The better method of dropping the voltage from a 12 V battery to 5 V for phone charging to use a 5 V voltage regulator. The circuit shown below does this. To put 2.8 V and 2 V on pins 2 and 3 respectively a resistor voltage divider can be used because very little current is drawn from them. The phone uses these pins to know if it is plugged into a computer or a wall charger and so only weak strength is required. R5 in the circuit corresponds to an extremely high resistance circuit that the phone would use to measure the voltage of the data pins. The load resistor corresponds to a phone.
It is important to note that the circuit diagram and simulation uses a LT1086-5 voltage regulator which is not what we actually used. From Radioshack we bought the 2 LM7805C ($2 each) regulators which are a more general purpose component. The plot below corresponds to the simulation of this circuit which shows that the the nodes have the correct voltages and that only 1 A is being drawn from the battery. Since 1 A is drawn through the regulator and 7 V are being lost there is a power dissipation on the regulator of 7 W. This means it gets very hot very quickly which is a loss of power but there does not seem to be many ways of getting around this effectively. We actually put two of these regulator in parallel for testing to dissipate the heat better.
This 1 A is essentially the maximum current that a phone will use to charge and to my knowledge only the iPhone uses this much current. It is more likely that in our project we will encounter older (non-smartphone) Samsung and Nokia phones which usually do not use more than 500 mA. The charger for the Samsung Intensity U450 is rated an 800 mA output but upon testing it seemed that the phone actually only uses 415 mA to charge. The photos below shows the small charge monitoring circuit which we built. It is simply a USB cable connected to a phone charger which feeds its 4 pins into a bread board. We can then make measurements of current and voltage while rerouting the pins into the phone for charging.
The voltage regulator circuit was quickly tested on a bread board and was able to test below. The photo below shows the initial circuit.
While this circuit works and it was necessary to learn about how phone charging is done to day there is an alternative way to charge phones. It is very easy to buy small phone chargers designed for cars which therefore run on 12 V. These devices usually drop the 12 V to 5 V in a more efficient way too. They use larger a full PCB to drop the voltage rather than one integrated circuit. We are considering using the charge featured in the photo below and it would be simple to modify the unit to use a cable and connector to plug into the node rather than the usual car socket. This device, although more expensive, "should" be less prone to breaking.
It is important to note that the circuit diagram and simulation uses a LT1086-5 voltage regulator which is not what we actually used. From Radioshack we bought the 2 LM7805C ($2 each) regulators which are a more general purpose component. The plot below corresponds to the simulation of this circuit which shows that the the nodes have the correct voltages and that only 1 A is being drawn from the battery. Since 1 A is drawn through the regulator and 7 V are being lost there is a power dissipation on the regulator of 7 W. This means it gets very hot very quickly which is a loss of power but there does not seem to be many ways of getting around this effectively. We actually put two of these regulator in parallel for testing to dissipate the heat better.
This 1 A is essentially the maximum current that a phone will use to charge and to my knowledge only the iPhone uses this much current. It is more likely that in our project we will encounter older (non-smartphone) Samsung and Nokia phones which usually do not use more than 500 mA. The charger for the Samsung Intensity U450 is rated an 800 mA output but upon testing it seemed that the phone actually only uses 415 mA to charge. The photos below shows the small charge monitoring circuit which we built. It is simply a USB cable connected to a phone charger which feeds its 4 pins into a bread board. We can then make measurements of current and voltage while rerouting the pins into the phone for charging.
The voltage regulator circuit was quickly tested on a bread board and was able to test below. The photo below shows the initial circuit.
While this circuit works and it was necessary to learn about how phone charging is done to day there is an alternative way to charge phones. It is very easy to buy small phone chargers designed for cars which therefore run on 12 V. These devices usually drop the 12 V to 5 V in a more efficient way too. They use larger a full PCB to drop the voltage rather than one integrated circuit. We are considering using the charge featured in the photo below and it would be simple to modify the unit to use a cable and connector to plug into the node rather than the usual car socket. This device, although more expensive, "should" be less prone to breaking.
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