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Using an NTC Thermistor as a Surge Suppressor

Using an NTC Thermistor as a Surge Suppressor

In this post we learn how to use an NTC in circuits for suppressing surge current during power switch ON. We also learn the datasheet and the electrical specifications of an NTC.

Today electronics is getting more and more compact and light weight, it's basically due to the involvement of compact converters which have completely eliminated the age old iron cored transformers.

However this had to come at a cost, these units became too vulnerable to switch ON power surges.

But electronics always has appropriate answers, whatever may be the issues. NTC thermistors were created exactly for tacking this, that is in-rush surge currents during power switch ON.

What's an NTC

NTC (Negative temperature coefficient) thermistor is a semiconductor that contains
metallic oxides. It displays an electrical resistance which has an extremely foreseeable alteration
with warmth.

The resistance differs substantially with heat, much more in comparison to
normal resistors.

These are incredibly perceptive to heat change, very precise and interchangeable.

They possess a broad temperature envelope which enable it to be hermetically
packed to be used in damp conditions also.

Main Features:

* Durability of service, superior stability
* Compactness, robustness, sturdy surge current resistance
* Quick reaction time to surge current
* Extensive operating spectrum
* Significant element constant (B value), minimal stay resistance.

How does an NTC Functions

An NTC is attributed with a special property through which it is able to raise its resistance significantly during power switch ON.

When used in electronic circuits this property helps blocking the initial surge currents in to the connected circuit.

However in the process, the NTC becomes relatively warmer, which brings down its resistance to lower levels such that the normalized safe power subsequently is allowed to pass over to the adjacent circuits.

Practical application:

Thermistors are commonly used as

* Inrush current limiters
* As Temperature sensors
* In the form of self-resetting over current protectors
* In self regulating heating elements
* Power Converters, switch mode power supply SMPS, UPS power protection
* Energy efficient lights, electronic ballasts and chokes,
* Many vulnerable electronic circuits, power supply circuits etc.

The following image shows an example NTC component:

Identifying the NTC Thermistor from its Print Mark:

The first digit "5" indicates the resistance of the part at normal conditions. Here it indicates 5 Ohms.
The subsequent alphabet and the digit indicate the diameter of the particular part, here it's 11mm.

How to Connect an NTC Thermistor in Practical Electronic Circuits

Normally in an electronic circuit an NTC is connected at one of the mains inputs, in series.

Alternatively, it may be also connected after the bridge rectifier, as shown in the following examples of surge controlled compact transformerless 1 watt LED driver  circuits.


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

84 thoughts on “Using an NTC Thermistor as a Surge Suppressor”

  1. Hi Swagatham
    Please clarify:-
    1). I think “275VAC, X2” BOX TYPE ( specially designed for AC supply ) capacitors safely inplace of highvoltage rated 400V dropping capacitor. What is your openion.

    2). Is it three terms are same or different….Surge current , Spike and Zero-crossing, related with capacitive supply…?

    I assembled the first circuit before one year, with my own modifications. Circuit worked perfectly. I used one 20mA high brightnes blue LED. My dropping capacitor was only a 224P, 630V Polyester type. I added a 9V, 1W zener diode at output and 1K resistance series with LED. 56ohms, 1W surge/spike limiting resistor replaced with 100ohms, 1W.

    VERY USEFUL MODIFICTION……..the capacitor 1mfd, 250v electrolytic parallel with LED, I USED A 2200mfd, 16V ELECTROLYTIC CAPACITOR. This modification is highly useful at sudden powerfalure situations. Due to the holding charge inside 2200mfd (you can use high value, such as 3300mfd, 4700mfd etc.) LED will stay ON morethan 2 minutes after you switch OFf circuit /powercut, and gradually OFF the LED.

    I removed all parts of a burned CFL and inserted your circuit inside it’s cover. If it is difficult to fix a single 4700mfd inside CFL cover, use 1000mfd x 5 pcs parallel. This will save space.

    (There is a printing mistake in your circuit. 1mfd, 250V is shown as NONPOLORISED. It should be an ELECTROLITC type.)


    • Hi Anil, here are the answers:

      1) There’s no specially created AC or DC capacitor, as long as the voltage rating of the cap is well above the supply peak it is well suited for that application…and for AC it must be a non-polar.

      2) A surge may happen during a spike in voltage, because current will only surge when there is a sudden increase in voltage. zero crossing is the period when the AC wavrform is passing through the zero level of its waveform, switching a gadget during this point is considered to be the safest because the AC is in its most weakest level at this zero crossing point.

      for enabling a emergency light effect you can increase the series resistor value of the LED, which will further increase the backup illumination of the LeD during a power failure.

      the 1uF in the diagram is correctly shown, it can be a polar or non-polar doesn’t make any difference, I have shown it as a non-polar because a non-polar capacitor is more commonly available in the market and is more compact than an electrolytic.

  2. sir is there any circuit which can supply dc to a circuit for 1 min only when allready connected dc supply is intrupted for very short periods like 40 sec ,1min .
    i want system like ups but for very small circuit like 555 timer 12v circuit. i want this supply system max for 1 min not more than that.
    please suggest me something
    thank u

  3. thank you very much swagatam, its very useful and can you suggest me how to improve the power factor for the 5 watts single LED driver

  4. sir can it help to reduce inrush current of motor ?if yes
    than can u plz suggest me ntc or ntc rating for given details.. 3hp motor inrush current is 40amp runnnig current is 14amp voltage is 230 single phase.

    • Gurmel, I don't think there's any NTC that may be designed to handle 40 amps….instead you can try connecting a high wattage resistor in series, in conjunction with a delay relay,

      when switched ON the resistor will allow 50% current to reach the motor causing a slow start, meanwhile the relay will trigger after a 1/2 second and bypass the resistor to enable an optimal current for the motor

    • but sir how to know rating of resitor to decrease current upto 50,
      is thereany formula you can suggest with example?

    • yes, through Ohm's law

      R = V/I

      220/15 = 14 ohms

      wattage = R x I^2

      14 x 225 = 3150 watts

      the wattage looks unnaturally huge.

      since you would want to use it only for a split second, a relatively smaller wattage resistor could be tried.

  5. Hello I have an Inverter with 1kva ,whenever I connect it with a Laser printer (600watt) the inverter takes more current as inrush current ,at that time the voltage fluctuation take place .Can i use this circuit for it

    • Hi, The inverter consumes more current because the printer asks for it, if you don't allow the inverter to take current from the battery, the printer will not start normally.

      moreover an NTC is designed to handle current only for a few milliseconds. beyond that it itself might get burnt or get destroyed so it cannot be applied for your application.

  6. Initially at power switch ON the NTC resistance will be higher, since its temperature would be lower, however after power switch ON the NTC will gradually heat up due to cuurent flow through it enabling it to lower its resistance until eventually it reaches to its specified level.

    A relay bounce cannot have any significant effect on an LED if the LEDs are protected with a limiting resistor or with some other form of current limiting stage.

  7. An NTC stats of with the rated Resistance and after the initial surge the current and the heat ( as a function of I Squared R) will heat up the NTC (that's why it is called a negative temperature co-efficient )and damage form the surge is eliminated . I have used properly rated NTC to protect Ac driven LED bulbs especially those that more prone to damage like LDR (Light dependent Resistor) based relay circuit that turns ON the light during nightfall . Here the problem was a bit more acute because of the relay bounce . all relays bounce as they make contact . In this particular case I lost couple of 220 v ac based LED bulbs. Since then no more LEds Bulbs lost. Was able to fix those LED Bulbs too .. because it was noted that 1 among the 36 , 3 Volt LED was blown out within about a month less than 20 turn on's associated with relay bounce which i would say was the main contributor

  8. Great i take a lot of my time and moments to find how block surge current in sircuit but when i see this article i am got woundered really a ntc can be candidate as final option in capacitive power supply i understand it and make it good.

  9. This article is very useful to me as i have seen only on PC SMPS and i was wondering what this component was but now i know what it is, thank to you sir. Sir i have a SMPS @450watt which the fuse was blown, so i replaced the exact fuse rating(5amp-250V) but when i turned the SMPS on the NTC was blown, i can't find the exact NTC rating which is "1F-72 5D-9" so sir can i replace it with this one "NTC 10D-9" Thank you.

    • Thanks Lima, if the fuse and the existing NTC blew up then you must first find what caused these hazards, until you have determined the fault it wouldn't be a wise idea to replace new NTC or fuse…….check by connecting a 40 watt lamp in series, if the lamp glows brightly that would indicate a possible short circuit or some other similar fault inside the SMPS

  10. you are welcome -S…a NTC would surely help to keep the LeDs safe…additionally you can try adding an inductor wound over an iron nails for the same, in series with the LED….use around 200 turns of any thin super enameled copper wire on an iron nail…inductors can be excellent surge suppressors for DC circuits

  11. According to me it's only during the initial power switch ON when the capacitor is fully empty, and acts like a short circuit when introduced to mains inrush.

    During this time it gets charged to the mains level with the first cycle, and subsequently the opposite cycle only discharges the capacitor to the level as determined by the load specs, if the load is 12V rated then the capacitor is now discharged and charged at this limit, until it's switched OFF and switched ON again or if there's a major voltage fluctuation.

    Most probably You can find the current of the NTC in its datasheet

    LED got burnt with NTC included?? or without an NTC

  12. Thank you -S,

    Here are the answers to your queries:

    1) The surge current will be always equal to the maximum AC mains input current…so it could be 200amps or even 1000amps, but it's not the amp value which becomes crucial rather it's the period for which it's allowed to pass through the capacitor, this in turn depends on how quickly the capacitor gets charged. Lower value caps will charge quicker than higher value caps producing quicker charging and consequently lower average surge current….which could range normally in microseconds.

    2) I am not entirely sure about the NTC current but mostly it could be within 1 amp at 220V…so it's better to install at the 220V side of the circuit.

    For your application you can effectively use the first circuit from above…use two 105/400V in parallel for achieving 100mA

  13. Sir
    Can i use ntc directly in series with my half bhp water motor? Presently i have ntc 10d9 22d9 and 33d9 in my stock. Can i use them? If not please suggest any suitable.

  14. Sir
    Can i directly connect one ntc in series with my half bhp water motor? Presently i have ntc 10d9, 22d9 and 33 d9 in my stock. Can i use them? If not please suggest any suitable.

    • No, the NTC is for low current applications, it won't be able to handle 0.5 hp motor….you may have to employ a PWM soft start kind of circuit for implementing the high initial current surge protections.

  15. Sir
    Thank you for your advice on how i say it does not compensate. The only thing I can do. Can I pray for you. You always have wealth.

    • there's no way to check whether an MOV is actually working or not,,,,but in most cases it will perform as expected from it.

      pray for the world!…God bless you!

  16. Hello sir, I have a charging fan that uses NTC 10D-9 in its power supply. It was a new fan and it worked fine for one month and after that its fuse burnt out so I replaced it with new one but the new one burnt out as soon as I turned on the switch. I have tried to search for the short circuit but there is any short circuit. now I believe that it is because of NTC it is not limiting the current. can you please guide me that what ever i m believing is right ?

    • Hello Aneel, an NTC is connected in series to the input line, so it cannot be a cause of a short circuit, still you can remove it and check by connecting a 100 watt bulb in place of the fuse, if the bulb glows and the fan does not move, then surely your fan could be the culprit.

  17. Sir i want to run 300 leds of 5mm or 8mm to 230v AC main of 6 series in parallel means each series of 50 leds so wat will be the changes for the circuit

  18. Sir can I use 50 nos of 5mm leds in series for this circuit I have blinking leds (dual color led) like red blue and red green

  19. Tanks Sir'
    1.can I put only 20 or 25 LED(5mm) in the circuit?
    2. Sir can you more better (1or2watt)LED circuit ask me?

    • yes 25 to 39 5mm LEDs will also work, but you may need to change the input capacitor to 0.47uF/400V

      for higher watt LEDs you should opt for SMPS driver

    • Dear Hasan,

      I think your local retailer will know better regarding a standard equivalent, or alternatively you can try using a hand wound coil for obtaining the same results.

      wind 500 turns of any thin super enameled copper wire over an iron bolt and use in series with the LED.

  20. Hello swagtam , bro I have 1 9w led driver I want to add ntc in this for better protection. Can u plz tel me where I can ad ntc in main connection on phase or nutrel actually on line point it already have fuse resistance so cn I ad ntc on nutrel point. ?plz clear my doubt.

    • Hello Manoj, An NTC can be added on any of the input terminals, whether it's phase or neutral doesn't matter, you can even connect it in series with the fuse, nothing is critical as long as the device is in series with the supply line.

  21. thanks sir another question is that if i use 4 no of 5 watt led instead of 20 no of 1 watt leds for making an illumination of 20 watt cfl can this circuit work

    • May be you did not understand what I suggested in the previous comment. In capacitive power supplies using less number of LEDs will reduce efficiency and light….using more number of LEDs will improve.

      Therefore it would be better to use 20nos 1 watt LEDs instead of 4nos 5 watt LEDs which will not give any illumination

  22. thanks Sir,I do as your answer ita woring very well.
    Another Question is that if i connected single 3 watt Led instead of 4 1 watt leds can this circuit work
    Another ? is that can single 3 watt led is equal to 3 1 watt leds

    • Harjot, using less number of leds in series will make the circuit less efficient and using more numbers in series will improve its efficiency.

      a single 3 watt led will produce an illumination of just 0.5 watt LED with the mentioned circuit

      for perfect results you should go for an SMPS AC/DC adapter with a current control stage.

      yes a single 3 watt LEd = 3 x 1 watt LED

  23. which circuit isbest for home usage with 4 1 watt led

  24. Hi,
    I need this circuit use for 7 or 9 High Power LED in series, so what I need to changed. only the resistance power increased or change the value as well?????

    • I can't interpret its resistance value, you can try it, because slight variations in the resistance won't affect the circuit outcome shown above.

    • Avik, The one which is shown in the diagram is exactly what is required for this circuit….. in fact it's suitable for all 220V operated circuits.

  25. sir i want to ask you two questions the first one I have deleted all of the thermistor how to identify the value and power of it, the second I want to know how to lock the increased flow of tiristor thermistor protected thanks

  26. When it comes to capacitive power supplies there are many things that needs to be considered. Just taking care of the zero crossing won't help

    here we need to exploit the voltage rather than the current, because here we have plenty of voltage and less current.
    Avoiding current means avoiding surges, so we have to thing of using minimum current which can be achieved by connecting LED in series.
    For 20 watts instead of using a single LED, we can go for 20nos 1 watt LEDs in series, that would restrict the max current need to 300ma, quite achievable with smaller caps.
    Next we also need to employ some sort voltage regulation by including 7812 or similar device in the circuit.
    But ultimately as you have mentioned an smps is always good for anything that exceeds 100mA usage….and I have always recomended the use of an smps for making high watt LED drivers

  27. yes it can be a good solution to the ever present surge problems in capacitive power supplies, I'll try to come up with a suitable circuit soon, and update in this blog.


  28. Thanks for ur reply sir,
    Can we add more 1u400v cap in parallel. If yes then any change with 56 ohm resistor and 1u250v in output

  29. Hello sir,
    How r u.Its long time to have some discussion with u
    I want to use ten 1watt led.can the above circuit is capable of this.
    I had 10d-9 ntc can i use it for above circuit.i had seen some smps for powering dth and similar using
    this value 10d-9 ntc.


    • I am fine Avijeet, thanks!

      The above circuit will illuminate 10 one watt LEDs in series but the glow will not be optimal due to lower current from the cap, yet still it would be considerably dazzling.

      yes the referred NTC will do the job

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