Portable phone charger

Posted on Feb 5, 2014

The phone is a Siemens S65. The official charger that came with the phone says 5V at 420mA. Because these things do not always actually deliver what is printed on them I measured the voltage myself. This is actually quite hard with the tiny contacts on the connector, especially when it also has to be plugged into the phone. Without a phone connected the voltage

Portable phone charger
Click here to download the full size of the above Circuit.

was much higher, around 8V if I remember correctly. When the phone was actually connected the voltage would drop to around 4V and even lower. The label says output: `5V-11V DC 350mA`. Of course I opened this thing to look what was inside. I found a cheap looking PCB with a step-down converter based on Motorola`s well known MC34063. Based on the component values it was configured for a 8. 75V output and a current limit of 320mA. When connected to the phone the voltage drops below 5V because the current limit is reached. To see at what voltage the phone starts charging I cut off the connector from the car charger and put 5V on it. The phone did not started charging. At 5. 5V still nothing, but at 6V it started charging. As was shown with both the official charger and the cheap car charger, once the phone starts charging the voltage may get well below 5V without problems. But it needs around 6V to get started. To get 6V out of two AA batteries I used a LT1949 switching step-up converter from Linear Technology. An AA battery typically delivers about 1. 2V-1. 5V but the voltage drops even lower under load and as they discharge. The LT1949 already runs at a voltage of 1. 5V so two AA batteries are enough to keep it running, even at low battery voltages. The IC has an internal switch current limit of about 1. 1A. This will limit the output current. In fact, the phone tries to use so much current that the current limit kicks in and...

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