Electronic Circuits

USB Power Bank Adapter

USB Power Bank – popularly known as mobile phone emergency charger – is an ideal solution for powering small microcontroller based hobby projects. Even the cheapest power bank in the market has so many useful features like auto power off and over current/short circuit protection which make them a safe and compact power supply for everyday electronics experiments.

However, one minor disadvantage (not a design flaw) is that the device switches itself off if the output load isn’t drawing current above a minimum level (usually 100mA or so). Because of this auto shut off mechanism, it’s very hard to power most basic microcontroller projects from the power bank as there’s a possibility of an annoying power cycling (restart-restart-restart) activity. One effective resolution is the insertion of a low-value resistor across the USB power supply output (VCC & GND) lines as a dummy load. Usually, a 47-Ohm/1 Watt resistor (a 100mA dummy load) will do the job well.

For most of us, constantly connected 47-Ohm resistor across a 5V power supply rail will raise some fears and doubts as there’s a remote risk of magic smoke from the ‘burning’ load resistor. So I did some experiments and found that presence of the said load resistor only for a finite time will keep the power bank happy to run without any issues. Later, I rigged up an ‘active load’ with a few scrap laying around my workbench. Shown below is the tried and tested circuit diagram of my USB Power Bank Adapter (v1).

USB Power Bank Adapter

Schematic

Parts List

T1: BC557
T2: BC547
R1: 100K ¼ w
R2: 22R ¼ w
R3: 1K ¼ w
C1: 4.7uF/25v
LED1: 5mm Green
Misc: USB ‘A’ Male plug, USB ‘A’ Female socket

My experimental setup was tested on my new “mI 10400 mAh” power bank and it worked great for me. If your fussy power bank still turns to sleep after a couple minutes, just try to replace the 4.7uF capacitor with a 2.2uF one as it looked very worthy with most power banks.

Note: Naturally, if you use your power bank for charging a phone or tablet it will work as intended, hence this external adapter is not necessary!

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