#104 Reverse engineering an old linear power supply
Recently I came into possession of two working ups devices from 1993. both of them had old capacitors and old 12v7a lead acid batteries inside the devices.
First thing I did was clean the cases and the PCB boards. Once that was done I replaced the old capacitors and the 12v7a batteries, then I tested both devices. both work fine but the design is old and a bit dangerous.
So I decided to reverse engineer the circuit in order to better understand the design and to see if I could make any improvements to a design I would like to make.
While reversing the PCB I noticed that the mains earth and the GND of the circuit were connected together. I also notices sone discoloration from what looks like heat between the regulator and the transformer. Also the 330 ohm resistor for the led appeared to be discoloured from what also looks like excessive heat.
With these issues in mind I also noticed that the heatsink for the LM317T was very small and close to the transformer and the mains 1A fuse was placed after the choke and varistors instead of before them.
In conclusion I decided to choose between a different regulator at a fixed voltage or a chain of 4 LM317Ts providing around 6A of peak current, Since I do not need to adjust my voltage like the original circuit I should be able to get 13.75v by using a fixed 1k and 10k resistor. I also wanted better heat dissipation and Amps so I will definitely install good heatsinks with thermal compound. Depending on the size of the enclosure I get for the project I may add a fan.
The project files and components list etc. can be found on my Github here.
While looking for a new multi port USB charger I came across the Model BK-357 sold on Takealot by OQ Trading. This charger had many positive reviews with a 4.1 star rating at the time of writing this article and for the low price of R149.00 I had to give this device a try.
Once I received my charger I noticed That the fast charge USB port was working flawlessly but the 3 normal charging USB ports seemed to have current divided between them.
So I decided to open the charger to investigate further. I noticed the bottom part was glued into place and could be pried open carefully with a small screwdriver and spudger.
Once opened the PCB was in good condition and contains 2 small switching transformers and a nice fusible resistor that also acts as an inrush limiter but that’s where to positives end. The interference capacitor was skimped on also the electrolytic capacitors are all different colours and brands It’s possible that they have been taken off old junk and re-purposed which is okey but they may have a diminished quality which is almost as alarming as the gap between the primary and secondary sides of the transformers. The biggest gap is around 4.8mm which is not to bad but right in the middle the gap closes all the way to 1.3mm!!!
This is very dangerous since there is 1.3mm of PCB space between you and mains voltage!!!
I will be posting my findings as a review on Takealot.
When purchasing multiple socketed USB chargers go for the larger more expensive ones.. As you can see in this case the tiny transformers just can’t output enough current on the cheapies.
After testing the generic Andowl UPS device I decided to find out if there were any alternatives in the same price range with similar features. After a few google searches I came across the SINYE TECH UPS device which has very similar features but is black in color and uses a different PCB.
After purchasing one and opening the ups I was disappointed to find out that the 4 18650 batteries had no markings on them whatsoever.. Also the main battery management IC has no identifying text on the chip. Seems like the mystery chip can of worms opens once again.
The good news is that the ups device has battery protection and has some decent components however… the actual black PCB board looks cheap unlike the good quality Andowl PCB…
Another major shock was the soldering (or should I say smoldering) of two of the transformer coil wires.. just looks ugly…
I also noticed the UPS device had a low battery LED flashing when I first switched it on and it never reaches the 100% LED even though the batteries are charged to 4.2V
Overall the UPS does function but I much prefer the Andowl Q-UP1000 UPS with similar functions it’s far more superior from what I’ve seen.
When looking for a suitable min ups for my CCTV cameras and WiFi router I found a wide selection on takealot. After doing some research I noticed that the Q-UP1000 mini ups was listed by many vendors but at varying prices. Many companies had rebranded the device and increased their price quite dramatically.
Finally I came across a deal selling the device for under R500.00 and I purchased 2 Andowl devices. Once I received the devices I was impressed with the quality of cables and the plastic mold of the box. (There are no screws to unscrew the box simply pops open)
Once the box was open I noticed a decent PCB with all the necessary protection circuits. There are two mystery chips though.
The product was made recently (at least at the time of writing this article) and the designers made the batteries easily accessible for swaps which is awesome.
Now a negative thing I noticed is the 12000mAh claim… clearly the batteries are four 2.200mAh INR18650’s and this adds to 8.800mAh
There can be many reasons for this capacity “lie” but other than that the mini ups is looking good. Time will tell if this was a decent investment.