Power supply teardowns reveal safety issues

A power supply is an electronic device that supplies electric energy to an electrical load. Most of the power supplies you use every day are the AC-DC power supplies that convert mains AC power (typically in 100-240V range) to low voltage DC (typically 5V to 24V). In those power supplies the output will be electrically isolated from the mains; this feature is essential for user safety. Many modern power supplies (switch mode) often include safety features such as current limiting or a crowbar circuit to help protect the device and the user from harm. The power supplies also typically need to have some form of fuse in the mains side to protect against short circuit or severe over-load inside the power supply (to avoid potential fire).

Power supplies come with various specifications, built using different technologies and with varying quality (from cheap unsafe counterfeit knock-offs to good quality).  Take a look at the nearest power supply labeling. It will hopefully have a slew of marks on it that indicate what standards it abides to (or at least promises to).

But not all product do what they promise. Many finding culminate to this: An engineering/project management triangle. Pick 2 of 3!

We all love a good tear-down (at least me), so here are some interesting tear-downs of power supplies:

Sparkfun Tears Apart Power Supplies for fun and to learn. Enginursday: Supplies! article takea a look inside some common power supplies. It walks us through how the basic circuit works and then points out why various other elaborations are made, and how corners are sometimes cut, in a few power supplies that he’s taken apart.


Dangerous Chinese power supply article tells about a cheap, Chinese, 12V 1A power supply, designed to be used in LED lamp retrofit project. It’s marked CE (meaning it respects European norms, which essentially have to do with EMI interference), but it still has a serious safety problems. What we learn of this: Remember that the the CE marking is stamped by **the manufacter himself**, which basically means “I tell you that this product respects regulations”. But of course, that does’nt mean at all that it does. Usually this is the case with cheap chinese crap where you usually get what you pay for. To safely use this power supply several corrections/modifications needed to be done.

Cheap switch-mode power supplies video a quick look as some cheap £15 generic Chinese switch-mode universal power supplies and what is wrong with them. Quality is as expected… I wouldn’t trust this at all for anything serious!

There are many cheap bad-quality USB chargers.  I have made Safety analysis of one USB power supply  and saw something to worry about! Take a look at the separation between mains carrying parts and low voltage side: There is less than one millimeter between them (I measured shorted distances to be in 0.5-0.7 mm range). This is not how to build a safe power supply.

I also made Teardown of cheap USB charger that failed after quite short use. The power supply is built using very simple circuit (simpler than expected). There are two worrying safety issues here: The insulation distances on the circuit board are very small (less than 1 mm on several places between mains side and output). Too small to be safe! There does not seem to be any fuse in this circuit!

Autopsy of an exploded USB power supply. (With skidmark) video shows what is inside pre-detonated multi-port USB power supply. Given the isolation between the mains voltage primary windings and the low voltage secondary windings, perhaps it’s a good thing that it exploded (and did not put people in more danger).

Don’t buy a cheap replacement power supply! video warns about counterfeit batteries and power supplies. eBay and Amazon.com are loaded with cheap replacement power supplies and battery chargers for digital cameras, camcorders, laptop computers, and other devices. But as you might expect, these power supplies are built using cheap components with very bad build quality, and use no shielding and very poor filtering. This creates a tremendous amount of radio interference.


Why You Should NEVER Buy Generic Chinese Laptop Power Supplies by XanderDarien

Remember also this danger with adapters if you plan to use power supply with different powewer connector than you have on your wall: Deadly Chinese plug adapter

For more interesting tear-downs take a look at Teardown category on this blog.


  1. Tomi Engdahl says:

    Related video:

    How to open a laptop power supply and modify its output voltage

    This video provides a few tips on opening up a laptop power brick, as well as how to go about modifying its output voltage.

  2. Tomi Engdahl says:

    Teardown: 12V AC adapters – The Horror

    In the name of science, I introduced three of the worst 12V adapters (from My kingdom for a 12V adapter) to my bandsaw. What I found may shock you.

    Yes! Can you believe it? This board used to be in another adapter housing, or more likely, in an end-product. It seems Chinese manufacturing has sunk to a new low: repackaging used and/or surplus power supplies. On the plus side…recycling.

    The board was held in place by two pieces of foam, which you’ll see is pretty high-end packaging, relatively speaking.

    Next up, the “XY1205A”, again with its fictitious approvals & specs. Nothing horrifically wrong here, unless you count the tacked-on wires and lack of mounting. Good clearance between primary and secondary sides, except where there isn’t.

    This is good quality guts paired with a botched repackaging job.
    As far as I can tell, the production date is ca. 2000-2001 (board:0093, AC rectifiers:0062). Ugh. Wonder if the board had seen service before being repackaged.

    The two blue caps at the upper left are 2.2nF parts in series, bridging primary to secondary sides in parallel with the 88MΩ of resistance. What’s the goal here?

    None of the boards have isolation slots cut in them between primary & secondary sides

    There you are. A look at a very dirty side of Chinese manufacturing. One wonders how deeply this sort of thing permeates the industry,

    Home> Community > Blogs > BenchTalk
    My kingdom for a 12V adapter

    Our new house is inching towards 100% LEDness, from LED “bulbs” in standard light fixtures, to custom LED lighting, to flexible LED light strips. It’s the latter I’m having some problems with.

    After some preliminary research, I decided that the only affordable sources for LED strips would be from among the numerous Shenzhen & Hong Kong mega-retailer Websites. There, you’ll find standard 5m LED strips in the $4-$20 range, instead of the $50-$100 range typical of other sources.

    After some preliminary research, I decided that the only affordable sources for LED strips would be from among the numerous Shenzhen & Hong Kong mega-retailer Websites. There, you’ll find standard 5m LED strips in the $4-$20 range, instead of the $50-$100 range typical of other sources.

    With the LEDs themselves taken care of, I turned to power sources. In the basement, I’ll probably use some centralized high current 12V power supplies..

    While I’m willing to accept some creative LED specs, I do expect a 5A adapter to be a 5A adapter. Silly me. They turned out to be more like 2A, and to add insult to injury, the output cables appeared to be vastly undersized

  3. Tomi Engdahl says:

    Horrific USB power supply fault. (Electrocution risk.)

    Although I’ve come across some really dodgy power supplies with poor insulation between the mains and low voltage sides, this is the first one where the USB ports have carried full mains current. (via a rectifier)
    It’s a Swees QY08-05010 with this model and style carrying various other branding as well. Oddly it does appear to be relatively sensibly designed inside, but this one has a serious manufacturing fault that suggests others from the same run may also pose a risk of serious electric shock.

    Sadly, this just reinforces my doubts about the poor separation in many of these small transformers. I’d rather have split bobbin transformers even if it meant efficiency taking a slight hit.

  4. Tomi Engdahl says:

    Searching for USB Power Supplies that Won’t Explode

    USB power supplies are super cheap and omnipresent. They are the Tribble of my household. But they’re not all created equal, and some of them may even be dangerous. I had to source USB power supplies for a product, and it wasn’t easy. But the upside is that I got to tear them all apart and check out their designs.

    In order to be legitimate, it’s nice (but not legally required) for a power supply to have UL approval. Some retailers and offices and building managers require it, and some insurance companies may not pay claims if it turns out the damage was caused by a non-UL-approved device. UL approval is not an easy process, though, and it is time consuming and expensive. The good news is that if you are developing a low voltage DC product, you can pair it with a UL approved power supply and you’re good to go without any further testing necessary.

    Sourcing cheap electronics in large quantities usually ends up in China, and specifically Alibaba. First, we started with a how-low-can-you-go solution.

    Quality control could not be a high priority. After cutting it open, it wasn’t terrible, and it had all the necessary parts. It was surprising how much of it was through-hole, which indicates that the assembly was done mostly by people. That happens when factories are cheaper, hire inexpensive labor, don’t invest in technology, and don’t care as much about quality.

    There are certain things you should look for in a power supply to determine the level of risk:

    Isolation Distance – This is how much space there is between the primary (AC) and secondary (DC 5V) sides. UL requires a few millimeters, and often you’ll see two separate PCBs. On many single-PCB solutions you’ll see a white line meander across the board to distinguish between the two. The smaller this separation, the closer your USB power is to AC line voltage, and if the gap is bridged somehow, you’re in for a world of hurt.
    Fuse – if there is a short, a lot of current starts flowing, components heat up, and things get dangerous. A thermal cut-off (TCO) fuse (also known as a resettable fuse or a PTC) is a component that breaks the circuit when it gets too hot, like a circuit chaperon. When it cools off, the TCO resets and you can plug the device back in with no harm done. Without the fuse, the supply heats up and current keeps flowing until a component fries, sometimes explosively.
    Connectors – You don’t want bare leads hanging out in space where they could move and touch something. You don’t want the USB port to be soldered only by its four pins. You don’t want the power pins to be loose.
    Decent Label – “Adaptep”? Yes, to someone who uses a different alphabet the “P” and R are very similar characters. But still. Also, fake certifications abound. Look for the difference between the CE (China Export) and the CE (Communite Europeanne) labels. And the UL Logo should have a number. So should an FCC label.

    Check out a much more thorough analysis of this and pretty much every USB power supply cube by [Ken Shirriff]. It’s surprising how little has changed in four years with these supplies, and his analysis goes into how the circuits behind these supplies work, identifying each component and its purpose.

    Tiny, cheap, and dangerous: Inside a (fake) iPhone charger

  5. Tomi Engdahl says:

    Water damage

    Water and electricity don’t mix

    Scary stuff! – if you look closely, you can see that the USB adapter was cracked open, probably by the pressure released as it finally gave up and let out the magic smoke

    It has obviously been going on for a long, long time – that amount of corrosion couldn’t possibly have been caused by just a few hours of moisture!

  6. Tomi Engdahl says:

    Cheap bench power supply tear-down (it’s bad!)

    cheap MCH-K305D 30V, 5A 150W switching bench supply
    the current limiting has never worked so I figured I’d tear it down and take a look inside. It’s *awful*

    EEVblog #828 – Siglent SPD3303X Precision Lab PSU Teardown

  7. Tomi Engdahl says:

    Test of bare eBay 12V 1A PSU module, including safety tests.

    There are a few of these little power supply modules on ebay, so I thought I’d get one and give it a thorough test. Voltage holds relatively well, with just a slight drop under overload conditions, and the module layout and filtering is actually very good.

  8. Tomi Engdahl says:

    Crappy PSU analysis
    Pulling apart a too-cheap-to-be-true PSU. Guess what… it was too cheap.

    Inside a reasonable quality Chinese PSU

  9. Tomi Engdahl says:

    Wall plate death-dapter tests including fire and 1kV.

    Why settle for a wobbly death-daptor with flimsy electrical contacts and inadequate shuttering when you can have one permanently fixed into the fabric of your home. Better still, one with always-on electronics inside too. This product falls into the category of “Jack of all trades, but master of none.”

  10. Tomi Engdahl says:

    There were many flaws in the Led floodlights

    European market surveillance authorities investigated the safety and compliance of LED bulbs with a joint control project. Indeed, 47% of the products were removed from the European market as non-compliant.

    Of the 82 LED floodlights tested, only two met the technical and administrative requirements of both legislation. As a result of the project, 47% of the tested LED spotlights were pulled out of the EU market. A total of 87 floodlights were tested for electrical safety requirements. 87 per cent of the tested products did not meet the requirements. Indeed, 71 percent of the products had flaws that could endanger the user’s safety.

    Tukes closely monitored the ten LED spotlights sold in Finland, of which five were withdrawn from the market

    Source: https://www.uusiteknologia.fi/2017/09/12/led-valonheittimista-loytyi-paljon-puutteita/

  11. Tomi Engdahl says:

    Home> Power-management Design Center > Teardown
    Teardown: A better Chinese 12V adapter

    Rather than admitting defeat after uncovering some horrid Chinese-made 12 V adapters, I only found my curiosity piqued. I decided to keep looking.

    Giving up on China-direct sources like AliExpress, I turned to Amazon. After browsing the reviews for many an AC adapter, it seemed pretty clear that the Amazon name wouldn’t guarantee a quality unit, but I still felt my chances were greater than with Ali.

    Some PSUs on the site are shipped from a third party, others by Amazon. I chose one of the latter, a couple of 12 V 6 A adapters, ostensibly made by a company called Selectec

    My initial Impressions were positive. The unit felt solid, and would supply up to 6.9 A before shutting down. At the rated 6 A, cable drop amounted to 0.75 V. Not great, but I’ve seen much worse.

    Letting it thermally stabilize with a 3 A load resulted in what I’d call acceptable temperature rise.
    I’d estimate a case hot-spot of maybe 50 °C, with most of the case being cooler. At a bit over 5 A however, I found the case became unacceptably hot

    The 6 A unit is admittedly in a small case for its rating (30×50×109 mm).

    But you came for a teardown. Let’s have a look.

    Notice the EMI filter? No, neither do I. Do you believe the FCC and CE certifications? No, neither do I.

    The AC connector is not IEC, but two-terminal “shaver” style. Yet there’s a ground connection on the PCB, which is connected to a couple of those blue ceramic HV caps, and…the negative DC output.

    A 12 V adapter that, while not a total horror, isn’t anything to get too excited about either.


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