Hi,
I'm hoping to get a clearer understanding of question number 7
here:
http://www.allaboutcircuits.com/worksheets/latch.html

Specifically, I'm trying to understand the separation of power sources for gate logic and the motor (in the answer portion of question 7). Do the two sub-circuits share a common ground?
See attached graphic for clarification of my question.

If anyone can point to some good sources for further study on motor control, that'd be great too.

Thanks in advance for any input.
- Randy

 

all ground symbols are assumed to be interconnected (same potential)

 

Thanks for the help!
Does the approach shown effectively reduce motor noise on the circuit?
I guess the only alternative is complete separation via a relay, or something similar.

 

In most cases, if two circuits have just a single connection point in common, then noise from one getting into the other via that point is not too much of a concern because there is no closed circuit path in which currents can flow. But if there are two or more points in common (it doesn't have to be the supplies, either) then it is much more of an issue, because now currents can flow between the two circuits. How much interference one can cause the other is largely determined by how much of a noise signal appears between any given pair of connections, the impedance seen by that signal as it tries to enter the other circuit, and how sensitive the other circuit is to noise between that pair of connection points. Notice that in this circuit, there are two points of connection - the ground and the transistor base. But the impedance seen by the motor circuit looking back into the control circuit through the transistor base is very high and the control circuit is not very sensitive to any signals that do make it through.

 

That's helpful. Thank you for the explanation.