About Posistor (Positive Temperature Coeeficient-PTC)

By on October 29, 2008

thermister

If you have been in the CRT Monitor and TV repair, i’m sure that you must have come across the square black plastic with two or three legs at the input AC area just after the EMI/RFI Filter circuit. That component is posistor (PTC) and the function is to control the degauss. When you switch On the CRT Monitor or Television, the PTC resistor is cold and low resistance. When heated, it becomes very high resistance and turns off the degauss coil but gradually – the current ramps down to zero rather than being abruptly cut off.

Usually when this posistor have problem, the degauss function would not work and many times it would blow up the main fuse (but the bridge rectifier was okay).  It is quite hard to test the posistor and sometimes when it has problem you could hear some loosen parts in it when you shake the posistor. Since this part is easily available from electronic stores and from junk Monitor or TV, i would usually make a direct replacement and retest the set again.

Tips: If you can’t  demagnetize the CRT tube that have color patches even with the help of a degausser (demagnetizer) then chances is high the tube may have problem. If the color turned back to normal after using the degausser, then look for dry joints in the degaussing circuit and replace the posistor and test it out again.

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43 Comments

  1. Bambang Suhardjo

    November 29, 2010 at 3:09 pm

    About PTC what's ?

    Likes(1)Dislikes(1)
  2. admin

    November 30, 2010 at 7:10 am

    Hi Bambang,

    What is your question?

    Jestine

    Likes(3)Dislikes(1)
    • Dranreb Adviento

      October 14, 2016 at 6:54 am

      sir is there a difference between 2 and 3 pin posistor? the tv that i am trying to fix originally has a 3 pin posistor then i tried to replace it with 2 pin only and still doesnt have any result the problem was the demagnetizer before the original posistor with 3 pins were become so hot when i turn on the tv but then i use the 2 pins its cold

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      • Jestine Yong

        October 20, 2016 at 9:12 pm

        Hi Dranreb,

        Yes there is a different between 2 and 3 legs posistor. A 3 legged PTC will permits the residual current to reduce further. The best you put back 3 legs posistor.

        Jestine

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  3. Emerson P. Mante

    November 6, 2012 at 10:36 pm

    Hi Jestine,
    if the posistor is defective is it possible that the TV set will not work or no power at all?
    Thank you,
    Emerson
     

    Likes(1)Dislikes(1)
    • Jestine Yong

      November 7, 2012 at 9:22 pm

      Hi Emerson,

      If the posistor is open circuit the tv can't have the degauss mode. If the posistor is shorted it will blow the main fuse bu the bridge rectifier is good.

      Jestine

      Likes(4)Dislikes(3)
      • filjohn collamat

        July 19, 2014 at 9:56 am

        ahh ok sir tnx for the tips,,i encounter already that kind of problem in KOLIN KCP-2190 model,, question sir but if ever sir i dont have a new or good posistor then im going to hang it up or no posistor can be replcd it is ok sir?

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        • Jestine Yong

          July 19, 2014 at 10:01 am

          Without posistor the color impurity can't be removed unless if you have a demagnetizer. You can easily find one from junk board.

          Jestine

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  4. Smulik mops

    March 16, 2014 at 7:46 am

    Hie pass me here,am a qualified electrical technician with close to 20years in machineries and controls,about 7years in tvs,and smps,but not fully in tvs,so a have replaced my telefunken tv flyback tx,after a year it doesn't switch on until i adjust focus & g2 (2 nobs)on flyback and is almost 4months with ths prob,i wanted to substitute flyback what could be the problem here,i have tryed with new hot and surge protector but stil no ans?thaks guys

    Likes(1)Dislikes(0)
    • Jestine Yong

      March 17, 2014 at 9:38 am

      HI Smuliks,

      Just to verify your question again- Do you mean you want to look for a substitution of the flyback?

      Jestine

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  5. Kath

    January 11, 2015 at 2:45 pm

    Hi there Im an ECE student and I just want to know if Posistor can also be a heat sink ??

    Likes(0)Dislikes(0)
    • admin

      January 13, 2015 at 6:50 am

      HI Kath,

      Posistor is not a heatsink.

      Jestine

      Likes(0)Dislikes(0)
  6. harley

    April 14, 2015 at 5:38 am

    hello thank you for your article, could you answer, for TV's degauss circuit, why do some PTC's have 2 legs and some have 3 legs?

    Likes(1)Dislikes(0)
    • Jestine Yong

      April 14, 2015 at 9:52 pm

      Hi Harley,

      The function of PTC thermistor is to degauss the shadow mask of colour picture tube by reducing the alternating current flowing through the degaussing coil within a short period of time. A big difference between inrush current and residual current is crucial for good degaussing. A 3 legged PTC will permits the residual current to reduce further.

      Jestine

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      • harley

        April 15, 2015 at 2:16 am

        that's very interesting thank you for your excellent response.

        Likes(0)Dislikes(0)
  7. sandeep

    June 14, 2015 at 5:03 pm

    Sir I have problem I have a crt tv when I switch on the tv play green I can see the image but a lot green colour what the problem sir?

    Likes(0)Dislikes(0)
    • Jestine Yong

      June 15, 2015 at 1:07 pm

      HI Sandeep,

      I suggest that you check on the green color circuitry in the crt board. You can also compare the blue and the red signal.

      Jestine

      Likes(0)Dislikes(0)
  8. sandeep

    July 17, 2015 at 1:58 pm

    what can cause this green colour problem

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 20, 2015 at 10:58 am

      Hi Sandeep,

      What green colour you are referring to?

      Jestine

      Likes(0)Dislikes(0)
  9. sandeep

    July 23, 2015 at 2:14 am

    I fix it sir one of the three resistance in the pcb board have drop i change it work with no problem thx for the help

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  10. sandeep

    July 23, 2015 at 2:22 am

    hi sir i have a sony trinitron when plug it power on it make hissing noise then no picture stanby light blink 1 time what's the problem u can help me sir

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 24, 2015 at 8:43 pm

      HI Sandeep,

      Have you checked if the output voltages from the power supply giving the right and stable voltages or not?

      Jestine

      Likes(0)Dislikes(0)
  11. christopher

    January 4, 2016 at 10:31 pm

    Fantastic comments and brilliant answers. Thanks Mr J. Yong.

    Likes(1)Dislikes(0)
  12. Anil

    May 3, 2016 at 1:12 am

    Hello sir ,
    I was facing the same problem so I change the ptc.All is ok but while increasing contrast screen becomes reddish & decreasing it going greenish but picture is ok.
    what would be the problem ...

    Likes(0)Dislikes(0)
    • Jestine Yong

      May 4, 2016 at 8:09 am

      Hi Anil,

      It could be due to weak picture tube or the color section have problem. If the OSD menu is good then it is not related to picture tube.

      Jestine

      Likes(0)Dislikes(0)
  13. ARIJIT DAS

    September 6, 2016 at 2:37 pm

    HI JESTINE SIR, I AM ELECTRONICS STUDENT , I HAVE COLOUR GREEN AND PURPLE PATCHES TOP OF MY LG CRT TV, IS THIS PROSISTOR PROBLEM??

    Likes(0)Dislikes(0)
    • Jestine Yong

      September 6, 2016 at 11:02 pm

      Hi Arijit,

      If the color patches can go away with the help of a demagnetizer then the problem is in the posistor.

      Jestine

      Likes(0)Dislikes(0)
  14. Hari P

    October 2, 2016 at 4:33 pm

    1)does posistor heat up even when the circuit is open ?
    2)my sony crt shows yellowish color and red color is blackish but when i increase color intensity red can be seen blue and green are okay but red is not ok.

    Likes(0)Dislikes(0)
    • Jestine Yong

      October 5, 2016 at 9:54 am

      Hi Hari,

      If the circuit is open the posistor would not heat up. The color problem try check for any bad E-caps in the CRT board. If possible use ESR meter. I have not tried using the led to check on the current flow to the posistor. If you have the time you can do the test and remember to connect a resistor in series with the LED.

      Jestine

      Likes(1)Dislikes(0)
  15. Hari P

    October 3, 2016 at 8:13 pm

    3) can we check tge wirki6of posistor by connecting a small led bulb to degauss coil output connection, so we can figure out weather current is flowing from posistor to the output?

    Thanks

    Likes(0)Dislikes(0)
  16. Hari P

    October 6, 2016 at 10:26 pm

    one more question sir,
    i have noticed that now there is no static on crt glass when we bring hand near crt front glass no hair is attracted towards the glass of crt, but whereas i have always noticed static in every crt tv, so why this sony wega is not attracting hair ?

    Likes(0)Dislikes(0)
    • Jestine Yong

      October 10, 2016 at 3:19 pm

      Hi Hari,

      Is the Sony Wega TV working?

      Jestine

      Likes(0)Dislikes(0)
      • Hari P

        October 11, 2016 at 8:27 am

        yes tv has no problem in working but the led blinks 5 times continuously since tv is turned on.

        the vertical ic was replaced a year back and now again vertical ic has problem, there are lot of horizontal lines when we turned on and disappear after few minutes and picture becomes clear.

        Likes(0)Dislikes(0)
      • Hari P

        October 11, 2016 at 3:22 pm

        https://drive.google.com/file/d/0B4HwoUr6x54gdnZXTFY0RzlyaWc/view?usp=docslist_api

        please see the image in the google drive link above

        1)the picture on screen is yellowish green
        2)the bottom line displaying message in hindi language is red but is black
        3) there are lines on picture due to vertical ic problem
        4) 5 blinks led but tv works no shutdown just keeps on 5 blinks of small interval
        5) no static to attract hair.

        thanks

        Likes(0)Dislikes(0)
        • Jestine Yong

          October 14, 2016 at 2:47 pm

          Hi Hari,

          Saw the photo-it seems this TV have many problems. You have to find out from the TV manual what does 5 blinks means. If it is referring to the vertical section then concentrate on this vertical circuit first. Just make sure the dc supply line to the vertical ic is good and no dry joints or even decayed glue in the circuit.

          Jestine

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          • Hari P

            October 14, 2016 at 9:28 pm

            hi,

            found this from techlore

            Five blinks - AKB circuit (automatic kine bias), the timer/standby indicator blinks for about 30 seconds then goes to the self-diagnosis function. Something is wrong with the video. Check video out, Q705, 732, 761 and other components on the C board , also check Q218, 219, 220 on the A board. In addition, unit could be in IK blanking, try turning up screen slightly.

            but my screen is not blanking

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            • Jestine Yong

              October 20, 2016 at 9:07 pm

              Hi Hari,

              Sorry I do not have a direct solution for this.

              Jestine

              Likes(1)Dislikes(0)
  17. judith

    January 22, 2017 at 10:59 pm

    what the effect of resistance value of positor on picture color of crt lg tv

    Likes(0)Dislikes(0)
    • Jestine Yong

      January 24, 2017 at 4:14 pm

      Hi judith,

      Do you mean you want to know the resistance of a posistor?

      Jestine

      Likes(0)Dislikes(0)
  18. Peter

    March 11, 2017 at 10:50 am

    How do u test a chopper transformer with a multimeter?

    Likes(0)Dislikes(0)
    • Jestine Yong

      March 13, 2017 at 11:27 am

      Hi Peter,

      You need a ring tester such as the one in the below link to test the primary winding:
      http://www.electronicrepairguide.com/blueringtesterresult.html

      Normal multimeter with resistance checking can't test it if there is a shorted winding.

      Jestine

      Likes(0)Dislikes(0)
  19. ajay

    May 18, 2017 at 12:07 pm

    Hi sir jestine,

    Posistor has no specific value right? We can substitute it from other junkboard as long as it is ok, it will work fine? and as long as it has three legs in it. Right?

    Likes(0)Dislikes(0)
    • Jestine Yong

      May 18, 2017 at 8:43 pm

      Hi Ajay,

      It has part number and you can use junk posistor to test it out.

      Jestine

      Likes(1)Dislikes(0)

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