AC Power Cables

[HDTV Forum]

January 31 2003

I would love to unravel this whole power cord mess

For sure. They can be a nightmare. Here are some tidbits for you to ponder. Perhaps one of them will offer a clue to explaining some of your past observations:

* Power cords are usually unshielded two or three conductor cables. They are excellent antennas for radiating noise above 30 MHz. They conduct noise from low-freq power line harmonics up through hundreds of MHz in either direction.

* Power cords are hard to shield. You can buy a foil-shielded cable but it has unpredictable results. There's no good low-impedance metal connector on the ends like a signal cable, plus there's no shielding on the house wiring anyway (steel conduit might help). The safety 'green wire' is poor for RF termination. Shielded cords lower the cords impedance and actually make it act like a transmission line to conduct the noise at high frequencies.

* Equipment design affects power cord noise. Good equipment uses a shielded, filtered IEC male plug on the back with a detachable cord. Look for a metal can around the power connector. Cheaper stuff uses no filter component shielding, and EMI filter components are exposed inside the equipment or supply. Cheaper equipment may just barely meet limits, and could exceed under various cabling configurations.

* Noise coming out on the line cord is broadband power supply switching noise, and current switching harmonics of digital logic passed through from the DC load side.

* Changing the impedance to ground at the line cord plug can greatly affect radiation off the cord. A power conditioner/filter might have a low output impedance with capacitance to ground or high output impedance with series inductance. Either way, the line cord's inductance is part of an L-C circuit. Moving the line cord changes the inductance, and moves around the resonant frequencies. It is possible for a line conditioner to affect the system, not by filtering noise coming through on the power but by providing a different source impedance at the plug which in turn changes how it radiates to other cables. You've basically isolated all the inductance of your house wiring at the same time.

* All it takes is one noisy piece of equipment and line cord to trash all the other cords and signal cables in a system.

* It is good to keep all your signal cables as far away from line cords as possible. Segregate them, bundle all excess cable length to reduce radiation pick-up or radiation.

* Wrapping a noisy power cord, or a sensitive signal cable around a ferrite core (toroid) or clamping on a split toroid can sometimes help attenuate common-mode noise, if it exists.

Hope the above might help explain some of the voodoo. Some things help a lot and they are cheap.

emc guy

[HDTV Forum]

About 2-3 years ago I researched this with another engineer but was unable to come up with anything definitive. Thanks for your time.

* Power cords are usually unshielded two or three conductor cables.
* Power cords are hard to shield.

Just like we have a standard of 75 ohm cable and connectors for video is there a standard for accurately transmitting AC power using AC cords and connectors?

Cheaper stuff uses no filter component shielding, and EMI filter components are exposed inside the equipment or supply.

This is how nearly 95% of consumer equipment is designed.

Noise coming out on the line cord is broadband power supply switching noise, and current switching harmonics of digital logic passed through from the DC load side.

This is due to power supply design or simply the nature of the beast? Power transformers are the best and switched mode supplies can do well if designed correctly?

All it takes is one noisy piece of equipment and line cord to trash all the other cords and signal cables in a system.

Is this a subtle detectable reading or should I be able to easily see this effect with a spectrum analyzer by simply monitoring the output of the AC synthesizer?

Wrapping a noisy power cord, or a sensitive signal cable around a ferrite core (toroid) or clamping on a split toroid can sometimes help attenuate common-mode noise, if it exists.

Again, should I be able to easily see this effect with a spectrum analyzer by simply monitoring the output of the AC synthesizer?

Curiously what about hospital grade AC outlets and connectors - any input?

Thanks,

Richard

[HDTV Forum]

is there a standard for accurately transmitting AC power using AC cords and connectors

There are numerous standard plugs and receptacles depending on the max. voltage, max. current, # phases/neutral, and country reqmt's. They are all designed with the primary goal of transmitting power and do it safely. They all accurately transfer power, but I think maybe what you are asking is, are there designs to 'only' transfer power and nothing else? Basically, no. I have used special shielded power cables with military type shielded connectors. Very expensive and everything shielded on both ends. All custom designs.

This is how nearly 95% of consumer equipment is designed

I think you're right. I seriously doubt if much consumer equipment uses the shielded EMI filters (see http://www.cor.com/ for examples). It adds cost but they work the best.

This is due to power supply design or simply the nature of the beast? Power transformers are the best and switched mode supplies can do well if designed correctly?

Yes on all counts. Some switcher topologies are noisier than others, but they all generate lots of broadband noise into the 10s or 100s of MHz. The harmonics are close together (several kHz) and can move around because they are not based on a precise clock, plus they are pulse width modulated. They are great noise sources to corrupt audio and video. The designers have to take extreme caution to filter it so it doesn't affect the product's operation. The question is: How good a job are they doing on the AC side? Just meeting the FCC (and other country) limits may not be good enough when there are lots of other video/audio equipment around. Another thing, the noise not only comes out conducted on the cord, but radiated as a magnetic field. The switcher's magnetics (chokes, transformers) all radiate mag fields several inches to a couple feet away. The lower frequencies pass right through plastics and even some metals. These fields will couple into the magnetic components of other equipment nearby if they are not properly shielded (adds more cost). Stacking equipment therefore increases the chances of all these components to 'talk' to each other. The quietest power supplies are linear - using bulky transformers. These are heavier, bigger, and more expensive that's why they aren't used much anymore. The linear's transformer has little noise above a few harmonics of the AC power frequency (rectifier diode noise) however it could pass through high-freq. noise from the DC load if it is noisy.

Is this a subtle detectable reading or should I be able to easily see this effect with a spectrum analyzer by simply monitoring the output of the AC synthesizer?

Spectrum analyzers are the best tool, if it covers the frequency range of interest. The best thing to look at noise on the cords is a common-mode current probe (see http://www.fischercc.com/instframe.html for examples). You clamp it on the cord and look at the spectrum. You can see which cables are noisiest, but sometimes it's hard to tell. One noisy cable circulates the noise on all the other cords and signal cables. You have to disconnect cables or turn off power one at a time to see which one is the worst.

(Re: ferrites) Again, should I be able to easily see this effect with a spectrum analyzer by simply monitoring the output of the AC synthesizer?

The analyzer will instantly show a drop in the noise levels when a ferrite is clamped on, assuming it is going to work. Ferrites need a low impedance to work effectively since they have impedances of maybe a few ohms to a couple hundred ohms. The 'low impedance' is the common-mode impedance of the cable - not the characteristic impedance (like 75 ohms). Ferrites don't do anything to affect the signal if the signal and it's return conductor are both exclosed. That's good, because they only affect the common-mode noise - the noise current that is on both conductors going in the same direction.

Back to the original question - you might see the effect on the AC using a scope but only at higher frequencies. You won't see a ferrite even put a dent in harmonic distortion of the AC power. These frequencies are too low, plus the current is differential mode between the phase and neutral. The amount of harmonic distortion is dependent on how well they design and filter the supply (more money).

Curiously what about hospital grade AC outlets and connectors - any input?

These have better insulation and maybe other safety-related characteristics. They don't do anything for noise.

I know some of this is getting fairly technical but I hope it is useful.

emc guy

[HDTV Forum]

I know some of this is getting fairly technical but I hope it is useful.

You haven

[HDTV Forum]

You can get a lot of noise attenuation down into the kHz range in the small current (1A - 3A) filters. Not quite as much attenuation with filters in the 6A - 10A range, but this would be the high power amplifier range and it's probably less susceptible to noise anyway. So if you modified all your equipment with these filters, you would have practically no measureable noise on the AC cords - at least coming out of the equipment. Shielded cords and ferrites would really be unnecessary. This would be sure to clean up anything circulating around on the audio or video signal cables, which have a finite amount of shielding effectiveness. The filter really needs a box with a metal chassis. If the device is in a plastic box the filter won't be nearly as effective.

The advantage to this over the line conditioners is that it blocks the noise right at the source. Trying to block it at the plug end of the cord is too late. The horse is already out of the barn by then. No matter how great your filter is located several feet from the box, the noise is already circulating on all the other signal cables (except for fiberoptic cables) attached to that box.

emc guy