I don't have a lot of experience with mains voltage and designing protection circuits for them. I'm not sure:

A) What do I need to protect against? I know I don't need to worry about 10kV voltage transients on the mains line, but what about 300V spikes? Mains voltage is 120Vrms nominal (in the US, for residential users), but what is the maximum that should be expected and planned for.

B) How do I protect against it? For voltage transients I can use a TVS diode, or a MOV, but I don't know how to select one. When do I use one over the other? For overvoltage protection, is there an AC version of a crowbar?

What are good guidelines about what to protect against, and how to do it for mains circuits?


To try and head off the inevitable "close for too vague" votes, I want to mention that I'm only interested in:

  • "what are the common voltage issues that residential mains wiring will present to a circuit on it" and
  • "what are the basics of protecting against these issues"

I realize that there are probably many books written about circuit protection circuits, but I think that a very basic answer would be very appropriate here.

  • 1
    \$\begingroup\$ Your question will probably be closed as being too broad. You need to ask a specific question. Start with defining what you want to protect. Light bulbs or medical life-support systems? \$\endgroup\$
    – Transistor
    Sep 27 '16 at 21:07
  • 3
    \$\begingroup\$ @Transistor: The relevant part of the "how to ask questions" page is about vagueness, not broadness. This isn't vague is it? If we close everything that is "too broad" this site is going to become uselessly filled with questions that don't have any meaning beyond their specific intent. \$\endgroup\$ Sep 27 '16 at 21:12
  • \$\begingroup\$ @op I partially agree with you. A too broad question cannot be answered, while something too specific is less reusable. Procedure on this board is to close too broad questions anyway, and we can't change that, and your question is quite broad indeed. You could write a book about it broad. Try to be more specific while also leaving room to generic and useful answers. \$\endgroup\$ Sep 27 '16 at 21:18
  • \$\begingroup\$ Are you interested in protecting your equipment from abnormal behavior of the line, or are you interested in protecting your users from getting electrocuted? \$\endgroup\$
    – The Photon
    Sep 27 '16 at 21:26
  • \$\begingroup\$ @VladimirCravero: I'm looking for the simple "I want to plug in a circuit to my mains voltage, what do I need to watch out for, and how do I watch out for it" answer. \$\endgroup\$ Sep 27 '16 at 21:27

Your wattmeter only has arc suppression gap set at 6kV standard. So you can expect anything less than that and possibly more , likely in Florida , with transients that do not ionize the gap.

This is why burnt out LEDs in Florida are so common with cheap PAR Lamps.

One way to suppress peak voltage is via line filter using two X1 caps and a choke or a 3rd order LPF. This reduces the 10kV spikes to <3kV or even as low as 300V but stretches them out in time.

So Optoisolator products assume you know about this and are all rated for 3kV isolation.

Any protection you add to clamp these voltages must be able to,handle the current that follows.

Gas tubes can handle 10kA surges but expect you to fuse the input.

MOV's are cheap and good but can only absorb occasional surges and will fail after so many accumulative Joules in energy. Chokes allow the rise rate in current to be limited until they saturate , then they behave like wirewound resistors.

Here is some industrial design background for line filters. Line filters in LED bulbs must pass these tests, and not all,suppliers are legit in saying they are compliant.

There is no "one solution fits all" because of other requirements for low cost, high reliability, and new EMC compliance rules for devices >= 100W must have power factor >=0.9 using active PFC etc.

  • \$\begingroup\$ Thanks! What is the frequency of surges on the line (is there a national average)? How much energy is in a typical surge? If I use a MOV, do I need to spec it against 1 surge a day, 1 surge a week, a year? Is there a reason you didn't talk about TVS diodes? \$\endgroup\$ Sep 27 '16 at 22:44
  • \$\begingroup\$ I see that the link you provided mentioned that, thanks. \$\endgroup\$ Sep 27 '16 at 23:04
  • \$\begingroup\$ @AndrewSpott, why you are so quick to mark answers as answers? \$\endgroup\$ Sep 27 '16 at 23:46
  • \$\begingroup\$ @Ali Chen: because he answered all my questions. I was a little quick for this one, but I'm not yearning for more information. \$\endgroup\$ Sep 27 '16 at 23:49
  • \$\begingroup\$ Ali < if it's any consolation I'll vote for your link, which describes all the test PLT and ELF standards that I used to write for a Corporate environment decades before. \$\endgroup\$ Sep 28 '16 at 0:52

Protection of electronic equipment is normally defined in IEC 61000 standard. It includes protection against ESD (61000-4-2), immunity to EFT (61000-4-4), 61000-4-5 (lightening surge immunity), etc. The standard is a collective wisdom of what kind of surges are typically there, and defines several classes of equipment protection. Here is an example from Texas Instruments, with examples of what to protect against, and partially how. There are many other good write-ups, with examples of protection solutions. The keyword is "IEC 61000".


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