When reaching for a power supply design it’s normal here in 2022 to reach for a switching design. They’re lightweight, very efficient, and often available off-the-shelf at reasonable prices. Their benefits are such that it’s become surprisingly rare to see a traditional linear power supply with a mains-frequency transformer and rectifier circuit, so [ElectroBoy]’s dual voltage PSU board for audio amplifiers is worth a second look.
This type of linear power supply has an extremely simple circuit consisting of a transformer, bridge rectifier, and capacitors. The transformer isolates and steps down the AC voltage, the rectifier turns it into a rough DC, and the capacitors filter the DC to remove as much AC ripple as possible. In an audio power supply the capacitors have the dual role of filtering and providing an impulse reservoir for the supply in the event of a peak in demand imposed by the music being played. Careful selection is vital, with in this case a toroidal mains transformer and good quality capacitors being chosen.
The choice between a linear power supply such as this one and a switching design for high quality audio is by no means clear-cut, and may be something we’ll consider in our Know Audio series. The desirable properties are low noise and that impulse reservoir we mentioned, and it’s probably fair to say that while both types of power supply can satisfy them. With the extra expense of a toroidal transformer a linear supply is unlikely to be the cheaper of the two, but we suspect the balance tips in its favour due to a good linear supply being the easier to design.
Recently [Big Clive], everyone’s favorite purveyor of anything electronic that’s dodgy, cheap, cheerful, decidedly crispy or any combination thereof, got sent a very dead external power supply unit. Being clearly a third-party PSU with poorly written and many (likely not truthful) safety approval markings on its label, this PSU had the dubious honor of having destroyed a Microsoft Surface computer as well as the monitor that was connected at the time.
In [Clive]’s video (also embedded after the break) the black and very crispy board is examined, showing a wealth of vaporized traces and plenty of soot. What’s however most fascinating is the failure mode: instead of something obvious like e.g. the main transformer between the primary and secondary side failing, here it would seem that an inductor (see heading image) on the secondary side had its insulation rubbed off and shorted on a nearby heatsink. A heatsink that just happened to be also electrically connected on the primary (mains-level) side.
Judging by the former owner’s report and aftermath, this led to a very sudden and violent demise of the PSU, with mains power very likely making its way into the unsuspecting Surface system and connected monitor. The number of ‘very nope’ design decisions made in this PSU are astounding, and a lesson for both aspiring EEs and anyone considering getting a ‘cheap’ third-party replacement PSU.
The build is a simple mashup, starting with a ZY12PDN USB Power Delivery board. This board talks to a USB-C supply that is compatible with the Power Delivery standard, and tells it to deliver a certain voltage and current output. This is then used to supply power to a pre-built power supply module that handles current limiting, variable voltage output, and all that fancy stuff. It even comes with a screen built-in! Simply slap the two together in a 3D printed case with a couple of banana plugs, and you’re almost done.
All you need then is a USB-C power supply – [Ricardo] uses a portable power bank which allows him to use the power supply on the go. It’s a great alternative to a traditional heavy bench supply, and more than enough for a lot of hobby uses.
Old electrolytic capacitors are notorious for not working like they used to, but what exactly does a bad capacitor look like, and what kinds of problems can it cause? Usually bad caps leak or bulge, but not always. In [Zak Kemble]’s case, a bad cap caused his Samsung HT-C460 Home Cinema System to simply display “PROT” then turn itself off. Luckily, replacing the troublesome cap fixed everything, but finding the problem in the first place wasn’t quite so straightforward. A visual inspection of the device, shown open in the photo above, didn’t reveal any obvious problems. None of the capacitors looked anything out of the ordinary, but one of them turned out to be the problem anyway.
The first identifiable issue was discovering that the -5 V supply was only outputting about -0.5 V, and there was a 6 V drop across two small 0805-sized resistors, evidence that something was sinking far more current than it should.
Testing revealed that the -5 V regulator wasn’t malfunctioning, and by process of elimination [Zak] finally removed the 470 uF output capacitor on the -5 V output, and the problem disappeared! Inspecting the capacitor revealed no outward sign of malfunction, but it had developed an internal short. [Zak] replaced the faulty cap (and replaced the others just to be safe) and is now looking forward to getting years more of use out of his home cinema system.
When a PSU gives up the ghost, bad capacitors are almost always to blame, but we’ve seen before that it’s not always easy to figure out which ones are bad. One thing that helped [Zak] plenty in his troubleshooting is finding a full schematic of the power supply, just by doing a search for the part number he found on it. A good reminder that it’s always worth throwing a part number into a search engine; you might get lucky!
The venerable ATX standard was developed in 1995 by Intel, as an attempt to standardize what had until then been a PC ecosystem formed around the IBM AT PC’s legacy. The preceding AT form factor was not so much a standard as it was the copying of the IBM AT’s approximate mainboard and with it all of its flaws.
With the ATX standard also came the ATX power supply (PSU), the standard for which defines the standard voltage rails and the function of each additional feature, such as soft power on (PS_ON). As with all electrical appliances and gadgets during the 1990s and beyond, the ATX PSUs became the subject of power efficiency regulations, which would also lead to the 80+ certification program in 2004.
We know what you’re thinking. It’s a bad power supply, of course it was capacitors to blame. But even if we all intuitively know at this point that bad caps are almost always the culprit when a PSU gives up the ghost, it’s not always easy to figure out which one is to blame. Which is why this deep dive into a failed ETK450AWT by [eigma] is worth a look.
The first sign of trouble was when the computer would unexpectedly reboot with nothing in the system logs to indicate a problem. Eventually, [eigma] noticed a restart before the operating system even loaded, which confirmed the hardware was to blame. A quick look at the PSU output with a voltmeter showed things weren’t too far out of spec, but putting an oscilloscope on the 12 V line uncovered a nasty waveform that demanded further investigation.
By carefully following traces and comparing with common PSU diagrams, [eigma] was able to identify the SG5616 IC that checks the various voltages being produced by the PSU and generates the PWR_OK signal which tells the motherboard that everything is working normally. As before, all of the DC voltages at this chip seemed reasonable enough, but the pin that was measuring AC voltage from the transformer was showing the same ripple visible on the 12 VDC line.
Even more digging uncovered that the transformer itself had a control IC nestled away. The 13 VDC required by this chip to operate is pulled off the standby transformer by way of a Zener diode and a couple capacitors, but as [eigma] soon found, the circuit was producing another nasty ripple. Throwing a few new capacitors into the mix smoothed things out and got the PSU to kick on, but that’s not quite the end of the story.
Pulling the capacitors from the board and checking their values with the meter, [eigma] found they too appeared to be within reasonable enough limits. They even looked in good shape physically. But with the help of a signal generator, he was able to determine their equivalent series resistance (ESR) was way too high. Case closed.
You may have seen the Ruideng range of programmable power supply modules from China: small and relatively inexpensive switch-mode buck converters, with microprocessor control and a front panel featuring a large colour OLED screen. Given 30 volts or so they can supply any lower voltage with the extra bonus of current limiting. They’ve been so successful over the several years they’ve been available that they’ve even spawned their own Chinese clones, and countless hacker projects, for instance on the DPS300X and DPS500X models.
Late last year a new module came from Ruideng, the Riden-branded RD6006 combines the basic idea of the previous modules with an extremely flexible front panel with full keypad and rotary encoder, creating something like the front panel to a decent bench power supply but without the accompanying power supply. I ordered one, waited for it to clear customs, took it to my bench, and reviewed it. Continue reading “Review: The Riden RD6006W DC Power Supply Module”→