Testing SMPS Transformer Without Load
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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vijay ubale
February 9, 2011 at 11:17 am
smps transformer sp-4203
+5V 40A
+12V 35A
admin
February 9, 2011 at 11:18 pm
HI Vijay,
May i know what is your question?
Jestine
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.
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
D.N.Senarath
October 22, 2017 at 9:39 am
Sr,i want to know about check method a switching transformer in CRT Tv
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
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?
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
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?
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
Samuel
May 29, 2012 at 11:48 pm
Hi sir, how can i test SMPS Transformer without voltage checking.
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
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
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
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
Jestine Yong
July 25, 2012 at 6:45 am
Hi Silvertana,
Did you replace with the same componenst or substitution components?
Jestine
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
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
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
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
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
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
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.
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
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,
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
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).
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
Chris Matheis
September 10, 2015 at 6:50 pm
Yes
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
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
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
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
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
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.
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
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.
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
Arnel balbido
December 21, 2018 at 8:14 am
Sir what happen to the circuit specially in CRT TV if the primary of smps transformer is open winding ?
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
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 ?
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
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.
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
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
Jestine Yong
May 20, 2017 at 11:52 am
Hi Emmanuel,
What have you checked?
Jestine
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
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
Deepak
December 25, 2017 at 1:54 am
I have a philips SK 4.0 chasis. The problem is the B+ volt reads 46-59 volt. other volts in smps reads zero '0'. I tried removing all loads from the smps and still the output is same. I tried removing the smps and checked the STR output it shows pulsating 300 - 420 volts.
I removed the flyback jumper and still the same. removed the hot transistor and still the same. I am unable to understand where the fault lies. I checked there is no short in the board. checked most of the resistance, diode and capacitors.
Please give me some tips to move forward with my troubleshooting.
Raytech
March 1, 2018 at 10:13 pm
Hello Sir..... Please would like to know how to read part number for Smps transformer in order to no the secondary voltage for proper replacement..... Thank you sir
Jestine Yong
March 2, 2018 at 10:29 am
Hi Raytech,
SMPS transformer part number did not represent the output voltages. You need a service manual in order to find the right output voltages. Usually it is very hard to find the replacement for smps transformer .
Jestine
somar
April 13, 2018 at 10:27 am
hi i have a ps3 and keeps shutting down when press power button, anyone can help
Hy
August 22, 2018 at 1:15 am
Hi sir!
I was checking smd transformer by multimeter, at the secondary side there are 5 pins and all are short to each other, what does it mean?
Jestine Yong
August 22, 2018 at 3:31 pm
Hi Hy,
You got the reading on all pins due to that one is a reference pin to ground. This transformer probably have 2 to 4 output voltages.
Jestine
Christina
October 6, 2018 at 12:04 am
hi Sir ,
I have an smps to repair..When I tested with multimeter I can get the dc around 220 on the input of the smps transformer..But I didn't get any reduced output.I am also confused about the auxillary winding..Can You please explain?Also tell how to test the transformer by multimeter.
Jestine Yong
October 9, 2018 at 8:39 am
Hi Christina,
The input should be AC and not DC. Many factors can cause no output voltage such bad fuse, bad power IC, shorted secondary output diode, open start up resistor and etc. Auxiliary winding is the secondary winding in primary side. You can't test the primary winding using multimeter. You need a blue ring tester. I suggest that you get my ebook http://powersupplyrepairguide.com/ to understand about the safety, function of each SMPS circuit and how to troubleshoot and repair.
Jestine
Christina
October 16, 2018 at 12:38 am
Thank You sir..One more question..The electrolytic capacitor gives dc output.Then it is given Mosfet..How the transformer gets ac input?Does the mosfet switches it to ac?..Can you explain in detail about that and the how the process goes further in remaining components(schottky diode,optocoupler) after switching?
Jestine Yong
October 16, 2018 at 9:20 am
Hi Christina,
There are many websites explaining how SMPS works. Below is just one of the websites:
https://www.efxkits.us/switch-mode-power-supply-smps-works/
Jestine
sadegh
November 12, 2018 at 1:54 am
hi sir
can i ask you something?
i read your SMPS book and it is awesome
but i do not understand resistance testing method
i read your translated book
it may translator does not good.
any way can you explain more to me??
Jestine Yong
November 13, 2018 at 4:55 pm
Hi Sadegh,
The resistance method is to check if the DC output rail have any shorted components or not. Sometimes a shorted load can cause the DC outout rail to have low resistance.
Jestine
Efijemueh Philip
August 14, 2021 at 3:10 am
Hi sir,l am new to electronic and am working on a crt TV that has standby red light on but it won't turn on whether with remote or pressings the button at the front. The smps transformer output at the diode are B+98v,35v,5.5v and 8v instead of b+126v,35v,6.5v and 14v.please does it mean that the transformer is bad.thanks
Peter gay
January 30, 2022 at 8:29 am
Dear Sir I built a 700 watt Smps designed by Long Technical it supposed to get 7amp with 50-0-50 volts I get the voltage but I cannot get the Amps Max3.3 amps The original trans was ER42 I changed it to ETD49 (Cause I could not fit all the wires in the ER42) If try to get more than3.3amps the fets blow up, Could you help. Thx.
John Patrick
September 24, 2024 at 10:04 am
Hi, Can you help me how to measure the output power of smps transformer including the test setup, test equipment and schematic diagram?
Also, can you give me an Idea how can I overload SMPS Transformer.