Current limiter add on

[Yossef Gofer]

Dear Friends,
I purchased a simple step-up converter (2.5-5V in, %V out) with USB output socket. My intention was to use it with 3 AA batteries for portable, on-time extender or charger for smartphone. This is the simplest circuit with no protections of any kind. When I used it, hooked to my HTC, it did the job: the phone was charging. However, after several minutes the batteries, as well as the converter became VERY hot. I'd guess that the phone draws too much current, although the DCDC converter is rated to 1 A.
Now, what I would have liked to do is to add a simple, though energy efficient (if possible) current limiter. I don't know it it will be wiser to hook such a limited after the battery pack and before the converter, or after the converter, before feeding the USB. I found some interesting circuits based on L200C regulator, but I have no idea how to harness it to my needs. I thought of limiting the current to 800 or 700 mA, but I will have to tweak it for best results. Maybe better off to go simpler and add serial resistor, and loose bit energy.
I will be happy to hear your comments and suggestions, of any kind.
Best regards,
Yossi

 

[tpetar]

I'm not sure that AA batteries can hold 700-800mA of current. In start you attack these batteries very hard, if you use alkaline chemistry maybe you will see some results, but with cheap AA I dont think you can get that current. plus on all of this alkaline AA have capacity 1500-2000mAh (better products), and phone battery capacity is something between 700-2000mAh, usually around 1000-1100mAh, when you add StepUp converter efficiency (losses), I'm not sure what you will get. With MC34063 3,7V to 5V for around 450mA of output current, you need around 1A of input current (current from AA batteries).

I want to say, better forget on fast charging with StepUp design, better use slower charging 150-350mA but with 96% of efficiency. :wink:

As additional solution consider usage of more AA cells in serie to avoid losses in StepUp converter, this can give higher charging currents with stable 5V. Use LDO or switcher regulator to ensure stable 5V.

Look to buy solar case for phone, that is better solution. You can charge phone everywhere where you have light.



Best regards,
Peter