flow is inhibited because it has already had to cross a semiconductor junction. The anode needs to be more positive than the cathode to work as a diode.

The rest of your question seems to be about "why does the 9V not break the 5V half of the circuit via the common ground connection?". I will only say... at the common ground point there is no longer a potential difference... just a desire for a balance of electron flow... (what goes in... must come out)

wmorr: Good Day,

I've attached an image that will clarify (and has text which poses) my questions. My questions are a bit obtuse without the referenced diagram in the image.

After reviewing the image, would somebody please provide your insight and expertise?

  • What's forcing the flow to go to the transistor's drain instead of looping back to the power bus via the diode's anode. My understanding is that the 1-way path functionality of the diode is only stopping any back voltage from (+) on the power bus spiking back into the transistor. Current should be able to flow from the motor ground into the anode of the diode though, no? Is this path of least resistance in action?
  • How are both of these circuit's completing their loop?

Thank you very much in advance for your time.

-- Chris

Hi,

You need to post a schematic.

In lieu of that however, the diode will not conduct until the voltage on the green wire goes higher than +9v because diodes dont conduct until their anode is about 0.6v higher than their cathode. In reality they might start to conduct very little at around 0.4v but the current gets more significant as we get to 0.6v or so. Some people prefer to assume 0.7v. So if you go by the 0.6v standard then you need 9.6v on the green wire before you see the diode conduct.

We can say more if you post a schematic.

Hi,

Thank you thus far for your input.

Sorry not posting the schematic. I've added it to the original post. Won't make that mistake again.

pwillard and MrAl answer explains the confusion around the diode. Electrons won't hop onto the anode because the cathode has the higher positive charge. Thank you. Karma given to both.

With some of my semantic mixups (e.g. calling the common ground a floating ground) and lack of schematic there would be some confusion around my other question about the looping of circuits. I actually think pwillard's response here may also satisfy my question, but I'm going to take time to digest that and read more about the impact of common grounds.

In the meantime, I'm definitely interested in reading your follow-up responses (thanks again) with the schematic added. Just having difficulty seeing a return path of electrons back to both power sources.

Regards,

Chris

See this page I wrote about using two power supplies http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

wmorr:
Hi,

Thank you thus far for your input.

Sorry not posting the schematic. I’ve added it to the original post. Won’t make that mistake again.

pwillard and MrAl answer explains the confusion around the diode. Electrons won’t hop onto the anode because the cathode has the higher positive charge. Thank you. Karma given to both.

With some of my semantic mixups (e.g. calling the common ground a floating ground) and lack of schematic there would be some confusion around my other question about the looping of circuits. I actually think pwillard’s response here may also satisfy my question, but I’m going to take time to digest that and read more about the impact of common grounds.

In the meantime, I’m definitely interested in reading your follow-up responses (thanks again) with the schematic added. Just having difficulty seeing a return path of electrons back to both power sources.

Regards,

Chris

Hi again,

Now that we can see your schematic it is more obvious what the diode is for.

The diode is there to prevent the inductive kickback from the motor coil which acts like an inductor. When the MOSFET is turned on, the current flows downward in the motor and the motor runs normally. But when the MOSFET is turned off, the coil in the motor has stored energy that it wants to release in the form of a reversed voltage. That’s the same idea that makes a boost circuit work, but here we dont want a higher voltage so a diode shunts the kickback current around the motor so it can eat up that stored energy. If it were not for that diode, the motor could develop a very high positive voltage at it’s lower terminal in the schematic, which could easily blow out the MOSFET.

In the drawing you’ll see the motor replaced with an inductor. A motor has series resistance too which isnt shown, but the coil acts very similar to that inductor and when the MOSFET turns off the inductor creates a very high reverse voltage as shown with the polarity symbols plus and minus. That makes the diode conduct, and that eats up that stored energy so the ‘inductor’ does not blow anything out.

Note that if one lead of that diode becomes disconnected, the MOSFET might blow out unless we get lucky and the inductor saturates before the voltage gets too high.
That diode is a common thing to see in motor circuits and relay circuits too as relays have inductance too.

You can see the path the current takes by looking at that red arrow. It loops around back to the other terminal of the motor. The motor has been replaced with an inductor because that is the aspect of the motor we are most concerned with when analyzing the action of the diode.

As a final note, you might want to notice the file size of the attachment, which is very small. That’s because the jpg format works better for drawings that have a very varied color scheme. The original was something like 800k bytes while this one looks almost the same but only uses less than 40k.

I noticed that the thread title refers to "conventional" current flow, which is from positive to negative, but you're describing the "true" (electron flow) current direction in the body of your opening post here:-

Current should be able to flow from the motor ground into the anode of the diode though, no?

The arrow in a diode is in the direction of "conventional" current flow, ie pos to neg, so current cannot flow from negative to positive through the kickback diode.

I'm rehashing part of what's already been said, but just wanted to clarify the point on "conventional" flow vs electron flow.

|500x333

Note: Should probably be a current limiting resistor in series with the gate.

Note: Should probably be a current limiting resistor in series with the gate.

WHY ?

The series gate resistor with HEXFET® MOSFET and IGBTS controls the switch on and off times.

http://irf.custhelp.com/app/answers/detail/a_id/215

It is generally a good idea to include a gate resistor to avoid ringing

http://electronics.stackexchange.com/questions/68748/question-about-mosfet-gate-resistor

A gate resistor limits the instantaneous current that is drawn when the FET is turned on

http://www.electro-tech-online.com/threads/does-a-mosfet-need-a-gate-resistor.87419/