I was curious about the internals of the 110V AC phone chargers and I wanted to see if I could modify one to change the output voltage. So I did some investigating and this instructable will show what I found.
Before we go any further, I have a few important points to cover ..
1) I do not recommend that anyone modify any AC phone charger. There are high voltages involved in these devices that can be dangerous if handled improperly.
2) Do NOT attempt any modifications to anything if you are not trained to handle the modification safely.
3) There are a wide variety of AC phone charger chips and circuits. I do not claim that the mods that I made for the AC charger in this instructable will work for other AC phone charger circuits.
4) I have another instructable on 12V DC phone charger circuits that can be found here:
Step 1: Take It Apart
The charger I chose is a Samsung TAD137, I picked it up at a thrift store for 25 cents. Before disassembly I measured the output voltage at 5.0V. I used a couple of screwdrivers to pry the case apart, but it was not easy to disassemble and I sort-of mangled it a bit so I'll probably need to find another project box to house it after completing the modification. I added a short AC line chord so I could power up the circuit on my test bench.
The key components to look for here are the controller chip and the feedback resistor. The controller chip in this case is a DH0165 chip. Look up the data sheet online (Google it). The feedback resistor (R13 on the data sheet schematic) is a 5k Ohm resistor.
Step 2: Review the Data Sheet
The data sheet (at least the one I found) is not very informative about the application circuit or how to modify the output voltage, but for the TAD136 I was able to deduce (guess) that the formula is
Vout = 2.5 * (1 + (R13/5k))
for the original circuit this is verified since
5V = 2.5 * (1 + (5k/5k))
I wanted to change the output to about 3.6V, so wit a little math we get
R13 = 5k (Vout - 2.5) / 2.5 = 5k (3.6 - 2.5) / 2.5 = 2.2k
We can verify this by plugging back in to the original formula for Vo
Vo = 2.5 * (1 + (R13/5k)) = 2.5 * (1 + (2.2k/5k)) = 3.6V
Step 3: Modify the Value of Resistor R13
As outlined in the previous step we need to replace the 5k feedback resistor R13 with a 2.2k (1/4 W) resistor.
As can be seen in the photos, the output voltage changes as expected from 5V to 3.6V after making this change.