Modifying the output Voltage of an Adjustable DC-to-DC Converter (Phone Charger Hack)

Picture of Modifying the output Voltage of an Adjustable DC-to-DC Converter (Phone Charger Hack)
I had a project that needed a 12V to 3.5V DC-to-DC converter and rather than building one from scratch I decided to just modify a 12V (automotive-cigarette-lighter type) phone charger.  Most newer phones have standardized on 5V USB-type charging systems, but before this standardization it seemed that every phone manufacturer had their own unique plug and voltage combination.  Because of this variability, these older charger were almost always built with a adjustable DC-to-DC converter chips (where the output voltage is set by the ratio of 2 resistors).  Thrift stores are a good source for these older-style chargers and these chargers are easily modified to generate nearly any voltage that you need (within some limits as will be discussed later in this instructable).

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Step 1: Take it Apart

Picture of Take it Apart
I picked up this charger at a thrift store for 25 cents.  Before disassembly I measured the output voltage at 5.5V.  It was relatively easy to disassemble; there are no screws at all, just use a screwdriver to pry it apart at the seam and it will click apart.  In my project, I was not planning to re-use the housing, so I was not ultra-careful about saving the housing or the cigarette-lighter components.

The key components to look at here are the controller chip U1 and the voltage-setting resistors R1 and R2.  U1 is a PJ34063 chip and the data sheet can be found online (Google it).  I measured R1 and R2 at 2.47k and 8.2k respectively.

Step 2: Review the Data Sheet

Picture of Review the Data Sheet
PJ34063 -3.jpg
From the data sheet we find that we can modify the output voltage of the converter by using this formula

Vout = 1.25 * (1+ (R2/R1))

Set Vout to 3.5V we will modify R2

After a little math and we see that

R2 = R1 (Vout - 1.25) / 1.25

And plugging in the values for Vout and R1 we get the required value for R2

R2 = 2.47k (3.5 - 1.25) / 1.25  = 4.44k

I didn't have a 4.44k resistor but I found a 4.7K so let's see if that will work

Vout = 1.25 * (1+ (4.7k / 2.47k))  = 3.6V   .. which is close enough :)

Phil B1 year ago
I appreciate your very practical Instructable. It was interesting to me to see the formula you gave is the very same one used with the LM317 variable voltage regulator chip.

I recently wanted to drop the voltage on an old 120 VAC to 5 VDC switch mode phone charger for an output of 3 VDC. I had once tried an LM317 chip on a switch mode power supply, but the output shut down completely until I removed the chip and its discrete parts. I am not sure if the chip shut down or if the power supply shut down.

This time, though, I added diodes in series until the output was what I needed. I did describe it as an Instructable here.
scd (author)  Phil B1 year ago
Hi Phil,

A circuit mod similar to this instructable should be possible with a 120-VAC-input phone charger and I am kind of curious to try modifying one of those sometime. I'll post my results in another instructable once I try that (if I can get it to work ;)

I have an older instructable (posted in July 2011) where I used a simple diode-referenced battery-replacement circuit for a wall clock. This mod has been working fine for the past few years.
Phil B scd1 year ago
Thank you for the response. Your circuit is more sophisticated than mine and you showed me I have some things to learn. Thanks.