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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
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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

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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 :)


dog8127 days ago

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?

scd (author)  dog8127 days ago
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.

Regards,
Scott
dog812 scd7 days ago
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?
http://www.amazon.com/20000mAh-Aluminum-Portable-Multi-Voltage-Technology/dp/B005NGLTZQ/ref=sr_1_1?s=electronics&ie=UTF8&qid=1428866468&sr=1-1&keywords=anker+9v
scd (author)  dog8127 days ago

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 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.

http://www.instructables.com/id/Quartz-Clock-Power-Supply-Hack-AA-battery-to-AC-p
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.