AC Mains Surge Protector

Voltage spikes  can sometimes be a big nuisance as far as the safety of the various electronic appliances are concerned. Let's learn how to make a simple AC Mains surge protector circuits at home.

What is Voltage In Rush

A sudden voltage spike is basically a sharp rise in the voltage lasting not more than a few milliseconds but enough to cause damage to our precious equipment almost instantly.

It thus becomes imperative to stop or block these from entering vulnerable electronic gadgets like our personal computers.

Commercial spike busters are though available pretty easily and cheaply too, cannot be trusted and moreover have no reliability test arrangement so it becomes just a "assuming" game, until it's all over.

Working Design

The circuit of a Simple AC Mains Surge Protector Device below, which shows how to make a simple homemade AC mains high current protector device is based on very simple principle of "speed breaking" the initial jolt through components who are well equipped in the field.

A simple iron resistor and MOV combination are more than enough to provide the protections we are looking for.

Here R1 and R2 are 5 turns of iron wire (0.2mm thick) over a 1 inch diameter air core each followed by an appropriately rated varistor or an MOV connected across them to become a full fledged spike protector system.

Sudden high AC entering the input of spike  are effectively tackled and the "sting" absorbed in the course by the relevant parts and a safe and clean mains is allowed to go through the connected load.

Transient Protector Using Inductors and MOV

surge suppressor using inductor and MOV varistor

Question Regarding Surge Prevention in Electronic Ballast

Hi swagtam, I found your email address from your blog. I really need yr help. Actually my company has customer in china we make UV lamps and we use electronic ballast for it. now the problem is in china because of Over Voltage the ballast burn out so i design circuit which is in attachment which dosen't help either?

so i found your Ultimate High/Low Voltage Protector Circuit which i wants to build. or can you tell me the update if i can do in my circuit that will be great. sorry if i am bothring you. but i really really need yr help to save my job thanks Thank You Krishna Shah


Hi Krishna, According to me the problem may not be with the voltage fluctuations, rather it's because of the sudden voltage surges that's blowing of your ballast circuit. The diagram shown by you may not prove very effective, because it does not incorporate a resistor or any kind of barrier with the MOVs. You may try the following circuit, introduce it at the entry point of the ballast circuit.

Hope it works:

surge suppressor using 2 MOV, resistors, and fuse

Using an NTC and MOV

The following image shows how two different sudden high voltage suppressor devices could be tied up with the mains line for achieving a double edged safety.

The NTC here enables an initial switch ON current in rush protection by offering a higher resistance due to its initial lower temperature, but in the course of this action its temperature begins increasing and it begins allowing more current for the appliance until a normal working conditions achieved.

The MOV on the other complements the NTC output and makes sure that in case the NTC is unable to stop the up-surge onslaught correctly, it switches ON itself shorting the residual high transient content to ground and as a result establishing a safest possible supply for the connected load or the appliance.

surge suppressor using MOV and NTC

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Swagatam said…
You can incorporate thermistors and MOVs together, resistors can be avoided then.

for knowing more about thermistors, you may refer to this article:
Swagatam said…
For what application do you want to use it?
Swagatam said…
No it's not due to spike, it's due to a split second late response from the UPS, try with a new good quality UPS and you won't find this issue happening.
Swagatam said…
telephone surge protector?? i didn't get the application, can you pls explain it?
Hi....Swagatam can u give the VA rating of mov & fuse of above ckt u used above....when my ckt I/p current is 130mA and o/p current is 170mA & o/p voltage is 167 when I/p voltage is 230V for electronic ballast. please share it with me asap....
Swagatam said…
Hi Shersingh,

you can use a 350V RMS, @5amp rated MOV.
chris.sorbi said…

Is it possible to use this diagram for a single wire ground return system? My power line is a single phase 240V and the neutral is coming from a copper rod in the ground. The voltage fluctuates a lot and i would really like to protect my devices (Mainly motors and VFD's) and some computers. Do you recommend any other systems like high and low surge protection? Also, what protection rating should i expect (high voltage vise)

I appreciate your response in advanced.

Swagatam said…
the above circuit will only help to control surge in-rush to some extent but won't control the voltage fluctuations.

you would need a voltage stabilizer circuit for tackling the issue effectively, one design has been discussed below which may be tried or procured readymade:
Nilesh said…
Hi Swagatam ,

How are you ? hope doing well.

If want to draw constant 220 v AC at any point

Generally input is
Low - 110v / 130v AC
High - 230/260 v AC

Required Output
220 V AC (Stabilized)

Can you please suggest best circuit . if Tranformerless will preferable.

Nilesh Patil
Swagatam said…
Hi Nilesh,

I am fine, thanks!

You can try the design that's shown in the following article:

a transformerless design could be very difficult since it would involve complex mosfet drivers, ferrite inductors etc, so I think using the traditional iron core transformer would be much easier, although a little bulky.
Makhon Barooah said…
Plz. value of MOV and best one company product......thnx.
Makhon Barooah said…
Value of MOV and best company product....thnx.
Swagatam said…
company is not important as all are good, the clamping voltage rating should be 50 V more than the input mains supply voltage.
Roberto Carmona said…
Hi Mr Swatam. Cogratulation for your extraordinary electronic web page.......

I write from México. My question is: For a 120v nominal voltage, what should be the value of the resistors, and the maximun peak voltage and Joules for the MOV's ?

Thanks in advance for your help.......
Swagatam said…
Thanks Roberto,

for 120V you can select 200V as the clamping voltage for the MOV, the resistors are not critical they are just for complementing the MOVs, any value between 0.5 ohms to 1 ohms will do the job.

10 ohm is far too big and must not be used, it's incorrectly shown in the diagram.
Hi Mr. Swatam, do you have a schematic diagram for TVSS or SPD for resedential use only. pls include the value of materials.

thank you so much.
Swagatam said…
Hi Cloyd,

you can probably try using the devices explained in the following article:

put these directly across the wires that enter the house wiring.

the value can be of 350V/50amp
Hi Sir Swagatam, thank you for your response. i want to ask you more question sir about TVSS/SPD , This is for our thesis sir and i'm hoping that you can give us more details about the connection. we are troubling of our design sir because we would like to make a TVSS device that will connected parallel to the source (40A Breaker), thus we doubted our design due to its connection if it can really suppress the voltage surge. i will show you our design sir and were hoping that you can give us some advice on how to fully get the right design.
thank you for your time sir Swagatam, best regards.

this is the diagram of our device sir.. the source is Line to Line. we just put the ground into the device for the MOV's.
The upper diagram sir is what we wanted to do (parallel to the source) but we are not sure if it can supress the voltage surge thats why we decide to design another (lower diagram) which is connected in series from the source. i'm begging for your expert suggestion and advice sir that can help a lot to our device to function and for us to pass the subject. thank you sir SWAGATAM.
Swagatam said…
Hi Cloyd,

The link which I suggested you in the earlier comment can be effectively used as a transient voltage surge suppressor.

Other option is in the form of diodes which is explained here, these also need to be connected parallel to the input mains supply:

The image provided by you is not opening, please make it "public" by selecting the share option.
Swagatam said…
...please toggle the share option...
Swagatam said…
normally all TVSS devices are extremely reliable and can be trusted for the necessary actions.

you can refer to the following article for more info:
Mark Mare said…
Dear Mr Swagatam

Firstly thank you for this great website, i have built a few small projects of your design, i have become extremly interested in transient voltage and the long term damage it does. I will test the above circuit and give some feed back.

What i would like to ask is (if you perhaps have an existing) a design where one can "monitor" the transient voltage on a incoming line (the main grid). I understand that there are "in line counters". This can be a great tool to take the readings before and after the unit install.

Will this be possible?

Again thank you for the great site and info.

Best Regards
South Africa
Swagatam said…
Thank you dear Mark,

The simplest way could be to connect a digital ammeter in series with the MOV that may have a feature to latch up to the reading corresponding to the transient value each time power is switched ON.

If it's possible I'll try to design and post it for you.
Mark Mare said…
Thank you very much Sir.
Makhon Barooah said…
ko good, but we want buy pcg/ components/ etc in india. is it possible in india?
Swagatam said…
yes, it' possible.
Lim said…
A friend of mine was thinking of building a surge suppressor and voltage regulator by using ARM microprocessor. He was thinking of using inductor plus MOVs together. Do you think it is feasible?
Thank you.
Swagatam said…

MOVs don't require any external triggering circuit for activating, it's a self-actuated device so I don't think an MCU could be used for operating an MOV
Narciso Calzo said…
Hi Swagatam,

I was looking for a surge suppressor device circuit diagram and was directed to your site. Upon reading about you and going through the comment and replies between you and your readers, your patience in answering all the queries really put me in great admiration to you. I came across so many people like you in the web but you are different from them, they can't respond.

Recently I just decided to become a hobbyist in electronics although I don't have a formal education or training about it but a steward/stewardess can land a commercial plane with the guidance of the ground controllers( this is from a movie).

I was inspired by a friend that showed me his DIY audio amplifier and it works well. I asked him for the details and he gave me copies of schematics, pcb layout, parts list as well as procedures of adjustments but it is limited to a 100W into 8ohm power. I want to evolve this to a 350W,8ohm design. The design uses 2SC5200 output devices and TIP41C drivers.

Can you give me suggestions, advices or even a new design from you, or, is it okey to publish the circuitry since it was designed by an EE in order for you to go through it and come up with solutions and recommendations to meet the desired power for my first project.

Actually I have already etched the pcb the very conventional way. By the way I am from the Philippines, work as a material eng'r in a construction company.

Nice knowing you Swagatam.

Your new reader/jollower,

Swagatam said…
Hi Osic, thanks so much! I appreciate your enthusiasm and wish you the very best!

Amplifiers can be quite difficult circuits as they require precision and balance at every node and stages of the circuit and so far I have not been able to master these gadgets due to their complex concepts and configurations.

So I am really sorry I won't eb able to assist you with amp designs, therefore I would recommend you to select a schematic which is originally designed to suit your specifications instead of modifying a design having a different specifications.

I have a 100 watt simple mosfet amplifier circuit in this site designed to work with around 35V normally which is capable of generating upto 300 watts power simply by using a higher voltage input of around 100V...however it requires a 4 ohm speaker, If you wish you could try that design also!!
Narciso Calzo said…
Okay Swagatam I will take a look at your design. Thanks Swagatam, more power.
shivtej nirmal said…
What MOV VALUE and part no
patrick vaz said…
Hi Swagatam,

How are you,
A friend of mine had his TV friend due to a voltage surge,
Voltage Input from the mains line is 230V, sanctioned current is 6amps. want to help him out.
Can I use the circuit mentioned above 2 MOV with 10ohms 2 watt resistor,
-what should the value of the MOV be.
- what should the value of the fuse be in terms of AMPs.
- should a Indicator lamp be added in the circuit after the fuse,
Kindly advise.
warm regards

Swagatam said…
Thanks Patrick, I am good!

yes you can try it, just replace the resistors with 1 ohm 2 watt, because 10 could be a bit too high.

MOV could be rated at 350V (clamping voltage)

fuse could be an 8 amp rated....

indicator lamp is optional, it can be added for indicating a blown fuse
patrick vaz said…
Hello Swagatam,

Thank you so much,

Apart from the Mains, it could be added in different sections of the house, where sensitive equipments are such as TV/ Music systems etc., just to be safe.

Is this okay ?

warm regards
Swagatam said…
Thanks Patrick, yes you can use an MOV with every appliance but MOV has its own restrictions too, for example it might not withstand the surge if it lasts for more than a millisecond, and could cause a short circuit and fire, therefore a series fuse also becomes mandatory.

Alternatively you can use a single powerful MOV near the main DP of the house, the MOV specs are explained below

By the way all modern equipment already include a built-in MOV in their system
Ravi kumar said…
Hi Swagatam,

I'm planning to install a surge suppressor for the whole house at the mains AC panel (next to the meter board) is there a standard part available available off the shelf or do we need to design one and install. i was able to find CHSPT2MAX from Eaton, but its too expensive, any suggestions...
Swagatam said…
Hi Ravi, you an industrial MOV, in conjunction with the following delay ON relay crcuit, adjust the delay time to may be a second or so, and connect the relay contacts with the house wiring.

Industrial MOV sample is discussed below
RAO said…
I have an electronic digital calendar clock, its supply unit has bagged off three times due to surge voltage in the mains. I want to incorporate a simple sure suppressor for it. it may be drawing less than on ampere current. what value components are recommended. please advise me if any simplest circuit which I can assemble myself.
Swagatam said…
Normally any god power supply will never burn with minor supply fluctuations....what kind of supply unit have you been using? Is it SMPS based or transformer based?

I would suggest you to use your cellphone charger for the purpose which has all the protections in-built

still if you are interested to build your customized surge suppressor, you can try an NTC in series with any of the input mains supply lines, and additionally connect an MOV across the two supply lines after the NTC.
RAO said…
I am talking about the " Ajantha make wall clock : OLC Quartz 104, Model. computer century Calendar", which I am using. There is a supply module inside the clock. Every time there is a power shout down in our area and the power comes up, the clock fails. The watch repairer says supply unit is burnt. So I just wanted to incorporate a surge suppresser to this clock externally.
Swagatam said…
OK I understood, the in-built power supply could be an SMPS expressed in above comment you can try connecting an NTC in series with any of the supply lines, and also connect an MOV across the two supply lines after the NTC.
I'll try to update the diagram soon for your reference
Voon Yongjian said…
Hi sir,
I am designing a surge protection device for lightning. Is the design above is suitable for it? Can I replace the resistor with an inductor because i saw it somewhere in other website?

Basically my input supply will be 240 Vac, what is the value of clamping voltage of MOV is suitable? Can I use MOV with 240V clamping voltage?

Swagatam said…
Hi Voon, if the lightning strikes directly into the house wiring then no surge suppressor would be capable of stopping it.

MOV can suppress only minor electrical surges, not lightening related suppress lightning related surges the best idea would be to employ a triac based crowbar circuit as described here

the high voltage capacitor and diode can be ignored and removed.

you can also include an MOV/inductor suppresor as per your initial requirement in conjunction with the above design.
Gajendra mishra said…
hi sir,
first i try to explain you my system....which is regarding to operate 3 phase submersible pump from gsm based remote and data accusation also....
in that metal sheet enclose contain electro-mechanical contactors, smps for gsm data logger for sms and gprs connectivity.
problem is ---
1.some time when pump is off due to non power we found that when power comes suddenly electrical blast create in contactors at ac input terminals.
2. some time when starter strat, gsm electronic immediate shutdown or restart, due to this pump cant start.
3. in solar operated gsm logger , found that in rainy season modem became hang.
i think all above are due to electrical surge..
sir please guide me how to solve it, i want to make a spd solution for above .....
Swag said…
Hi Gajendra, for controlling surge you can try a delay ON timer circuit, as explained at the bottom section of this article:

You can replace the relay with a 3 contact relay for connecting the 3 phase through the relay.

the delay ON could be set by adjusting the values of C2/R2. The emitter zener diode is not required and could be replaced with a wire link
subramanian said…

Iam using a device whose load is 5A, 220V, it already has builtin Mov. Its getting busted if any surges occurs. I want to avoid busting of inbuilt Mov, because replacing inbuilt Mov is little bit complicate.
So suggest me a circuit, which discharge the surge before that inbuilt Mov. Also specify the ratings.

Swagatam said…
Hi, An MOV is not supposed to bust unless the voltage spike is much higher than its rating or may be its quality is bad.

Because MOVs are voltage based devices not current based.

Anyway please try implementing the last circuit from above which includes an NTC.

The NTC resistance will be higher initially preventing the initial surge, and as it gradually heats up due to the current, it will start passing the normal amount of voltage and current.

This will safeguard your MOV during the initial surge.

Please tell me the voltage rating of the load, I'll provide your the NTC resistance according to the specs.

Or you can calculate yourself through Ohm's Law:

NTC Resistance = V / I

subramanian said…
Thanks for your immediate reply!!

Rated voltage of the load is 220v.
1. What ratings of NTC and MOV can be used?
2. Should i use any resistors along with it?
3. Somewhere i have seen using 3 Mov's (ie. parallel to phase neutral, neutral ground, phase ground) should i follow that?
Swagatam said…
I don't think connecting MOVs in parallel can be very effective, because each may have slightly different specs, so at any instant any one of them would conduct... they may never conduct together at the exact same moment, so that won't work

However you can try an MOV having higher current rating, this may be useful in case the surge stays for longer periods of time (in milliseconds though)

The NTC suggested by me is actually a resistor, whose resistance will be high initially and as it gets warmed by the load current, its resistance will normalize.

Therefore you can try an NTC in series, and this technique is commonly employed in most systems.

If the load current is 5 amp at 220V you can go with a 20 Ohm NTC. This is an approximate and should work well.
subramanian said…
can you suggest me the model number for MOV....

since iam a beginner, please bare with my questions and guide me in detail.
Swagatam said…
No problems! You can use a high energy MOV capable is absorbing high peak currents at the rated camping voltage.

You can try a 400V MOV from Little fuse.

Please note that if your load is rated at lower peak voltages than 400V then you may have to select the voltage according to that voltage level...400V is an assumed minimum rating by me.
subramanian said…
Is there any circuit to indicate the failure of surge protector/ MOV? or which can disconnect the load when MOV fails.
Swagatam said…
I am afraid there isn't anything that may detect the failure of an MOV. If you are looking for a failproof surge suppressor, you could try a triac based "crowbar circuit", with fuse.

You can find an example in the following article;

the triac along with its associated zener, resistor and the fuse forms an effective high voltage surge suppressor.
subramanian said…
In the above method, because of fuse the load is disconnected whenever surge occurs. why cant i use same fuse along with MOV? because even MOV becomes short circuit whenever surge occurs, right?
Swagatam said…
The fuse will blow only if the surge lasts for more than a few microsceconds, and in that case the fuse should blow, otherwise the load could get destroyed.

Normally the surge spikes don't last for more than a few microseconds, so there's nothing to worry.

The same is true for MOVs, if the surge lasts for more than microseconds it can destroy the MOV itself.

You can use a fuse with MOV also, but chances are that the MOV might burn before he fuse does...
subramanian said…
In that case when does TRIAC gets damage or stop protecting the surge.....
Swagatam said…
Triac will never get damaged because before that the fuse will blow. Triac is many times stronger than an MOV therefore it can withstand the high current until the fuse is blown

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