Since I seem to be the local go-to for any dead electrical equipment, this brand-new Silverline polisher has landed on my desk. Purchased cheap from an auction this was dead on arrival. Checking the fuse revealed nothing suspect, so a quick teardown to find the fault was required.
Above is a photo of the commutator with the brush holder removed, and the source of the issue. The connection onto the field winding of the universal motor has been left unsecured, as a result it’s managed to move into contact with the commutator.
This has done a pretty good job of chewing it’s way through the wire entirely. There is some minor damage to the commutator segments, but it’s still smooth, and shouldn’t damage the brushes.
A quick pull on what’s left of the wire reveals the extent of the problem. It’s entirely burned through! Unfortunately the stator assembly with the field windings is pressed into the plastic housing, so it’s not removable. An in-place solder joint was required to the very short remains of the wire inside the housing. Once this was done the polisher sprang to life immediately, with no other damage.
This unit probably ended up at an auction as a factory reject, or a customer return to a retail outlet. If the latter, I would seriously question the quality control procedures of Silverline tools. 😉
A few months ago I did a teardown on this Anker PowerPort Speed 5 USB charger, but I didn’t get round to detailing the conversion to 12v I had to do, so I’ll get to that now I’ve got a couple more to convert over.
Here’s the internals of the Anker charger once I’ve removed the casing – which like many things these days, is glued together. (Joints can be cracked with a screwdriver handle without damaging the case). There’s lots of heatsinking in here to cool the primary side switching devices & the pot core transformers, so this is the first thing to get removed.
Once the heatsink has been removed, the pot core transformers are visible, wrapped in yellow tape. There’s some more heatsink pads & thermal grease here, to conduct heat better. The transformers, primary side switching components & input filter capacitor have to go.
Here’s the PCB once all the now redundant mains conversion components have been deleted. I’ve left the input filtering & bridge rectifier in place, as this solves the issue of the figure-8 cable on the input being reversible, polarity of the input doesn’t matter with the bridge. I’ve removed the main filter capacitor to make enough room for the DC-DC converters to be fitted.
Installing the tails to connect everything together is the next step, this charger requires two power supplies – the QC3 circuits need 14.4v to supply the multi-voltage modules, the remaining 3 standard ports require 5v. The DC input tails are soldered into place where the main filter capacitor was, while the outputs are fitted to the spot the transformer secondary windings ended up. I’ve left the factory Schottky rectifiers in place on the secondary side to make things a little more simple, the output voltages of both the DC-DC converters does need to be increased slightly to compensate for the diode drops though. I’ve also bypassed the mains input fuse, as at 12v the input current is going to be substantially higher than when used on mains voltage.
With a squeeze both the boost converter & the buck converter fit into place on the PCB.
I recently used a Vishay Spectrol Precision 10-turn potentiometer in my latest PSU project, and since these clip together instead of being ultrasonically welded like Bourns potentiometers, I decided to do a quick teardown for the blog. I didn’t find much in the way of how these pots worked from a search, so here we go!
Here’s a pot of the same spec as I used in the previous project, again from my random used junk bin. This is a 100Ω, 5% wirewound potentiometer. The shaft is secured in the centre bushing with a snap ring, this is easily removed with a pair of needlenose pliers.
After unclipping the back cover, the stationary part of the wiper contact is visible in the back plastic cover.
Inside the back of the potentiometer shows the inner workings. These devices have a large helical winding of resistance wire around the inner diameter of the casing, the wiper tracks this helix as the potentiometer is turned.
As the wiper must move axially as it winds around the spiral of resistance wire, the contact is mounted on a pair of guides so it can slide back & forth. The electrical connection is made via another spring contact that runs down the side of the plastic shaft insert. Two notches cut into the black plastic wiper frame engage with the round profile of the resistance winding like a screw into a nut, keeping the wiper in perfect alignment as it travels the full length of the winding. I suspect all these moving contacts are made of Beryllium Copper in this rather expensive component as this alloy is very flexible, as well as being a very good electrical conductor.
With the centre shaft & it’s wiper contact removed from the shell, the helix of resistance wire can be seen inside. Oddly, the former the resistance wire is wound around appears to be metal, possibly copper, but to keep the entire thing from shorting itself out this must be coated in insulation.