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DY294 Supertip To Safely Test Small SMPS Adapters

By on January 16, 2021
dy294 repair tips

I always wondered if there was a safer and easy way to test small smps power adapters without having to connect them to the dangerous deadly AC power lines anymore. And with all the analyzing tools in the electronic engineering world if such a tool maybe already existed?

The best tester we know about which already safely can test 3 pin voltage regulators, diodes, capacitors and transistors without even destroying them is the DY294 universal HV tester. The great thing about the DY294 is that it can test components up to their breakdown voltage level before normally an avalanche of electrons would destroy our component making them useless parts.

This breakdown maximum voltage limit is always mentioned in datasheets of transistors, and we can safely examine these with any DY294 tester.

But now anyone with a DY294 tester also can test these small smps adapters very fast and safe!

This article explains how anyone can use the High Voltage output of his DY294 to safely test these small power adapters like for instance any 5V USB charger.

Simply connect the diode bridge AC input pins of your hopefully fixed power adapter under test to the Capacitance Test socket (yellow arrow) of your DY294.

And then switch the function selector knob to the maximum test voltage position of about 1000V (which easily can reach up to about 1600VDC!), or use the max 200V scale and press the TEST button. Or use the C and E transistor HV output pins of your DY294 tester. But always observe the right polarity of the circuit under test preventing blowing up e-caps that immediately will explode when the wrong polarity is connected!

If you keep the TEST button pressed in and at the same time test the output of your (in this case a 5V USB charger) adapter with a Led Light connected to the USB port you will see the Led Light flash at any time the test voltage reached about 165V DC. After the voltage drops fast to a much lower voltage and the Led Light dims (it will very fast discharge the primary HV capacitor of your adapter) , and the process repeats itself after the voltage again reaches about 165V DC, after the 5V Led Light (uses normally about 160 mA in my case) flashed briefly again and so on. As long we press the TEST knob on our DY294 it will charge the primary HV capacitor until the charger activates like it would if it was directly connected to the 230VAC power line.

But if nothing happens you have to switch both Capacitor Test wires, or the E and C wires, to the AC pins of the diode bridge of your adapter, or turn the voltage selector to the opposite 200V or 1000V selection.

dy294 tester repair tips

But keep in mind that only if you have connected your DY294 output directly to both ~AC diode bridge inputs and chose the wrong polarity at best nothing happens and nothing gets destroyed. If you however connected the HV output of our DY294 to the + and – pins of the diode bridge and chose the wrong polarity it of course immediately will destroy your charger and the primary HV e-cap(s)! So ALWAYS check the right polarity BEFORE pressing the TEST button! To test these power adapters I used a 6V larger current 3A adapter to power my DY294, but probably a 2A adapter will work too.

how to use dy294 to test smps

usb led light

Above photo showed my Led Light that I used to test my fully repaired small 5V USB charger with mlx02367A PWM controller and 13005 NPN HV transistor on its small pcb.

But instead of the much consuming Led Light as load to the charger output we also can just connect any cheap USB 5V voltage tester dongle. That also will work longer on the energy that was pumped into the primary HV e-cap(s) by our trusted DY294 than the short flashing of the Led Light does.

The above presented small 5V USB charger started when the DY294 reached about 145V DC if the selected max voltage scale was 200V, and it reached about 165V DC if the 1000V scale was selected.

By the way another test I did just now was test the today received new (Aliexpress 0,50 euro/p) 5V 2,5A smps adapter on my DY294. With the 8 dip LY2223 pwm controller chip on it. (as mentioned in my recent article on Jestine’s blog about the 5V 3A adapter repair).

This time the smps LY2223 adapter circuit already started to work when the DY294 reached a voltage of just about 50V DC! It showed a superlit secondary blue led, which also kept glowing after the input voltage already had dropped to zero when the HV primary e-cap was discharged. Next photo shows this Raspberry Pi 5V 3A adapter that was fixed in a previous repair.

dy294 how to use

See also next photo of the component side of the with this method tested 5V adapter that already works at an input voltage on the AC input pins of about 50V DC.

So the voltage level at which the smps adapter under test activates depends solely on the type of controller used and design of the used circuit. The DY294 just outputs up to the max selected voltage.

dy294 test switch mode power supply

To conclude this SUPERTIP I also tested if I could extend the active working time of the 5V smps charger adapters under test. On the first presented adapter with the mlx02367A PWM controller.

I did this by parallel connecting a big discharged! 460V DC 220uF capacitor to the C(ollector) connected on the plus pole of my e-cap and the E(mitter) of the DY294 to the – pole. And connected these wires to the AC pins of the diode bridge and pressed the TEST button on my DY294 to see what happened.

No matter if the the 1000V or the 200V scale is selected, the adapter began always at about 0V until it reached about 165V on the 1000V scale, respectively 145V if the 200V max scale was selected. (unsure if the starting voltage difference is real because the LCD display probably takes some time to show its characters or here the 1000V indeed just reaches that voltage level sooner?).

And charging the capacitor to that level took about 2V per second in time. (so give and take about 70 seconds to reach the voltage at which the smps adapter starts working). My USB 5V voltage tester dongle worked on the charged capacitor for about 30 seconds until the DC input voltage discharged to about 50V and the 5V smps USB charger stopped working.

Pressing the TEST button while the USB 5V charger was working, did not help to keep the voltage at 165V DC or to 145V DC if the 200V scale was selected. But right after the charger stopped below about 50V DC pressing the TEST button started to raise the capacitor voltage on the Charger AC diode bridge input again up to about 145V respectively 165V DC after the process starts all over again. Of course only when the TEST button is pressed.

So although testing these smaller 5V charger smps adapters with an 5V USB voltage tester dongle already worked splendidly without the 460V DC 220uF capacitor added, it is possible to extend the working time of the smps adapter considerably! (about a factor 30! from about 1 second to about 30 seconds activation).

I also tested this Supertip with a special High Voltage Insulation Tester (often also called “Megger”) the AR907A+. This device is not as safe as the DY294 semiconductor tester is, and neither does it the things the DY294 is able to test but it does output a stable max selected voltage of 250V DC, upto a dangerous maximum of 2500V! And it is like the DY294 short circuit protected, but probably kills all semiconductors that it is connected to.

Sadly this not as universal AR907A+ device compared to the great famous DY294 only operates on 6 penlight cells because apparently for safety reasons there is no external adapter option and adding an external power jack therefore could be very dangerous! But for delivering a decent 250V test voltage this device did work like the DY294 did but since the AR907A+ eats batteries it is a bad choice if you already have a universal DY294 Tester that already works on a decent 6V 3A external power adapter and does not need any batteries! Also keep in mind that the DY294 safely starts at 0V, the AR907A+ however probably starts right away at the selected 250V DC! So do not use it to test caps with lower max working voltages! And the only thing that the AR907A+ also does is measure AC voltages up to 600V and it shows the Mega Ohm value of the connected device or circuit under test.

Just after I thought that I had finished this article about small smps adapters testing without the usual dangerous power line connection and the commonly used in series placed Light bulb I found a third option to safely test these adapters!

I decided to give my last bought TV LED Backlight tester also a try. I previously had bought another LED Backlight tester that had less adjustable voltage and current setting options why I bought this second one:

This one (US $38.95 59% OFF|Tkdmr 0 330V Smart ) also worked if the voltage was adjusted high enough!

It worked fabulous and incredibly fast! And if you also set the current tumbleswitch to automatic it tests the smps adapter even faster! The 160 mA Led light flashes on about once every second. Faster than it did with the AR907A+ or the DY294! And it also can test e-caps on max breakdown voltages like the DY294 can. And also a few other options on TV backlight powerboard testing.

And the USB voltage tester dongle now was constantly active! The input voltage is exactly adjustable to the point where the voltage tester dongle just switches off. And can be set at a voltage that it is constantly powering the smps output to the USB voltage dongle tester which is at an inputvoltage of around +140V !

I again also added a HV e-cap, a 390uF 400V type, to see how it would improve testing my smps adapter. To the Led backlight tester red and black output probes.

And this now helped to keep the 160mA Led on the output of the 5V USB charger constantly working!! It worked flawlessly instantly and Incredibly well!! I think this method is a SUPERTIP !!

So this is also a very interesting result in testing these smaller smps units! Depending on what device the engineer/reader of this blog can afford to buy and use, there are now at least 3 given options. Of which the last mentioned Backlight Led tester worked the fastest and the best!

Here a final photo of this Backlight Led tester with the now flawlessly and constantly working 160mA 5V Led light and the to the output of the Backlight tester extra added 390uF 400V HV e-cap.

Tkdmr 0 330V Smart led backlight tester

The model I used probably is best compared to other Led backlight testers because of the extra adjusting options this device has. I know that touching the probes is free of any real danger, although the instruction manual with my previous backlight tester warned the user NOT to touch the HV probes, but I know the backlight tester is also short circuit proof for short periods. The instructions included with my previous tester also warned not to keep the Led/Ledstrip under test too long connected to the probes because the current is raised in steps the longer they are connected. And with my new backlight tester there was no instruction manual included but all information is given online on eBay.

Anyway whatever device you decide to use as small smps adapter tester, know that it works and that especially in case you use the DY294 it is safe for engineer and electronic parts! But using the Led Backlight tester was also safe and surprisingly here also the best small smsp adapter tester!

I’m sure this Super Tip will safe time when fixing small power adapters and maybe even safe lives at the same time! This way you won’t need to place a safety Light bulb in series anymore either!

This way you always safely can test your small smps adapters and chargers without destroying anything as long as always the right polarity is observed! And as long as the primary maximum HV e-caps voltage never is violated!

But whatever you do, always double check your circuit under test because here are High Voltages involved! Because you certainly won’t like getting a shock from a 400V fully charged electrolyte capacitor!

albert from netherlands

Albert van Bemmelen, Weert, The Netherlands.

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Note: You can read his previous repair article in the below link:



  1. Parasuraman S

    January 16, 2021 at 9:33 pm

    Vow! What an innovative idea of testing SMPS! Awesome methods! Many, many thanks to you, dear for letting us all know about this method!

    • Albert van Bemmelen

      January 16, 2021 at 11:17 pm

      There is of course always a reason to save on expensive tools and methods by just using the old lightbulb 'trick', but I guess there never is a reason to save on our safety when we can avoid danger so easily!

  2. Paris Azis

    January 16, 2021 at 10:12 pm

    Hello Albert
    Your proposal is very interesting, nevertheless it’s rather complicated in comparison to the lamp test setup.
    I find much easier and cheaper as well the use of an ordinary three Watt incandescent lamp, used for corridor lighting during the night.
    It will not allow more than about 15mA current flow through a dead short circuited device under repair and costs less than 2€.
    I find your method technically correct in any aspect, but somehow exaggerated in terms of equipment used in reference to the device under repair...
    All I mean is that devices like these are not worth it even to power up your oscilloscope or use an expensive multimeter in order to troubleshoot them...Therefore the cheaper the repair method, the better it is...

    • Albert van Bemmelen

      January 16, 2021 at 11:08 pm

      I fully understand what you mean Paris! Indeed any light bulb is always cheaper but also in principle very dangerous with a live power line connection. Why for instance the Led Backlight tester method is so much safer and also easy. And it is not so much the price of the device or the cost of the used method that is important here but our safety is. And after the old light bulb is gone this method still works! I guess that if there was more safety in this world it probably wouldn't have created an extremely expensive world wide pandemic either?

      • Albert van Bemmelen

        January 17, 2021 at 3:19 am

        Speaking about using an oscilloscope to test our smps adapters. It would be a bad thing to do that when our circuit under test is still connected to the live 230VAC power line! The very nice thing about the 'offline' testmethod here is after all that we safely can use our oscilloscope without creating dangerous earth loops or other dangerous backfiring AC voltages that blow up the BNC inputs or cause other damages to our expensive oscilloscopes when we want to measure signals on our working smps circuit board.

      • Paris Azis

        January 17, 2021 at 7:14 pm

        Albert, talking about safety, the most important countermeasure which secures anyone involved in electronics repairs, remains the isolation transformer. This is a must when it comes to safety. And the combination of the transformer with the test lamp in series, I think, is the perfect match for such tests/repairs.
        Anyone involved in repairs who doesn't understand this simple truth, or, worsely, who refuses to use an isolation transformer when working with live circuits, is simply a fool.
        There is no excuse in such an attitude.
        By my comment I wanted only to highlight that your method is very "hi-tech" for such repairs.
        As for the small incandescent bulb that might disappear from the market, this is also possible, don't forget that capacitors will always be there.
        No lamp? No problem! A 470μF/400V (or X series) cap will do its job "lightlessly" (but quite safely) for such small loads and for sure!
        There are many alternatives anyway. A small variac for example could be used very effectively for such a troubleshooting procedure...
        For sure a) there is nothing wrong with your proposal and above all b) every repairer works with his own strategy and tactics!
        And I respect that!

        • Albert van Bemmelen

          January 18, 2021 at 12:13 am

          Understood Paris! Of course I also do have a heavy 320VA safety transformer with resettable 16A fuse but it is like you say 'overkill' in equipment value. (an original medical hospital cheap bought safety transformer that also can be mounted onto a wall). But don't forget that if you use the DY294 you also have a safety transformer by using a 6V 3A adapter that doesn't cost much either! And the AR907A+ is already power line free on batteries. Using a variac is still very dangerous!

    • Riaan Diedericks

      January 17, 2021 at 10:11 pm

      Good tip. Dankie Albert!!

      • Albert van Bemmelen

        January 18, 2021 at 12:01 am

        Glad you liked it Riaan!

  3. Suranga Bandara

    January 16, 2021 at 10:31 pm

    Very nice...
    Good Repair ..Mr Albert .

    • Albert van Bemmelen

      January 17, 2021 at 1:09 am

      Thank you...Mr.Suranga.

  4. Yogesh Panchal

    January 17, 2021 at 1:30 am

    Thanks! for sharing informative article.

    • Albert van Bemmelen

      January 17, 2021 at 3:52 pm

      Thanks Yogesh!

  5. Lynn Blakely

    January 17, 2021 at 5:40 am

    Safety first, that's important. I appreciate your Supertip, and the though explanation, of both testers. I enjoy reading all the repair posts, it seems that outside the USA people don't have the throw away attitude. That is good economics. Why should we support a country that contributes to cheap electronics being sold. Phillips tvs which is owned by Funai are manufactured in Thailand. I try not to buy products made in China, that is hard to do. Thanks to everyone that posts these interesting articles.

  6. Albert van Bemmelen

    January 17, 2021 at 3:50 pm

    Maybe it are the cheap things that contribute in making this world a little better Lynn? And I like most things that were made in China like the in the article mentioned DY294, and the Omnipotence Backlight tester. (Google: God's omnipotence). Expensive things without real value like jewelry were never my cup of tea. And in politics things often only shine on the outside with destructive ruling as result. We all came from nothing and go as we came. And we all had to make babysteps before reaching for the Moon (or Mars). Cheers!

  7. Lewis Spedding

    January 18, 2021 at 6:31 am

    Thank you for this as I am working on a Small SWMode PS adapter at present. How can i purchase a DY294 ? It could be a great asset to have this.

    • Albert van Bemmelen

      January 18, 2021 at 1:37 pm

      If you just want to buy it to test these smaller smps 5V and other PWM chip controlled USB chargers, the last presented Omnipotence Led backlight tester is capable in delivering more test current and always works faster and better Lewis. If you however want to mainly test breakdown semiconductor voltages the DY294 is the best choice!
      My previous three DY294s I bought on Aliexpress. The AR907A+ I bought in 2015 on eBay. The Omnipotence backlight tester can probably be bought on both Aliexpress and eBay. (see the Backlight tester link in the article for more info and an explaining video). Cheers!

      • Albert van Bemmelen

        January 18, 2021 at 1:55 pm

        By-the-way: I do not know how large your PS pwm unit is you want to fix, Lewis? But know that ATX and XT power supplies can't be tested with the presented safe testmethod because they require a too high supply current before they even start working!
        That simply would require a very large (100-300 Watt?) short current protected safety power supply capable in testing also the large(r) smps units! And testers like the DY294 are because of their protective nature mainly designed to 'carefully' test small semiconductors and capacitors.

  8. Waleed Rishmawi

    January 18, 2021 at 2:23 pm

    I never used or knew about such a meter but I did my search about it last night. thanks for the eye opening article and thanks for sharing. have a blessed day

    • Albert van Bemmelen

      January 18, 2021 at 7:19 pm

      Nice that it may have helped you to add it to your tools Waleed!

      • Albert van Bemmelen

        January 18, 2021 at 7:29 pm

        PS: Did you also see my repair on Jestine's blog on the DY294?
        It may also warn you never to use a unregulated 6V power adapter to power your DY294. That is how I killed mine in just seconds! But afterwards made a very usefull article about the repair that was never published before!

  9. Humberto

    January 18, 2021 at 3:11 pm

    What a lot of information in this tutorial. Thanks for sharing it.

  10. Albert van Bemmelen

    January 18, 2021 at 7:20 pm

    Glad I could share it Humberto!

  11. Albert van Bemmelen

    January 19, 2021 at 11:11 pm

    The in the article presented testing method with the TKDMR 0-300V Led backlight tester also works excellent on 12V 1.5A adapters. Tested successfully on a Sunny 12V 1.5A DC 18W 230VAC ~ max 1.5A adapter.

    • Albert van Bemmelen

      January 20, 2021 at 11:07 pm

      Testing the mentioned Sunny 12V 1.5A DC smps adapter worked perfectly even without the need of connecting a HV e-cap to the Omnipotence tester output! Higher max current adapters (2A and higher) probably will work too. But then the HV e-cap is probably needed and it is connected to the adapter only after it is fully charged upto at least about 300V DC to also successfully activate/test the more energy consuming adapters without much effort!

  12. Ulise Aguilar

    September 13, 2021 at 6:36 am

    grate job sir


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