The USB port of a Nokia N95 8 GB suddenly stopped working. A dig deep into the problem revealed a broken USB port in the N95. Read about it there.
Charging the Nokia N95 with a car voltage adapter
There are many USB car chargers on the market nowadays. It’s a small piece of electronics converting 12 – 30 Volt to the 5 Volt DC as specified for USB equipment. It’s a one-chip task and not so spectacular either, these step-down / voltage stabilizer chips have been around for quite some time.
Now, the household batteries in the machine are AGM cells. This battery type has a high float voltage of 14.1 Volts – meaning the battery charger keeps this high voltage as the level at which the battery is full. For lead-acid batteries this is about 13 volts. For large mobile homes, trucks, lorries and boats, battery voltage is usually doubled with 2 batteries in series. For AGM batteries this makes a high float voltage of 28 volts.
This high float voltage reaches the near maximum (30 Volt) of the one-chip voltage stabilizer normally found in cheap USB car chargers. The step-down conversion is almost 25 volts to make the 5 volt required for the USB port.
A switch-on spike of the AGM battery charger can easily reach the 30 Volts or above. Consequently the 5 Volt DC output of the USB car charger can shortly reach higher voltages when the input voltage spikes above 30 Volts during switch-on.
AC mains tripped and spiked the Nokia N95 USB port
The Nokia N95 8Gb was connected over the USB port when the spike occurred. At that moment 220 Volt AC mains tripped violently in a matter of a few seconds. The Nokia N95 was charging simultaneously over the USB car charger. The USB connection was dead straight after the spike.
Diagnosis of Nokia N95 USB port damage
Neither laptop or PC was able to connect to the N95 8Gb USB port. The familiar USB symbol at the N95 screen did not appear either. However when inserting the powered USB cable from a laptop in the N95, the Symbian dialog does still pop-up asking which USB connection type is required. Neither selection did make the USB connection work again.
USB car charger internals
The USB car charger is branded with FREE LINK. The product label has the following details
Input: 12 – 30 VDC
Output: 5.0V / 1 Amp
Made in China
USB car charger internals shows a one-chip voltage stabilizer / step-down with markings MC34063A SUM 6C20N