pic: A little teaser from 3571

[Caleb Sykes]

Originally Posted by TheThings

(this is the Electrical lead): By running the pwm signal wires next to large motor wires that could potentially have upwards of 40 amps through them, you risk electromagnetic interference across the pwm. For this reason we like to run our pwm cables as far away from our power cables as possible, and where they have to come near each other, we cross them at a 90 degree angle. This prevents the magnetic fields on the power cables from interfering with pwm communication.

Anecdote: I have never had a problem when routing PWM wires and power wires together.

Physics Note: Even if the magnetic field were substantial, routing PWM wires parallel to power wires should not have an appreciable effect on signal, since the magnetic field would only cause a Hall Effect on the signal wires. That is, the magnetic field would not speed up or slow down the electrons, it would just shift the electrons to one side of the wire or the other.

Just for fun though, let's calculate the magnetic field created by a 12AWG wire, and see how that magnetic field affects a 22AWG PWM cable next to the power cable.

Take:
I1 = current in the 12AWG wire
I2 = current in the 22AWG wire
r1 = radius of 12AWG wire = 1.03mm
r2 = radius of 22AWG wire = .32mm
d = distance from center of 12AWG to center of 22AWG wire

The magnetic field B(r) created by the 12AWG wire at a distance r (r>r2) from the center of the wire will be:
B(r) = (mu0)*I1/(2*pi*r)

The magnetic field will be perpendicular to the propagation of the electrons, which means the magnetic field will cause the electrons in the 22AWG wire to move either toward or away from the 12AWG wire (This is the Hall Effect). The difference in voltage VH between the near side and the far side of the 22AWG wire will be given as*:
VH ~= I2*B(r)/(n*t*e-)

Where B(r) is given by the first equation, n is the number density of charge carriers of conducting material, t is the thickness of the 22AWG PWM cable, and e- is the fundamental unit of charge.

Combining these equations, and taking mu0 = 4*pi*10^-7 (mks), n=8.49*10^28 m^-3 (copper), t = r2 = .00032m, and e- = 6.02*10^-19 C, gives:
VH = (4.6*10^-14*I2*I1/d (mks)) V

Plugging in some ridiculously high values for I1 and I2, and a ridiculously low value for d:
I1 = 1000A
I2 = 10A
d= .1mm

This yields a potential difference between the near and far side of the PWM cable to be 5*10^-6 V. Which could be measurable if you had a good sensor, but doesn't come anywhere near the 5V** high voltage transmitted by the PWM cable.

Please don't route your signal and power wires separately for this reason.


*At these small of distances, the magnetic field can not be assumed to be constant over the entire width of the cable, hence the ~ sign.
**Source needed