Addon to auto-Off-Powerbanks

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Addon to auto-Off-Powerbanks

Post by Orngrimm »

We all know and love them: USB Powerbanks
Often you plug in something and they switch on, charge the gadget and switch off if the current is no longer drawn.
Sometimes, a push-Button os needed to switch-on and charge, but the Auto-Off is almost always a thing.
All well and good, but i have some gadgets i want to charge from a powerbank but they draw a quite low current to charge. Often, this current is low enough to trip the Off-functionality on the Powerbank.

Now... What about a thing for in betwheen the powerbank and your gadget? Something that keeps the Powerbank from turning off as long as the gadget is drawing current but will let the Powerbank switch off as soon as the gadget is finished charging?
Basically an update to the sensitivity of the auto-off...

My basic idea now:
A microprozessor monitors via a current sensor the current flowing into the gadget.
The microprozessor draws (by itself and a resistor) enough power from the powerbank to not let the powerbank switch off.
As soon as the current to the gadget is low enough (Like lets say 0.1mA) the microprozessor cuts the main power from the Powerbank and therefore to the gadget, the microprozessor and the added load. This makes the Powerbank to switch off after a while as the curent is now = 0

As a current sensor, ill take a 1 Ohm resistor and as current-to-voltage-converter a simple good OpAmp: ADA4528-1
I simulated sucessfully down to 2 micro (!) amps and got the correct voltage out (+/- a reasonable amount):
>1mA: 5V
1mA: 5V
0.5mA: 2.5V
0.1mA: 505mV
50uA: 255mV
10uA: 57mV
2uA: 15.5mV
15mV is simple to measure with a uP.

Of course, the waste-current (uP, Sensor, additional resistors if needed) can be adjusted for powerbanks with different switch-off-limits... But with the ADA4528 we can serve even the lowest current-gadgets succesfully and have the powerbank happily supply current till the gadget is fully charged and stops its current.

The switchoff-Level where the microprozessor will switch off all can be adjusted in the FW of the uP. One just has to measure the charging-current of the gadget and set the limit a bit below that.

The microprozessor will be a small Arduino pro micro or something alike. Yes, the Ardiono-FW and -boards are not current-Saving, but thats the point: We need to waste a bit of current to keep the powerbank active.

As a switch i chose a simple analog switch, the PI5A3166 ( ... 542974.pdf)

Thsi switch will be OFF in the normal state and need to be switched ON by the uP with a positive voltage. To kickstart all, a push-Button is there, to initially bridge the switch, Power the gadget and the uP and current sensor and from that moment on, it will be self-holding until the output-current falls below the trigger-level defined in the firmware.

What you guys think? Worth a shot and a quick PCB?
LTSpice-Simulation current-Sensor
(595 Bytes) Downloaded 5 times
Builder of stuff, creator of things, inventor of many and master of none.
Tinkerer by heart, archer by choice and electronics engineer by trade.
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