Learn to reaverse engineering the 10Ah power supply

Posted by Mohammed 15/01/2016 0 Comment(s)

I personally love juice, and apparently, my circuits loves it too! that's why we have a 10,000,000,000,000pAh (10Ah) battery charger.

Although it is really awesome, but there is one catch, it is a charger and not a battery. So what?

Well, a charger is intended to "charge", yes really, I mean it! So, when the battery it is charging is full, the charging stops. Mainly the charger knows this from the current being sucked out through it.

The problem:

If you want to power a small microcontroller circuit, it will draw a very small current which the charger will consider it as a full battery and will hence auto-shutdown.

The solution:

Mr. Me decided to hack the charger to try to remove or adjust this current limitation, and ....

 

&&++++****---- HACK ON ----****++++&&

First thing of course is to open the case, a screw driver was enough

power-supply

Ok, the case is opened, we have a beautiful little circuit taking the output of a 4-paralleled 3.7v batteries.

 

power-supply-circuit

Zoom in to the circuit, I decided to start looking at this IC which seems suspicious because it is directly beneath one of the outputs.

 

power-supply-resistors

Ok so it is a: "YF8036" IC, what does the first result in Google reveals?

Woot, it is a charger datasheet!

http://universe-micro.com/upload/6570123426213.pdf

From the datasheet, the charger will auto-shutdown when the charging current is less than 1/10th of of the programmed charging current.

Scroll down to page 8, there I found the following paragrap

Adjusting Charging Current
The charging current is programmed using 1%
precision resistor from PROG pin to ground. The
charging current and the programming resistor are
calculated using the following equations:
Rprog=1000V/Ichg, Ichg=1000V/Rprog

And where is Rprog? well, it should be connected to Pin 2 "PROG", have a look:

pin-2-prog

Yes, it is the little R23 resistor with "122" written in top which is a [12x10^2] = 1.2K ohm resistor, plugging this value on the top equation gives us a charging current of ~ 0.8A. A little bit less than the stated output of 1A but that is OK.

So, if I can change this value, I can change the charging current for small circuits, let us do some math

power-supply-equation

 

Great! so I can use the already existing 1.2K resistor in series with a 10K adjustable resistor (Pot/Potentiometer). Heading to soldering station ...

power-supply-soldering-station

Ok, Pot is fixed and value adjusted to 100K, time to test. The test circuit is a simple LED with a pot to adjust the current for about 20mA

potentiometer-powersupply

Ok, Power ON:

potentiometer-circuit

Yay, LED is ON, but after few seconds....

potentiometer-dead

 it is OFF :(

Why? Why? Why???????

Back to datasheet and read on....

Wait, what is this? (page 9)

capacitance on the PROG pin, the charger is stable with
the programming resistor value as high as 20k.
However, additional capacitance on this node reduces
the maximum allowed program resistor. The pole
frequency at the PROG pin should be kept above
100kHz

Ohhh, man!!!!!!!!!!!!!!!!!

That means I can only adjust the current up to about 50mA, still more than what my circuit needs, though, this might be useful for many people though

For me, lesson learned: Always RTFM (Read The Full Manual)

Leave a Comment