/* If you are in a hurry, you can fast-forward to my conclusions right after the second picture. */
I used this one in my Arduino solar energy monitor and I noticed that AMS1117 regulators were getting pretty hot to touch. A Nano clone, an idle SD card adapter and an HD44780 display shouldn't drain that much current, and if they did they would be warm to touch too. [edit: the whole circuit draws about 50 mA except, I think, for those short moments it writes to the SD card once a minute.]
So I wanted to measure the input current, since that's the easiest point to insert an ammeter. I disconnected the positive lead from the 12 V PSU and inserted there my DVM ... the power adapter switched on but Arduino was behaving erratically. I did a power cycle and nothing happened anymore, no reassuring LEDs saying hello, not even on the YwRobot board.
The DVM internal fuse could have blown, so I restored the original 12 V positive lead and powered everything up.
Arduino LEDs were blinking awkwardly, or not blinking at all. Another wiring check confirmed that nothing had moved or shorted.
Just to be sure I measured if 5 V were still there and ... SURPRISE! I measured 12 V where I was expecting 5 V!
Since I was feeding 5 V directly to the Arduino board, it was fried (the FT232 USB-to-serial adapter survived, though!). The SD card adapter uses its own 3v3 regulator, so it was not damaged either.
I removed the power supply module and verified:
- 5V output is now few hundred mV below input voltage, that's 12V or so
- 3.3V output is not present anymore
What the heck is going on here? I decided to sacrifice my second power adapter module, that was equally heating up in my final application circuit.
This time I played it safer and removed the YwRobot adapter board from the solderless breadboard.
First of all, I wanted to know the idle current of the power adapter board alone. I removed it from the breadboard, added my DVM on the positive lead, fed 13V and the current drawn settled to ~20mA, which is fine according to AMS1117 datasheet (10 mA minimum load to keep regulating, and the board mounts two in series). After 5 seconds the board emitted the typical crackling sound of pine wood in the fireplace. And the unpleasant smell of burned electronics.
This time the DVM fuse didn't blow (10 A ;) ) and no other electronics were harmed.
|The two broken adapters. Notice silkscreen differences... that don't make a difference!|
Post-mortem analysis revealed that:
- the AMS1117-5 (5V regulator) shows a resistance of 14 ohm between input and output pins
- it passes through the input voltage
- the first board's AMS1117-3.3 blew up, the second is still fine
In both cases 220V AC to 12V DC was provided with a solid-state 5A PSU that mounts two 78M12 in parallel.
Curiously enough I find no mention elsewhere of this kind of failure. I do not own other YwRobot adapters and I do not plan to buy any more. Please let me know your thoughts in the comments.