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

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

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 :)

Step 3: Wrap Up

Use a soldering iron to remove the old R2 resistor (8.2k in this case) and replace it with the new R2 resistor (4.7k in this case)
Test it then button it back up and you're done.

With this technique, you can modify the output voltage of a DC-to-DC converter, but note that the voltage rating capacitor will limit the possible voltage output values.

Note 1: There are lots of different DC-to_DC converter chips (other than the PJ34063) , so you may need to look up (Google) a different data sheet, but the principle is the same.

Note 2: Some DC-to_DC converter chips are not adjustable. Phone chargers that output 5V may not be adjustable since that is a common voltage and there are dedicated chips that are set to output that voltage without using any external resistors. So try to pick a phone charger with an odd value as those are more likely to use adjustable converter chips.



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    11 Discussions


    3 years ago on Introduction

    good job. i looked around and most write ups don't talk about current supplied. the data sheet for the 34063 says Imax = 1.5 amps. I would like to do something similar to your project but I would want Imax > 2.0 amps. i'm tired of running out of juice with the converters sold at stores. Thanks.

    1 reply

    Reply 1 year ago

    You need to add a power transistor. Here is a link of an example using a LM317.


    4 years ago

    Hi, I live on a boat with 12v power. I bought a rechargeable device with an AC mains charging adapter that outputs 5.6v at 80mA. Would it be possible to make a compatible charger using your method?

    1 reply

    Reply 4 years ago

    You can probably find a (12V input voltage) car adapter with similar output voltage and current specs and then modify the output voltage as needed. If you're lucky, you might find a car adapter with "close enough" specs will work just fine without any mods at all.


    4 years ago on Introduction

    is it possible to go the other way? i would like to go from a larger capactiy external battery, say 25000 mah, that only has USB outputs, but be able to charge my gps battery , a 7.4v 2400mah lithium ion?
    is taht even a possibilty?

    3 replies

    Reply 4 years ago on Introduction

    Hi, A common cell phone charger (as used in this instructable) is classified as a "buck" converter which means that it converts from a higher voltage to a lower voltage. Your application would require a different type of converter known as a "boost" converter which converts from a low voltage to a higher voltage.

    So the short answer is yes, it would be possible, but it would be more complex that this instructable as it would probably require designing a boost converter from scratch. If you search the web for "boost converter" you can find more info on designing boost converters.

    A simpler solution might be to buy an 11.1V RC battery and then make an adapter to connect your GPS car charger. This could provide a portable charging solution for your GPS.



    Reply 4 years ago on Introduction

    Since the output of my chargers is 9.5v @1.4a wouldnt a 11.1v battery be too high voltage?
    sorry i dont know much about this kind of stuff..
    I see there is external batteries out there that output 9v.. would that work you think? the battery itself is only 7.4v?? why do they use a 9.5v charger?


    Reply 4 years ago on Introduction

    You misunderstood my comment. I did not suggest connecting the 11.1V RC battery directly to the GPS, I suggested using an existing car adapter for your GPS that plugs into a 12V accessory port in your car and power this adapter from an portable battery rather than from the car battery. The GPS car adapter generates the charging voltage needed by your GPS. DISCLAIMER: I have not tried this so I don't know that it would work. Caveat emptor !!

    As for your question about the charging voltage the actual "smart" charge controller is internal to your GPS and it controls the actual charging current and voltage to the battery. It requires some voltage "headroom" to do this.

    As for the Amazon link, it looks promising, but I would suggest contacting the vendor and verify that it will work for your GPS before purchasing ;)

    Regards, Scott

    Phil B

    5 years ago on Introduction

    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.

    2 replies
    scdPhil B

    Reply 5 years ago on Introduction

    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 Bscd

    Reply 5 years ago on Introduction

    Thank you for the response. Your circuit is more sophisticated than mine and you showed me I have some things to learn. Thanks.