7
\$\begingroup\$

Working on an enclosure in which there will be low voltage high precision circuitry (my area of more experience) coexisting with fairly high current (~15A) 120VAC load wiring (my area of less experience). Would be it be advisable to twist those AC wiring pairs which carry equal + opposite currents to reduce the amount of field radiated from these wires? Or is it considered sufficient just to route them together side-by-side?

Obviously I don't care about the susceptibility of the mains wiring, only its emission and the susceptibility of the low voltage circuits. Never seen this done before but don't work with mains powered devices much. Don't want to reinvent the wheel or do something dumb / non-standard so anyone's experienced take would be appreciated.

\$\endgroup\$
  • \$\begingroup\$ So I don't find your question totally clear. I would not run low voltage and mains next to each other (within several inches) or in the same conduit. Not only is there the issue of the AC acting as an aggressor for your low-voltage signals, but there is the safety issue as well (if they are in the same conduit). In your enclosure, there should be segregated high voltage and low voltage areas. But I don't think there is any need to twist the AC mains wires. Take a look at some thermostats and similar controls that have both mains and signal sections. \$\endgroup\$ – mkeith Sep 19 '17 at 4:16
  • \$\begingroup\$ I don't think I'm understanding your answer. The PCB itself has both the AC and the low voltage circuitry on it in the first place, with a suitable isolation barrier and a few components bridging it (a relay and some optocouplers). I imagine that tons of of compact equipment (power supplies for example) must have both AC and LV circuits and wiring in very close proximity in an enclosure, way closer than inches apart. \$\endgroup\$ – ChateauDu Sep 19 '17 at 5:49
  • \$\begingroup\$ I am talking about the wiring leading to the box. Don't run the signal wires adjacent to the mains wires leading to the box. Whether you twist the mains wires or not is probably not going to make any difference. \$\endgroup\$ – mkeith Sep 19 '17 at 6:00
  • \$\begingroup\$ Most of the thermostats and such that I see have separate screw terminal blocks for high voltage and low voltage, with separate covers. This makes it relatively safe to wire and probe the low voltage circuit stuff while the unit is "hot". That is what I meant by keep them separate in the box. Not really my area, though. And it sounds like you understand the importance of keeping high and low voltage separate. \$\endgroup\$ – mkeith Sep 19 '17 at 6:03
  • \$\begingroup\$ Ah okay, understood. Makes sense that the main defense ought to be distance rather than twisting though. Thank you for the insight. \$\endgroup\$ – ChateauDu Sep 19 '17 at 17:28
3
\$\begingroup\$

Lets model that twisted pair, assuming VERY SLOW RATE OF TWIST, as 2 wires of spacing 4mm (these are power wires, after all) with distance 100mm and 104mm from the vulnerable sensitive PCB loop of area 1cm by 4cm. We'll compute the induced voltage for distance 100mm and for 104mm, then subtract those for the presumed magnetically-induced trash in the sensitive circuit. We'll also need the slewrate of the power line currents; we'll assume the rectified diode peak currents: 15amps * 10X, and 1microsecond turnon time, to be the aggressor Hfield.

Math: Vinduce = [MU0 * MUr * Area / (2 * Pi * Distance)] * dI/dT

and inserting MU0 = 4 * pi * 1e-7, MUr = 1, we get the form

Vinduce = 2e-7 * Area/Distance * dI/dT

Vinduce = 2e-7Henry/meter * 1cm * 4cm /100mm * 150amps/1uS

Vinduce = 2e-7 * 0.0004 meter^2 / 0.1 meter * 150e^ Amps/second

Vinduce = 2e-7 * 0.004 * 150 e+6 = 2e-7 * 0.6e+6 = 1.2 e-1 = 0.12 volts for the 100mm distance.

For 104mm distance (the other wire of the twisted pair), the voltage is 4% lower; our residual voltage from the twisted pair is 4% of 0.12 volts, or 5 milliVolts.

Can your precision sensitive circuits tolerate 5 milliVolts of trash, with fundamental repetition of 120 Hz, with a few microseconds duration and 1uS risetime?

EDIT

How to mitigate this 5,000 microVolts of trash? We have all the degrees of freedom specified in the math: loop area, distance, dI/dT, and the UNSPECIFIED variables of (1) how tightly the twisted pair is twisted and (2) how uniformly are the twists. You can measure these effects in the lab. Make a loop of 1cm by 4cm (or your personal choice of loop area), and measure Vinduce for various sinusoidal drives (with 50 ohm resistor to avoid shorting the Function Gen), with untwisted wires, human-twisted (non-uniform) wires, and machine-twisted wiring.

Note the skin depth of copper at 60Hz is 8 millimeters, at 60MHz is 1,000X smaller at 8 microns (1/3 mil or 0.0003 inches), and at 6MHz that skin depth is 8micron * sqrt(10) = 25 microns, compared to 1 ounce/foot^2 foil of 35 (3-5) micron thickness.

Your 1microsecond Trise has period of 2uS, or 500,000Hz (if this is valid way to model a quick Trise with very slow 120hz repetition). Skin Depth of 500Khz is about 80 microns of Copper. You may want a steel tube around the power lines, or route the power lines through a steel trough.

EDIT#2

Should you decide to NOT USE TWISTED PAIRS, but use separate (color-coded?) wiring for 117vac, there is nothing to hold those wires at 4mm spacing, and your induced voltage can easily double or triple, to 10 or 15 milliVolts.

EDIT#3 April 2020

You will notice a big reduction in coupling, if the rate-of-twist is fast (many twists per inch) and the twisting is done by machine (so the magnetic field variations are very regular and thus mostly self-cancelling).

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ This is what I needed to hear. \$\endgroup\$ – ChateauDu Sep 19 '17 at 5:51
  • 2
    \$\begingroup\$ @analog: Your SI unit capitalisation is not right. "The names and symbols of SI base units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter. For example, the metre (US English: meter) has the symbol m, but the kelvin has symbol K, because it is named after Lord Kelvin and the ampere with symbol A is named after André-Marie Ampère." (en.wikipedia.org/wiki/SI_base_unit.) So, 'µs' not 'µS' (microsiemen), 'henry' (not 'Henry'), 'amps' (not 'Amps'), 'millivolts' (not 'milliVolts'), 'kHz' (not 'Khz') etc. \$\endgroup\$ – Transistor Sep 19 '17 at 17:31
  • \$\begingroup\$ I used to use cycles-per-second. \$\endgroup\$ – analogsystemsrf Sep 20 '17 at 3:28
  • \$\begingroup\$ @Transistor It looks like the full name of a unit named after a person is in lowercase (kelvin), the abbreviation in uppercase (K). Hence we should write microsiemens (NB always plural) but µS (µs would be microsecond). en.wikipedia.org/wiki/Siemens_(unit) \$\endgroup\$ – stevek_mcc Aug 6 at 13:13
4
\$\begingroup\$

There is a great paper on this written by Bill Whitlock of Jensen Transformers. He found that twisting the line and neutral pair 5 twists per foot and running the ground in parallel reduced the induced noise on the ground wire by over 1000 compared to the worse case scenario, so I think your idea of twisting the power conductors is valid as ground noise was dramatically reduced. Please see paper referenced here: http://www.jensen-transformers.com/wp-content/uploads/2015/02/AES-Ground-Loops-Rest-of-Story-Whitlock-Fox-Generic-Version.pdf

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.