Hi.
I have an old cell phone whose battery is dead. So I thought I just power it straight from a power supply so I can use it for some stationary tasks like listening to music on my desk.
I removed the dead battery and hooked up a lab power supply and set the voltage to 4.2V. Which worked without a problem. Since the lab power supply is bulky and I need it for other things I thought I would use a LM317 to build a little circuit to power the phone. On a breadboard I tested the standard LM317 circuit with Cin 100nF ceramic cab, Cout a 10uf tantalum, R1 330 Ohm and R2 a pot adjusted, so that Vout is 4.2V. For the test I used again the lab supply for Vin with 9V.
Weirdly the phone starts to boot but goes black after a few seconds. I measured the regulated voltage and it jumps around 4.2 - 3.8V as soon as the phone starts booting. I also measured the current, which jumps around from 100mA to 800mA. I also used a 0.1 Ohm resistor in series to measure the current with a scope. In infinite persistence, I can see that the current sometimes jumps up to above 1A. But only very shortly. You can not see that on the multi meter.
The output voltage of the LM317 does not seem to be very stable. Even at around a 600mA load, the voltage is already down to around 3.8V. I guess the high current jumps are causing the voltage to drop so low that the phone shuts down? Am I asking to much of the LM317? Or it's something else I don't see?
I mounted a little head sink to it. Because first I thought it would go into thermal shutdown. But the funny thing is if I increase Vin to 12V it seems to be more stable and I usually get the phone to fully boot up. Which causes to LM317 to get even hotter.
Any suggestions? Note, that I am a real novice. That is the first time that I try to use a voltage regulator. Maybe I am doing it all wrong...
Regards,
Norbert.