Recently I had an explosion on my desk. A 220uF 25V electrolytic capacitor just blown up shortly after attaching a load.
The high-level connection was: Phocos CA08 (solar charger controller) -> 12V-3.8V step-down -> uBlox Leon GSM modem.
Because the variable load generated by the GSM modem was causing the charger controller errors (it indicated an overload and cut off the voltage at the output socket) I put a 220uF 25V electrolytic capacitor on its exit and it worked for a week or so with no issues, also no overload indications were occurring anymore. The connection was not firm, it was just attached into the contacts of controller's output. There was an external power supply attached to the the solar cell so that to charge the lead-acid battery. The explosion occurred after ~5 seconds since attaching the K1 to the charger controller's output.
The schematic of a power supply for the GSM modem is the following:
The modem itself can generate a significant load when registering to the network (right after power up):
It came to me as a surprise that the capacitor like this exploded this way, especially when it is not the only capacitor on the path between the supply and modem, there are plenty of them.
Now I wonder what happened and what might have caused the explosion of this type. I did not change the polarization of the capacitor, I'd bet that its leads did not met each other. Do you have any ideas?