Ground Return Path
The negative line along which the motor current returns to the source. This path carries the same current as the positive line, so it must be run in the same cross-section and as short as possible. An indirect return through the vehicle body or the machine chassis is a source of voltage drop and interference.
A common mistake is to run the positive line thick and connect the negative to the nearest body point with a short cable. Paint, rust and bolted joints on the body create an undefined resistance; at 30 A even a few milliohms produce a measurable voltage drop.
Correct practice:
- Run the negative line directly to the battery negative terminal or to the common earthing bar.
- Use the same cross-section as the positive line.
- Run positive and negative side by side, preferably twisted; that way the magnetic fields they create largely cancel each other.
- Clean the paint at the connection points and use star washers.
The second effect of a weak return path is electrical noise. When the motor current circulates through the body, the signal and RF circuits that use the same body as their reference see the voltage difference created by that current as a shift in their reference. Reduced remote control range and false triggering on inputs often originate here.
Where this term is used: DC Motor Driver Selection Guide