What is dirty power?

I recently saw a question regarding this, and I never really understood what this meant.


4 Answers 4


Ok, this requires a little bit of trigonometry knowledge to fully explain. "Dirty power" generally refers to AC power. Ideal AC current waveforms look like a sin() function.

That is to say, kind of like this:

       .-.             .-.             .-.             .-.       
      /   \           /   \           /   \           /   \     
     /     \         /     \         /     \         /     \    
   /         \     /         \     /         \     /         \  
  /           \   /           \   /           \   /           \ 
-'             `-'             `-'             `-'             `

Dirty power, on the other hand, is not so even. It looks more like this:

      /   \       /\                   .-
     /     \     /  \                 /  
   /         ._.      \     .-.     /    
  /                    \   /   \   /     
-'                      `_.     `-'      

This can cause issues due to causing slightly erratic behavior in electronic components. They want the top to work with, and they get the bottom.

Thank you to http://asciimator.net/kangaroo/sinewave.html for the start with drawing the graphs. It saved me a lot of time.

  • 6
    Nice diagrams!!
    – nhinkle
    Aug 29, 2010 at 3:30
  • 8
    You sir, have not only answered the question, but drew, in ASCII, beautiful waveform diagrams, to do so. I am impressed. Aug 29, 2010 at 4:00
  • Can a big capacitor be used to smooth it out?
    – user148298
    Mar 23, 2016 at 14:51

Dirty power from a computing perspective is power that is not:

  • Always as the specific voltage (120 V in the US)
  • For AC systems, always at the specific Hertz (60 Hz in the US)
  • The waveform is 100% consistent (see Ryan's answer for a good chart of this!)

In general, dirty power has an increased frequency of violations of the above list over what the designers of a device designed it to expect. Yes, this is a bit wishy washy, but it is a good generalization. The kinds of dirty power:

  • Transient over/under voltages. These can be caused by a variety of things, but turning on a high load device (toaster) on a specific circuit can cause a momentary drop as perceived by other devices on that circuit. When that load stops a transient over-voltage can occur as well. I've also noticed over-voltages when power returns after an outage.

  • Bad frequency. Plugging the electric fan under your cube-desk to the same power-strip as your desktop computer. The electric motor in the fan can introduce its own waveform as perceived by other devices on that circuit. Older computers were more sensitive to this as it could cause clock-timing errors.

  • Chronic under/over voltages. For whatever reason, the power on the line just doesn't look good. Perhaps it is running very close to the rated load for the breaker panel. Perhaps the wire run to the outlet is old and is heating up.

Dealing with dirty power takes work and introduces electrical inefficiencies. The top tier of UPS devices will fully condition power, supplying load from battery when voltage drops, clamping voltage during spikes, ensuring consistent waveform. I have seen actual discrete power conditioners without UPS, but not recently.

Dirty power can affect the lifetime of computing equipment. Chronic under/over voltage is perhaps the most harmful as it really increases wear on components. A high frequency of transients can cause UPS gear to die sooner, as it has to work harder for longer relative to cleaner power. Power-supply quality affects how able they are to mask dirty power from internal components. Transients that make it past the AC/DC conversion inside the power supply can cause components to die sooner.

  • 2
    I like how the easy to understand answer with ASCII graphs gets upvoted 20 times, while the more correct technical answer only gets 2
    – TheLQ
    Aug 29, 2010 at 15:12
  • 4
    @TheLQ This is why presentations to upper management generally involve a lot of pictures ;) Aug 29, 2010 at 15:24

This is a pretty good description. Here's a clip:

Electrical pollution is not something you can see, smell, taste, or touch. It is not something you can sense, making it difficult for one to be aware of the presence of electrical pollution. With this in mind, it is important to understand what causes electrical pollution and what to look for in your everyday environment and home. Normally occurring incidents from everyday electrical use, electric and magnetic fields, earth currents and transients and high frequency noise are sources that cause electrical pollution. Electrical pollution can be one of these causes or a combination of the causes. Stray voltage is also a cause of electrical pollution. Stray voltage most often occurs on and off farms in localized areas. These localized areas can have poor grounding and utility infrastructure causing electrical failure. Stray voltage on farms has been detected by observing behavioral changes in farm animals and some health problems for humans. Stray voltage was one of first discovered sources of electrical pollution prior to the 1950s in areas of the rural United States.



Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a disturbance that affects an electrical circuit due to either electromagnetic conduction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. The source may be any object, artificial or natural, that carries rapidly changing electrical currents, such as an electrical circuit, the Sun or the Northern Lights.

EMI can be intentionally used for radio jamming, as in some forms of electronic warfare, or can occur unintentionally, as a result of spurious emissions for example through intermodulation products, and the like. It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television reception, although to a lesser extent.

(emphasis mine)

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