I bought some time ago a 119338 Dual Car Cigarette Sockets Power Adapter with USB Power Port – Black (DC 12~24V) to replace the power adapter in the car that started failing. In hoped this product would fix the situation, but it turned out this was not a good fix.
The specifications on the web page looked pretty good:
- Input Voltage: 12~24V
- USB: 5V 500mA
- 90 degree foldable
Here are the specifications as shown on the package:
The specifications say 5A current for the 12V outputs. That’s reasonable. Also 500 mA for USB output is OK. That folding was the feature I wanted and could not found on adapters in nearby shops.
In first tests the device worked well. But when I folded the device to the final position it turned out that it did not work anymore. I complained on the problem (got refund for money). The shop let me to keep the device, so I took a look what went wrong. Here is what I saw when I opened the device.
What I noticed when I opened it was that the the device came with 2A fuse! With this fuse this device will not be able to handle the promised. The question is did the manufacturer put a wrong size (smaller amperage) fuse in by accident? I don’t think so. The wires inside are very thin, would heat up very much if pushed to 5A, but would be OK for 2A. I would not put in 5A fuse to this. It seem that the marketing is lying on the specifications or the manufacturer has afterwards changed the product to cheaper parts that do not match the original specifications.
The original problem of device failing was caused by a bad soldering on the ground wire (cold solder joint) that had failed. It was easy to fix with a proper soldering iron. So the I got the device working again.
I was curious how this device had implemented that 5V 500mA USB output. I would have expected to see a small switch mode regulator to convert 12V to 5V for USB port (like many other similar devices are built). To my surprise there was no inductors, just one capacitor and a component that looked like a small transistor.
It turned out that the USB output was implemented with 78L05 linear regulator. Linear regulator approach as such would not be completely out of the question if you could accept poor efficiency and handle the generated heat (around 60% of input power would be converted to heat in 12V to 5V conversion). The power adapter specifications promised 5V 500mA output which can’t hold true with this approach.
78L05 is a tiny regulator in TO-92 case that is designed to provide 100mA output current (very far off from promised 500mA). 78L05 data sheet says that with adequate heat sinking the regulator can deliver 100mA output current (max peak output current 140 mA). When looking at the power handling capacity of the TO-92 case, it seem that the IC would just barely be able to provide continuous 100mA 5V from car 12V (and considerably less if powered from 24V). Current limiting is included in the IC to limit the peak output current to a safe value. Safe area protection for the output transistors is provided to limit internal power dissipation. If internal power dissipation becomes too high for the heat sinking provided, the thermal shutdown circuit takes over preventing the IC from overheating. All those are good safety features to have, but do not compensate the original problems that the output current is not what was promised! 100mA current output on USB is very low for example to change devices.
It seem that the marketing is again lying on the specifications or the manufacturer has afterwards changed the product to cheaper parts that do not match the original specifications.
So the end result there are several things wrong in this product. They have made many things well, but those mistakes makes this product pretty poor. If the soldering quality would have been OK the device would have not failed in the first place. The package current rating and the actual construction should also match (2A total current would have been maybe acceptable if listed as such on specifications). The USB output should have been built to provide the promised 500 mA current, that 100 mA output it now gives is too low for most uses (like charging cellular phone with USB charging cable).