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Charging iphone 'charger' from car battery


Dec 19, 2012
Dec 19, 2012
Bill of Parts

L7805 voltage regulator (L7800 series)
Heatsink for L7805
10uF 100v electrolytic capacitors x 2 (could be any voltage value over say 15v)
1 x 104 Ceramic Capacitor
12v battery
Micro USB Cable

Cut the Micro USB cable in half. Strip the wires down so you end up with only the red wire (+ve input) and the black wire (-ve ground). Same applies if you want to use the standard USB end of the cable.

The voltage regulator integrated circuit (with the 3 pins) will churn out a smooth precise 5 volt output pretty much regardless of the voltage you feed into it. Two main rules apply. First, always check the datasheet to find out what the maximum voltage parameters are, and what the dropout voltage is (of which more momentarily), and secondly, bear in mind that the higher your input voltage (compared to the output voltage), the more heat will be generated in the conversion process. Think heat-sink. Different types of voltage regulators churn out different though precise voltage outputs, so for example an L7806 would provide a smooth 6 volt output, and an L7812, a 12 volt output.

Dropout voltage is always shown on the datasheet. It means the margin by which the input voltage must exceed the regulated output voltage. If the dropout voltage is 2 volts, then you must supply a minimum of 7v to the voltage regulator integrated circuit in order to get a precise 5 volt output. The dropout voltage of the L7805 is about 1 volt, so in order to get a 5v output I had to supply a minimum of 6v input to the device, else the output would be below 5v and that would defeat the objective of building a smooth 5v output power supply.

The capacitors smooth out ripples and pretty much guarantee a rock solid 5v output (my output was 5.02 volts, which is within the margin of error of the digital multimeter I was using).

In any event, connect the red and black wires (forget about the green and white data wires when you are making a power supply) of your stripped USB cable to your breadboard as they will carry your regulated 5v supply to your external an iphone solar charger (not an iphone...I am charging an external 'charger' here, not an iphone, though you could just as easily make a power supply for an iphone in this way too).

Take your L7805 voltage regulator and insert it into a heatsink. You will need a heatsink because heat dissipation from the voltage regulator depends on the voltage difference between your input voltage (here 12v) and your regulated output voltage (here 5v) multiplied by the current in amps. 12v - 5v x (I) (current) = probably much more than 2 watts so I need a heat-sink even if I am not sure precisely how much current will be involved (300 milliamps was my best guess, in which event the heat dissipation would be about 2.1 Watts).

Stick your voltage regulator on the breadboard...the centre pin is the (common) ground, common to both input and output.

As the voltage regulator IC faces you (the heatsink being on the back of it), the pin on the left is the input voltage pin (+12v in), and the pin on the right is the regulated +5v output pin.

You will need two 10uF electrolytic capacitors that can handle more than 12v (minimum 15v needed where a fully charged car battery is probably in or about 13.7v...but I had 100v 10uF caps in my parts bin and they were fine.

Connect the positive (longer) pin of the first 10uF cap to the 12v input voltage pin of the voltage regulator IC (on the left). Connect the positive pin of the second 10uF cap to the 5v output voltage pin on the right.

Connect both negative terminals of each of the 10uF caps to the common central ground pin of the voltage regulator (the pin in the middle of the voltage regulator IC).

Then connect a 104 ceramic cap between the 5v output pin and the common ground central pin in the middle.

Next up, connect your 12v car battery (+ve red cable) to the 12v input terminal on the left of the voltage regulator IC... and connect the black -12v cable to the common ground (the middle pin) of the voltage regulator.

Then connect the 5v output of the VR IC to the red wire of your micro-USB and connect the black wire of the micro-USB to the common ground (use a separate ground wire even though it originates from the common (central) ground pin of the voltage regulator).

You will get a perfectly regulated 5v output and can then power 5v external appliances (such as an iphone charger) from a 12v car battery.

Note that the heatsink I used was too small. It overheated fairly quickly when connected to the 12v car battery, so be sure to use a larger heatsink than I used to avoid melting your breadboard.

Also, when I tested this power supply using a bench power supply to furnish a 12v input, the heat dissipation was minimal. However, much more heat was generated from the heatsink when I used the actual car battery. I think this was for two reasons. First, the voltage of a fully charged car battery is probably about 13.7 volts and secondly the current in amperes was no doubt quite high.

In contrast, I had set my bench power supply to precisely 12v and the current to 300 milliamps, hence less heat dissipation from the bench power supply version.

You can build a power supply for pretty much any device (using an appropriate voltage regulator) in this way.

Photos attached :D


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  • LM7805 + Heatsink.jpg
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