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.

power_supply_1_overviewIf you are going for FCC approval and are having unintentional emissions testing done (which is more likely than UL as it’s a legal requirement for products that meet certain qualifications), the testing has to be done on the whole solution, so the power supply must be included in the testing, too.

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. This wasn’t even a power adapter; it was a power “adapteP”, and the whole batch was mis-printed. 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 (Conformité Européenne) labels. And the UL Logo should have a number. So should an FCC label.

So this first adapter? Isolation distance was fine because it was two separate boards, but there was no fuse and no protective tape between components. The connectors were all secure, but the label didn’t make any promises. As for performance, output at 5.34V under my product’s load meant it was a little outside of USB spec (5.25V limit), but not dangerous. On the scope it was ringing with a peak at 5.5 V at 4 kHz.

Of course, sourcing this supply for a second batch proved tricky, and we wanted the USB plug to come out the side instead of the front so it would have a thinner profile against a wall. Additionally, we needed UL approval for a client. Our second attempt was surprisingly successful. This adapter had UL certification, with a number to look up. Note that just having a number isn’t enough; many companies will just put someone else’s number on their product and assume nobody will bother to check. So when you do look it up, and find a different manufacturer, a different enclosure, and it looks more like a refrigerator than a USB power supply, don’t be too surprised. But no, this particular one was great! The label had a company name on it, model number and specs, and certifications that could be verified. Let’s tear it open!

power_supply_2_overviewSweet sweet silicon meat inside an ABS shell! Components wrapped in protective tape, two PCBs for isolation, and even a special injection-molded plastic piece to add additional protection. Components are labeled, and what’s this, an IC to control the oscillation instead of a feedback winding on the transformer? Fancy! It’s pretty clear that this power supply is good, and I’d trust this one.

Comparing this one to the others, there were so many noticeable little details that are important and clearly thought-out. Take, for example, the connection between the prongs and the PCB. On the previous board, it was made with wires soldered by hand. Solid, but time consuming and prone to failure or quality issues. This adapter has metal contacts that snap into the case very solidly so that the prongs cannot get loose. The connection to the PCB is via the springiness of the metal, but notice that the PCB has pads specifically designed to maximize the surface area of that connection. On the next PCB you’ll see no such effort.

Some components were covered in shrink tube, tape, or non-conductive grey adhesive. The assembly was tight with no room for components to shake loose or accidentally touch. And the output was perfect. 4.9 Volts with nary a ripple.

But this is China, and component sourcing problems are a thing, so I guess I shouldn’t have been surprised when these supplies were no longer available. In retrospect, maybe these were unsold overstock, or possibly QC rejects. That would explain why they were only slightly more expensive than the others. And so we moved on to another supplier; one that could pad-print our logo on top.

power_supply_differencesAt first glance these power supplies appeared identical. But close inspection reveals slight differences in the style around the USB and the raised ridges on the underside. The label was completely different, and gone was the number next to the UL logo. There was no company name on the supply either, and the company we purchased from turned out to be a reseller and not the OEM. Also, why was the output 4.7-5V, and why did my scope say 5.5V (but surprisingly stable)?

Inside was a completely different beast. Using a single PCB, the creep distance was about a millimeter. You can see the white line meandering through the bottom of the PCB that shows the high and low sides. The USB port wasn’t soldered to the PCB except by the four signal/power pins (see the bottom side lower left and the hanging USB connection pins), and there was a capacitor with really long uncovered leads and the positive side dangerously close to the USB shell. There was almost no protective tape, no shrink tube on the leads, and no protection in case of a short.


In the end, I wouldn’t trust the two non-UL supplies with anything worth more than a few bucks, and certainly not my cell phone. I’d have really big reservations about reselling them to customers who don’t know the difference. The UL-approved one was great, but the other two are only good for powering low-current-draw devices that are not sensitive to voltage. Also, finding a reliable supplier in China is HARD.

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.

We also covered a Sparkfun teardown of some power supplies with similar conclusions, and a Fail of the Week in which a faulty USB power adapter was the likely cause of a fire.

Powering A Lot Of Nixie Tubes

[Limpkin] has an idea for a project that uses a lot of IN-9 Nixie tubes. Where a Nixie tube clock would only use four or six tubes, [Limpkin] is looking at fifty IN-9 bar graph Nixie tubes. These tubes only light up above 100 Volts and draw about half an amp. That’s 64 Watts, according to the math on the project page, so how does [Limpkin] plan on powering these tubes? With a big high voltage power supply.

The power supply [Limpkin] designed is more or less what you would expect to find in any power supply. There’s a transformer, a bunch of caps, and a rectifier. Going with a standard laminated core transformer would mean this power supply would be huge and heavy, but once again eBay comes to the rescue with a small, 150 Watt toroidal transformer. The largest output on the transformer was two 24 V outputs. Combining those outputs gets [Limpkin] to 48V AC, or 68V peak to peak. A full wave voltage doubler with two caps and two diodes gives [Limpkin] the 136V DC that will power the tubes.

Combine the high voltage circuit with a 9V AC tap, a small bridge rectifier, and a few more caps, and [Limpkin] had a supply that would power the tubes and the rest of the electronics in his multiple Nixie tube project. A few passes with a CNC mill gave the power supply a nice case topped off with a foreboding toroidal transformer ready to power a beautiful neon project.

Handy Power Supply with 3D Printed Case

You can never have too many power supplies around your workbench. It is easy to buy them or cobble something together for most purposes. But once in a while you see one that is simple and also looks good, like this one from [RegisHsu].

The project is simple since it uses off-the-shelf DC-to-DC converter modules, and good-looking LED meters to measure voltage and current. The dual supply can accept 5 to 16 V in (presumably from a wall transformer) and deliver 1.3 V to 15.5 V out at 2 amps. [RegisHsu] removed an adjustment pot from the converter board and replaced it with a 10-turn pot to allow voltage adjustment.

Given the parts, you probably don’t even need a wiring diagram. However, the part that brings it together is the 3D-printed case, which [RegisHsu] has on Thingiverse. We’ve looked at muti-turn pot replacements before, and this is hardly the first power supply project we’ve posted.

Repairing and Improving Cheap Bench Power Supplies

Cheap benchtop power supplies are generally regarded as pieces of junk around these parts. They can measure well enough under perfect conditions, but when you use them a little bit, they fall over. There’s proof of this in hundreds of EEVblog posts, Amazon reviews, and stories from people who have actually owned these el-cheapo power supplies.

One of the guys who has had a difficult time with these power supplies is [Richard]. He picked up a MPJA 9616PS (or Circuit Specialists CSI3003SM) for a song. It quickly broke, and that means it’s time for a repair video. [Richard] is doing this one better – he has the 3A power supply, that sells for $55. With a stupidly simple modification, he upgraded this power supply to the 5A model that usually sells for $100.

The problem with [Richard]’s broken power supply were voltage and current adjustments knobs. This cheap power supply didn’t use rotary encoders – voltage and current were controlled by a pair of 1k and 10k pots. Replacing these parts cost about $5, and [Richard]’s power supply was back up on its feet.

After poking around inside this power supply, [Richard] noticed two blue trim pots. These trim pots were cranked all the way to the left, and by cranking them all the way to the right, the power supply could output 5 Amps. Yes, the 3A version of this power supply was almost identical to the 5A version, with the only difference being the price. It’s a good repair to a somewhat crappy but serviceable supply, but a great mod that puts a beefier power supply on [Richard]’s desk.

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Build Yourself an Awesome Modular Power Supply

You may think you’ve built a power supply for your bench. Heck, we all do. But until you check out [Denis]’s bench power supply build, you may not even know what you’re missing.

[Denis]’s design is nearly entirely modular and targeted to the intermediate builder. It’s built on easily available parts and through-hole components. It’s got an Arduino running as the brains, so you’re going to be able to hack on the code when you feel like tweaking it. But easy doesn’t mean light on features. Let’s walk through the build together.


It starts off with a pre-regulator: a switching MOSFET that gets the voltage down to just a couple volts above the target value. Then it’s off to the post-regulator that includes all of the fine adjustments, the DAC and ADC interfacing to the microcontroller, and some fancy features like a “down-programmer” that turns the output off extra quickly.

On the user end of things, [Denis] made a very sleek board that incorporates a TFT touchscreen for the controls, Arduino connections, and the obligatory banana plug outputs. There’s opto-isolation on the SPI bus, a real-time clock, and a bunch more goodies on board. He’s in his third revision of this module, and that level of refinement shows. It’s even SCPI compliant, meaning you can control remotely using an industry-standard protocol.

So what would you do with a ridiculously fancy power supply under microcontroller control? Test out battery charging algorithms? Program test routines to see how your devices will work as their batteries drain out? We have no idea, but we know we want one!

Sparkfun Tears Apart Power Supplies

We love a good tear-down, and last week’s “Enginursday” at Sparkfun satisfied our desire to see the insides of AC-DC switching power supplies, accompanied by knowledgeable commentary. [MTaylor] 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.

lishin_bottomshieldedWhat struck us in the comparison was that some of the power supplies were very minimal designs, while others had “features” that were obviously added after the fact. For instance, the Li Shin supply (about half-way down the page) has an extra circuit board tacked on to the bottom of the real circuit board to act as EM shielding.

Rather than declare this a dodgy hack, as we would have, [MTaylor] declares it to be “Good News!” because it means that they’ve probably run an emissions test, failed it, and then added this bit on to make it pass. This is of course in contrast to the other makers who’ve probably never even considered emissions testing. Sigh.

If you’re interested in seeing more inner bits of power bricks, Sparkfun forum reader [sgrace] passed along this field guide to various power supplies, which is also worth a look. And if you’re interested in building yourself the ultimate bench power supply, look no further than this project by [The Big One].

Easy Power Supply Mod Takes Control

Inexpensive bench top power supplies are great for the home hobbyist, featuring wide voltage range and current limiting for a low price. What’s not to love? The controls; most have a single-turn pot that is typically very fidgety, especially at low voltage.

The solution is to replace the factory pots with nice wire-wound 10 turn units in order to gain 10x the precision. Of course nothing is ever drop in, the new pots didn’t fit the old holes, but that is nothing a few moments with a drill can’t fix. Also the original knobs no longer fit, but that’s just an opportunity for a knob upgrade.

The end result is still a power supply with fidgety controls, but instead of holding your breath, tippy tapping knobs to get within 100mV of your target, you can dial right in to within 10mV of your target. That makes life much easier, especially on low voltage projects that may not have power regulation quite yet.

Join us after the break for a video with all the info.

Continue reading “Easy Power Supply Mod Takes Control”