Testing SMPS Transformer Without Load

By on July 25, 2009

smps transformer

I believe many of you (electronic repairer) have tested the output voltage of a linear transformer before. Just set your meter to measure AC voltage and place the probes across the output of the linear transformer. If the linear transformer printed as 9 VAC output then you can expect about 9.5 to 10 Vac because transformer have tolerance too. Now this is different if you want to check the output of SMPS transformer. You can’t just solder out the output pin and power on the equipment and check the output voltage. Since there is no more load, the primary side will sense an overvoltage which could shutdown the power supply or totally blow up the power components. I have came across this situation (blowing up the power side components-power IC, power FET, resistors and etc)  before thus in my mind i always says no-no to removing only the smps transformer output pin and power On.

If you would like to test the output of the SMPS then place your probe after the secondary diode output pin and check the DC voltage. It is a common sense that if you get the right output voltage then that’s mean the SMPS transformer is producing good AC voltage thus it is not necessary to check all the output pin of the SMPS transformer.

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47 Comments

  1. vijay ubale

    February 9, 2011 at 11:17 am

    smps transformer sp-4203

    +5V 40A
    +12V 35A

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  2. admin

    February 9, 2011 at 11:18 pm

    HI Vijay,

    May i know what is your question?

    Jestine

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    • gangadharaiah.

      January 5, 2016 at 8:36 pm

      Dear sir,

      Please advice me how to test smps transformer using multimeter with load and without load.

      Likes(0)Dislikes(0)
      • Jestine Yong

        January 6, 2016 at 8:24 pm

        Hi Gangadharaiah,
        SMPS transformer the best is to test it off board using a blue ring tester. A multimeter not be able to check the shorted primary winding.

        Jestine

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        • D.N.Senarath

          October 22, 2017 at 9:39 am

          Sr,i want to know about check method a switching transformer in CRT Tv

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          • Jestine Yong

            October 23, 2017 at 12:33 pm

            Hi D.N.Senarath,

            You need a blue ring tester to check on the transformer primary winding. Read the article below:

            http://www.electronicrepairguide.com/blueringtesterresult.html

            Jestine

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  3. Sreeraj S

    September 25, 2011 at 12:17 pm

    Hi Jestine,

    I Sreeraj, am an electrical engineer from India and a Software engineer by profession.
    I was watching a movie on my LCD TV for which the sound connected to an iBall 5.1 home theatre. All of a sudden the HT stopped working no sound, no indicators nothing.

    I then decided to put to use my electrical/electronics engineering skills [which ofcourse is minimal :)] and took apart of the HT. I knew that something had gone wrong with the power supply board.

    I connected the board alone to the mains (230V AC @50 Hz) and there was no reading on the multimeter for the output side. I had no load on the output side.

    On the board it was written www.kelpower.com and KB-120A.

    However, while trying to measure voltage across a zener that was in parallel with a resistor of 2.7ohm to the emitter of 13500 [I used 200mV DC range on my digital multimeter], the circuit came to life and I could measure the o/p voltage as 21.6V. When switched off and later switched on, I couldn’t get any reading on the o/p side. If I repeat the procedure of measuring the voltage across diode I was able to activate the circuit 7 out of 10 times. What could be the issue here, any idea?

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  4. admin

    September 26, 2011 at 3:34 am

    H Sreeraj,

    Your meter could have trigger the standby circuit. Did the CPU of the tv got 5 volt?

    Jestine

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  5. Mayur

    January 4, 2012 at 5:47 pm

    Please tell me how to validate the reliability of the SMPS? Suppose I'm having ready made circuit of SMPS and if I want to incorporate it into my audio system then on what parameters should I test the reliability of that circuit?

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  6. admin

    January 5, 2012 at 8:13 pm

    HI Mayur,

    Sorry no info on this because I'm not into the power supply design. However, i believe the quality of the components and the circuit board use have a big impact on it. It depends on the load too.

    Jestine

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  7. Samuel

    May 29, 2012 at 11:48 pm

    Hi sir, how can i test SMPS Transformer without voltage checking.

    Likes(1)Dislikes(0)
    • Jestine Yong

      May 30, 2012 at 8:31 am

      Hi Samuel,

      Use a Blue Ring Tester to check on the primary winding. Secondary winding rarely have problem.

      Jestine

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  8. RAMESH

    July 4, 2012 at 12:22 pm

    Hi Sir,
    I need to check 400 - 500 - 600 and 800 W platinum series smps how to conform this smps having very good quality. I need explanation with out and with out load

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    • Jestine Yong

      July 4, 2012 at 9:46 pm

      Hi Ramesh,

      Do you mean the power supply is working and you just want to know if this is a good quality power supply or not?

      Jestine

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  9. silverthana

    July 24, 2012 at 2:30 am

    Hi Sir,I have repair 2 sets of Sony VCR with SMPS problem.the fuse was burned.I have replace the smd diode S1WB S60.the voltage output about 260vdc.but when connect to SMPS board the fuse burned again.have check others SMPs component such as small diode,resistor,cap but all in good condition also have replace new FET.If the SMPS transfomer can be shorted that causes the fuse burn.
    Thanks

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    • Jestine Yong

      July 25, 2012 at 6:45 am

      Hi Silvertana,

      Did you replace with the same componenst or substitution components?

      Jestine

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  10. silverthana

    July 29, 2012 at 8:21 am

    Hi Sir,me silverthana again.I have replace with the same part no esp FET..but result still NG.I have joint in one of your forum regarding SMPS transforner shorted.I think my self should have one set of Blue ring tester to check to the transformer.Thanks

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    • Jestine Yong

      July 30, 2012 at 9:59 pm

      Hi Silvertana,

      Yes a burnt fuse can be caused by a shorted transformer and the transformer need to be tested by a ring tester.

      Jestine

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  11. fahad

    August 12, 2012 at 1:02 pm

    i have replaced some parts in smps after replacing parts in tranformer short is coming when it is totaly off so what  the problem sir
     

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    • Jestine Yong

      August 14, 2012 at 1:18 am

      Hi Fahad,

      may i know what do you mean by short is coming? What equipment you are reparing and what is the complaint of the power supply?

      Jestine

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  12. Siva

    September 26, 2012 at 11:32 pm

    Hi Jestine
    Can you advise my project i assambeled (SMPS 12v,3A) alway fuse blown when im ON the SMPS with load, Fuse located in front bridge rectifier (on supply source fuse 3A, 250v), if turn on without load the choke is blown any advise?
    regards,
    siva

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    • Jestine Yong

      September 27, 2012 at 7:52 pm

      Hi Siva,

      Do you mean you design the power supply? What type of use did you use a slow blow or fast blow fuse? Did you get good output voltages without load? What is the main filter capacitor value?

      Jestine

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  13. Siva

    September 27, 2012 at 11:03 pm

    Hi Jestine 
     thanks for reply, the main filter capacitior is 220uF/250v, if the main fuse is 3A/250v mean slow blow, without load i can't test the frequncy mode choke was burn during the test in first time, some of them advise to start with load, load i connected to 35w. 

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    • Jestine Yong

      September 28, 2012 at 8:41 pm

      Hi Siva,

      3a /250v does not mean it is slow blow because a fast blow fuse also have 3A/250volt rating. You need a slow blow fuse. As for the filter if you are using only one filter cap and your country is running on 230vac then 220uf 250volt is too low. You should use a 220uf with 400volt rating.

      Jestine

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  14. Tshering Tobgay

    July 27, 2015 at 3:45 pm

    hello Jestine,
    i have smps power supply of led tv which was no power at all, TV is sumsung model no UA32EH4500M,
    I started to opened main fuse was burn i replaced it and found no power on Tv i checked voltages in SMPS,
    Main capacitor have 300 v dc= ok,
    No short in hot area,
    so mr Jestine help me with your kind advice how to go from here,
    Thank you,

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    • Jestine Yong

      July 28, 2015 at 7:14 am

      Hi Tshering,

      I suggest that you check all the components in the primary side. If all found to be good then direct replace the power ic and retest.

      Jestine

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  15. Chris Matheis

    September 10, 2015 at 3:59 am

    Hi Jestine, I have watched many of you videos online and you have taught me a lot, I have a project I am working on and I am at an impasse. I am testing a SMPS power supply for a LED TV model# LG 47LN5700-UH, SO just to give you a run down of where I am at the TV turns on, LG splash screen appears, TV goes black, shine flashlight at screen, press menu button, menu appears, unplug TV, take off back, isolate power supply.

    Note: LED driver and power supply are on same board and I am supposed to get:

    Input: 120VAC
    Output: 3.5VDC, 12VDC, 24VDC

    LED Driver Output: 100VDC, 111VDC Note: has a 4 pin connector for two separate outputs

    P201:
    GND GND
    12V N.C. (not sure what this is)
    12V 12V
    GND GND
    24V 24V
    GND GND
    3.5V PDMI2
    3.5V PDMI1
    PWR-ON DRV-ON

    Check standby voltages of 3.5VDC - OK, Closed 3.5VDC to PWR-ON circuit, power supply on kicks on, check 12V and 24V rails - OK, read 76VDC at LED driver output, closed another 3.5VDC to DRV-ON circuit, read over 170VDC on each LED driver output, then drains back to 76VDC.

    It seems as the circuit is overloading and shutting down to protect itself. I am a novice electronics repairer, I can get a new board for cheap but I would really like to fix this as I like to do this sort of thing, I just don't know where to go next and need a point in the right direction. I have a multimeter and soldering tools and am comfortable with most component level repair.

    Tested fuses and were good, capacitors look good, checked bridge rectifiers, found a couple diodes with lower resistance then they should have but nothing out of the ordinary (I guess).

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    • Jestine Yong

      September 10, 2015 at 4:24 pm

      HI Chris,

      Do you mean the complaint was turn Tv On, can see the LG splash screen for a while then shutdown. When use torch light can see dim picture. If press on the menu button the OSD menu appear but the picture remain dim?

      Jestine

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      • Chris Matheis

        September 10, 2015 at 6:50 pm

        Yes

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        • Jestine Yong

          September 12, 2015 at 10:12 am

          Hi Chris,
          If the Brightness and Contrast setting is normal then you may have to login to the tv service mode and change it to the factory mode default setting.

          Jestine

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  16. Tomas

    March 5, 2016 at 7:58 pm

    Sir Jestine Yong i have a 42 inches panasonic plasma TV blinking 10 times and the relay clicks 3 times i checked the SMPS only standbye 5 volts is present can you help me with it

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    • Jestine Yong

      March 7, 2016 at 11:55 am

      HI Tomas,

      You may need to refer to the service manaul what does it means of blinking 10 times and the relay clicks 3 times. If stand by problem it means either:

      1) MCU IC got 5 volt but cannot send out the "On" signal due to the ic itself have problem or the EEPROM IC have corrupted

      2) MCU IC could send out "On" signal but it can’t reach the main power supply section due to bad components in the optoisolator ic and surrounding components problem

      3) On signal can reach the main power supply but main power supply it self have bad components like shorted secondary diodes, bad power ic and etc.

      4) There are shorted components in the secondary side like mainboard/x/y sustain board and etc

      5) The DC supply input to the MCU/EEPROM IC may be improper. Make sure the DC supply is good.

      Jestine

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  17. Ragghavenddrah

    July 13, 2016 at 4:51 pm

    Hi sir,iam ragghavenddrah, biomedical engineer,my work is maintaining and troubleshooting of medical equipment like,monitors,ventilators, x_rays.I procured ur testing of electronic s and smps troubleshooting e books last year.it's really good.Sir Iam interested in troubleshooting of smps my machines are costly equipments and in manual he don't publish of smps schematic diagram.without schematic diagram how to check it.and it's a double side pcb

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    • Jestine Yong

      July 14, 2016 at 10:56 am

      Hi Ragghavenddrah,

      Medical equipment SMPS schematic is very difficult to find as compare to those consumer electronics equipment. This is why understanding how a SMPS work and the critical test points is very important. Once you have known this, I believe there should be no problem in tackling the medical SMPS power supply.

      Jestine

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  18. Luis Rodrigues

    August 3, 2016 at 4:09 am

    Hello Mr.Yong

    I have a PC PSU with bright light 100w lamp test. When i remove the transformer or disconect its secondary the lamp test pass. The blue ring tester guives two red leds on. I tested another trasnformer from other PSU (same model) and guive the same result. I tested almostall components on secondary and i cant find the issue. Can you guive me your opinion about the blue ring tester? Thanks.

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    • Jestine Yong

      August 3, 2016 at 10:13 am

      Hi Luis,

      First you should test the transformer off board and second you need to test on the primary winding. Do you know which transformer pin is primary winding? If you test on the wrong pin or the transformer is having problem, it will show two red lights.

      Jestine

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  19. Mohit

    December 4, 2016 at 2:41 pm

    Dear Sir
    fuse blown when i switch on the Smps of dvb set top box again, I checked diode, capacitor, transistor on primary side all are ok but no result. when fuse blown i test the power transistor and found it is defective so that i replaced it. But when i power it fuse blown again and powe transistor also got damage again. please help me in this regard.

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    • Jestine Yong

      December 5, 2016 at 9:00 am

      Hi Mohit,

      Probably the transformer primary winding could be shorted or the power IC shorted because you found a shorted power transistor. I suggest that you put in a light bulb to test the power supply to prevent it from blowing it again.

      Jestine

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  20. Ulises Aguilar Pazzani

    January 29, 2017 at 7:19 am

    good after noon Sir ,Im trying to diagnose a switch mode power transformer , I just remove out of a monitor mother board

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  21. Manoj

    March 9, 2017 at 2:13 am

    I am repairing SMPS board of Philips LCD TV 40PFL4707. After several days, the primary and secondary of the SMPS are properly working. It works in a low power mode and generates only 3.3v and when its connected inside the TV to other boards and when powered on using remote, the P-ON-H1 pin gets shorted to the 3.3v generated and the SMPS starts in full power mode ( 3.3v is the standby voltage).

    My questions is I have repaired the SMPS and I do get 3.3v. I have not yet connected it to the TV boards. So if I put a jumper and short P-ON-H1 and 3.3v the SMPS should start in full power mode so i can check the voltages. However, since there is no load connected, is there any risk of blowing components on SMPS ?

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    • Jestine Yong

      March 9, 2017 at 5:16 pm

      Hi Manoj,

      The power supply board output itself should have resistors connected between the output line to ground and this is also a load. I have tested on other board with no blowing of components. In your case, it should work.

      Jestine

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  22. sijo sunny

    April 30, 2017 at 3:25 pm

    I have one pc smps . when i continue press pushbutton it is on but i remove button it could hold the power .May i know were is the problem.

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    • Jestine Yong

      May 2, 2017 at 10:57 am

      Hi Sijo,

      The computer is suppose to work that way which is to push the button one time and it will turn on.

      Jestine

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  23. EMMNANUEL

    May 19, 2017 at 12:33 am

    i jestine, long time.i have a problem of not getting standby voltage and the 12volt out from an LG DVD HOME THEATER PLAYER MODEL NO HT357SD.PLS, TELL ME WHAT TO DO.THANKS

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    • Jestine Yong

      May 20, 2017 at 11:52 am

      Hi Emmanuel,

      What have you checked?

      Jestine

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  24. ade

    November 11, 2017 at 5:11 am

    pls jestine, can I take an smps transformer from circuit 'A' for example and connect it a functioning smps circuit 'B' in order to test the smps transformer from A whether it is shorted or not.
    Note: both circuit are not from the same manufacturer. this is bcs we Dont have access to the blue ring tester here in Nigeria

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    • Jestine Yong

      November 13, 2017 at 9:10 pm

      Hi Ade,
      No you can't. First the transformer primary winding have inductance value. Using another transformer primary winding with different inductance value may cause the power FET to get shorted. Next, the secondary pins have different inductance value as well and if there is no load is connected the power primary side may not function due to no feedback voltage to the primary side.

      Jestine

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