I have been doing some experimenting with phone charging. Unfortunately there is no one charger fits all when it comes to phones but what is good is that today every company seems to be gravitating towards the USB charging protocol because phones are now expected to be able to be charged by computers. The silver bullet for phone charging is to simply have an inverter which will step up the deep cycle 12 V up to 115/240 V AC. Then you can plug in the charger that you bought with your phone and away you go.
The problem with this system is that you step up DC voltage to high AC voltage and then immediately bring it back down to even lower (5 V) DC. This is hugely inefficient and since we are working off solar panels and deep cycle batteries we are aiming to be as energy efficient as possible. So the solution to this is to drop the 12 V from the battery to the most standard possible phone charging protocol which seems to be the 5 V USB plug.
Something well documented on the internet is what pins do what for USB phone charging. Most phones just charge when pin 1 has 5 V and pin 4 is grounded. Pin 1 is the right most pin when you are looking into the USB socket and the protruding pin board is in the lower half of the entire socket. See the Wikipedia page to double check. For something as picky as iPhone however the middle two data pins actually make a difference. The iPhone will charge at 0.5 A if it connected to a computer. The way it detects this is by measuring the voltage on the middle two pins which should both be 2 V. If the iPhone is connected to the wall then is should measure 2.8 V on pin 2 and 2 V on pin 3 and then charge twice as fast at 1.0 A. Keeping this in mind a charger should be made which gives pin voltages 1-4 as 5, 2.8, 2, 0 V.
So what sort of circuit can be used to drop 12 V from the battery to 5 V for phone charging. The first thing that any electrical engineer will learn is how to construct a simple resistor voltage divider. You can workout your ratio of resistors very easily if you want them all in series by deriving the voltage division equation for each node and then reducing the matrix that they make. So for a voltage source across the dividing resistors of Vs, output voltage for a specific node of Vo, you can write down the equation Vo = Vs( Rfuture/(Rpast+Rfuture)) where Rfuture represents the resistance which your conventional current is yet to go through and Rpast is resistance which is the conventional current in that branch has already traversed.
Very quickly with some simple analysis you will notice something. If you attach a load to a voltage divide which is of considerably lower resistance than the resistors in your voltage dividing circuit then you have completely changed the voltage that each node is supposed to have. LTSpice is excellent (and FREE) circuit modeling software and so I double checked my hypothesis and as expected, adding a low resistance load (or a low resistance resistor) completely changed the intended node voltages. It is like shorting some of the resistors in your divider out of the circuit altogether.
So the other option, if you are set on going down the voltage divider path, is to make the resistors in the divider comparable to the load resistance. Since the max load of a phone charging is roughly 1 A for a 5 V source, Ohm's law will give you an effective resistance of 5 Ω. Making your divider resistors comparable to 5 Ω will leads to your circuit catching fire if you are using a deep cycle battery. A simple model of such a circuit is shown in the image below. (click to enlarge)
This circuit maintained the desired voltages on each node but these are extremely low resistances to be letting a lead-acid battery get into contact with. The plot of the node voltages and currents which are a result of this circuit is shown below and as you can see the total circuit current is over 7 A! This will destroy your circuit if your using the common 1/4 Watt resistors and furthermore that is a huge waste of power to be charging a phone at 1 A. We are running our phone chargers from a 35 Ah (amp hour) battery which means we can produce 1 A for 35 hours roughly. If we were to use the circuit above we would get around 4 hours of operation...which is useless. (definitely click to enlarge)
The problem with this system is that you step up DC voltage to high AC voltage and then immediately bring it back down to even lower (5 V) DC. This is hugely inefficient and since we are working off solar panels and deep cycle batteries we are aiming to be as energy efficient as possible. So the solution to this is to drop the 12 V from the battery to the most standard possible phone charging protocol which seems to be the 5 V USB plug.
Something well documented on the internet is what pins do what for USB phone charging. Most phones just charge when pin 1 has 5 V and pin 4 is grounded. Pin 1 is the right most pin when you are looking into the USB socket and the protruding pin board is in the lower half of the entire socket. See the Wikipedia page to double check. For something as picky as iPhone however the middle two data pins actually make a difference. The iPhone will charge at 0.5 A if it connected to a computer. The way it detects this is by measuring the voltage on the middle two pins which should both be 2 V. If the iPhone is connected to the wall then is should measure 2.8 V on pin 2 and 2 V on pin 3 and then charge twice as fast at 1.0 A. Keeping this in mind a charger should be made which gives pin voltages 1-4 as 5, 2.8, 2, 0 V.
So what sort of circuit can be used to drop 12 V from the battery to 5 V for phone charging. The first thing that any electrical engineer will learn is how to construct a simple resistor voltage divider. You can workout your ratio of resistors very easily if you want them all in series by deriving the voltage division equation for each node and then reducing the matrix that they make. So for a voltage source across the dividing resistors of Vs, output voltage for a specific node of Vo, you can write down the equation Vo = Vs( Rfuture/(Rpast+Rfuture)) where Rfuture represents the resistance which your conventional current is yet to go through and Rpast is resistance which is the conventional current in that branch has already traversed.
Very quickly with some simple analysis you will notice something. If you attach a load to a voltage divide which is of considerably lower resistance than the resistors in your voltage dividing circuit then you have completely changed the voltage that each node is supposed to have. LTSpice is excellent (and FREE) circuit modeling software and so I double checked my hypothesis and as expected, adding a low resistance load (or a low resistance resistor) completely changed the intended node voltages. It is like shorting some of the resistors in your divider out of the circuit altogether.
So the other option, if you are set on going down the voltage divider path, is to make the resistors in the divider comparable to the load resistance. Since the max load of a phone charging is roughly 1 A for a 5 V source, Ohm's law will give you an effective resistance of 5 Ω. Making your divider resistors comparable to 5 Ω will leads to your circuit catching fire if you are using a deep cycle battery. A simple model of such a circuit is shown in the image below. (click to enlarge)
This circuit maintained the desired voltages on each node but these are extremely low resistances to be letting a lead-acid battery get into contact with. The plot of the node voltages and currents which are a result of this circuit is shown below and as you can see the total circuit current is over 7 A! This will destroy your circuit if your using the common 1/4 Watt resistors and furthermore that is a huge waste of power to be charging a phone at 1 A. We are running our phone chargers from a 35 Ah (amp hour) battery which means we can produce 1 A for 35 hours roughly. If we were to use the circuit above we would get around 4 hours of operation...which is useless. (definitely click to enlarge)
Th next post will be about better approaches to phone charging and after testing what is probably going to be used in mini power node.
No comments:
Post a Comment