The PSP Power Brick

40 Hours of Play for those Long-Haul Flights

Introduction

Recently my brother, Cake, attended a conference in Australia (we live in the UK). This involved a 22-hour flight in each direction.

To prepare for the trip, we looked online for the biggest most long-lasting PSP battery pack we could find. Imagine our disappointment! We couldn't find any that went beyond about 8 hours of play time. This is no use on a 22-hour flight.

I found myself wondering why they didn't just make a PSP battery pack that could take normal batteries that you can buy at any airport. Sure, it would cost a bit to run on Duracells, but it would solve the problem of PSP power on a long flight once and for all. I seemed to recall that Duracell 'D' cells store about 12,000mAh (compared to the PSP internal battery's puny 1800mAh). And thus, the concept for the PSP Power Brick was born.

Initial Design Phase

First we looked at the PSP's recharger. We would be plugging into the same socket in the PSP as this, so we had to know how it was rated. It says 5V, 2A so it's a 10W power supply. That's quite a lot of power!

Duracell have some nifty documents about all their cells on their website (http://www.duracell.com/Procell/productdata/). The 'D' cell is the MN1300. The key graphs are reproduced below.

The graph on the left shows working life against current drain. The different lines represent the final voltage put out by the cell - so if you can handle only 0.9V your battery lasts longer than if you need at least 1.0V from each cell.

The graph on the right show voltage against time for a given current drain. Each line represents a different drain (it's hard to read - black = 2A, green = 1.5A, red = 1A, blue = 0.75A). This graph shows quite directly how the cell's output will change over its life.

Now, there was a big worry at this point. The Duracell 'D' cell can only sustain a 2A current for about an hour or two - although a 1A current can be sustained for 6-10 hours! So if the PSP really drew 2A, we'd need to put cells in parallel to get reasonable life from them. The cells need to operate in a sweet spot of current drain in order to provide the 12,000mAh they are capable of.

But it's no problem to put some cells in parallel. Maybe 8 cells in groups of 4 giving 6V?

Unfortunately, a 7805 voltage regulator needs at least 8V across it to generate 5V output. So the battery we build from these 'D' cells must be at least 8V. So we're talking at least 12 cells in groups of 6 to give 9V.

However, cells will show a voltage drop as they lose power. If we have 6 cells giving 9V then each cell only has to lose 0.167V before the total voltage drops below 8V. At this point, our power brick will fail to provide 5V and we have reached the "dead battery" stage. From the charts, even at a 1A current drain it only takes an hour for the voltage to drop to 1.3V.

If instead we have a 12V battery (8 cells in series) then we get 8V output as long as each cell provides 1V. This increases the battery life from an hour to six hours using Duracell 'D' cells so we're getting there.

This illustrates that the design of a power supply like this is a balancing act:

In fact, if the PSP really draws 2A, it seems that a power supply based on 'D' cells is impractical

Fortunately, it doesn't!

Measurement Phase

A few questions needed answering about the PSP:

To get started I knocked up a simple 5V power supply from 8 Duracell 'D' Cells wired in series. This simply used a 78S05 voltage regulator (5V, 2A rating) to supply 5V from the 12V provided by the batteries. I put a big heatsink on the 78S05 (rated at about 6 degrees C per Watt). I wired a 2.1mm power plug to this circuit to connect to the PSP, and placed a multimeter in series with the battery.

After a lot of double-checking (I didn't want to kill my PSP) I finally had the courage to take the PSP battery out, plug my power supply in, and switch on ...

I was rewarded by the PSP powering up with its little tune, booting GTA from disc, and running the FMV. Superb!

What was even more superb was the fact that the current drain - once the game had loaded and was more or less "quiescent" - was only about 300mA. A far, far cry from the rated 2A of the power supply. This makes sense since the power supply must be able to handle recharging the battery while running the CPU at full speed with the disc spinning and the WiFi enabled, but in normal use the battery is not recharging, the CPU is not at full speed, the disc is not always spinning and the WiFi may not be enabled.

I played around a bit with doing various things in the game and in the PSP's front end. What amused me was that the PSP's front end uses 400mA of power while an actual full-blown game (GTA) takes 300mA. Go Sony!

ActivityCurrent
Charging Battery (switched off)840mA - 15mA
Charging + Play750mA
Quiet part of game300mA
PSP Crossbar UI400mA
Online in browser (WiFi active)500mA
Loading data from UMD300mA - 400mA
Cost of light80mA

I also looked at how the battery was charged. I drained my internal battery and then connected the power supply. The initial charging current was 840mA. As the internal battery was charged, the charging current dropped - apparently linearly. Eventually the charging current reaches around 15mA and then charging is complete. (If the PSP was swithced on during charging, the current usage actually drops - at this point the PSP goes to "trickle charge" mode instead of "full charge" mode and diverts most of the input power to the game. The PSP never seems to draw more than 840mA of current.)

I also attached a meat thermometer to the heatsink to measure the heat rise after a few hours.

Note that when charging the PSP's internal battery the temperature went up to around 110F (from 65F ambient - a 45F rise or 25 degree C). This demonstrates the need for such a large heatsink - if you build one of these, please do NOT skimp on heatsinking. This is especially important in view of the fact that the device may be used on a plane.

The one thing I did not investigate is how the 7805, and the PSP in turn, responds to a low battery. I'm guessing the 7805 will output about (V - 3) when V is less than 8V. The PSP asks for 5V but its internal battery is only 3.6V so presumably it runs on 3.3V or less internally. It may be possible that the PSP could run from less than 5V at input - this would extend the battery life of the Power Brick if true.

Based on the power figures, a 300mA drain is very low for a set of Duracell 'D' cells and the expected battery life might be 30+ hours (it's hard to tell from the chart). Assuming a little leeway in the input voltage to the PSP, a set of 'D' cells might last up to 40 hours. Superb.

I had also determined the best way to use the Power Brick is with a battery installed (preferably a fully-charged one). This allows you to play up to the limit on the power brick and then the PSP should switch smoothly to the internal battery once the power brick fails (or you can turn the power brick off to switch to the internal battery at any time). With the internal battery the PSP can also be placed into "sleep" mode without being attached to the power brick - handy when changing flights!

Practical Features

Maybe it would be nice if the PSP Power Brick told you when its batteries were drained? A simple expedient would be to measure the battery voltage. If it dropped to near 8V then we should light a red light; if it's more than (say) 8.4V we should light a green light; if it's in between maybe we should light an amber light.

To implement this I used a PIC 12F675, programmed using PICkit1. The program is foolishly simple, but replaces (what would be) a large amount of discrete electronics. The sense voltage is obtained using a voltage divider to divide by three so the output is in range of the ADCs on the PIC (which is run from the 5V supply). The output LEDs take the physical form of a two-wire bicolour (red/green) LED.

Finally, since lighting an LED takes power (around 15mA) I added an on/off switch for the power brick as a whole. (It's amusing to note that the 15mA draw is 5% of the PSP's requirement, so you might end up losing two hours of play time because of this LED!)

The final circuit diagram is shown below. To avoid mistakes note that the PIC pin layout is mirrored. The capacitor C1 was only 0.1uF in my design but this caused the PIC to crash sometimes as the PSP was plugged in (presumably there is a momentary dip in voltage). To avoid this I recommend using a much larger capacitor e.g. 47uF, but some experimentation should be done. You can also reduce the value of the LED series resistor if you want a brighter LED that uses more power.

The parts are all available from Maplin (something of a miracle, I must admit) including the PICkit1 if you need it - but DigiKey are cheaper if you can navigate their site. The cost of parts from Maplin is around 10 per power brick. Remember in addition to the parts in the schematic you need a heatsink, a box, a battery holder for 8 'D' cells, a tiny area of veroboard, a couple of meters of speaker cable and a 2.1mm power plug.

The code for the PIC is available as C source code for the Hi-Tech PICC Compiler, or as a .HEX file for direct download to a PIC 12F675. The program can be tested on the PICkit1 using RP1 to set the sense voltage and reading the results from D0 and D1.

[C Code] [HEX File]

The code simply sets the PIC up, and uses a timer to run a simple state machine to switch the red and green LEDs on alternately. An A/D conversion is read on every timer interrupt to determine the colours to use.

Note that the code is not brilliant - in particular there is no hysteresis on the LED colours so at close to the switching points the LED will flicker between colours. Depending on your point of view, this is a brilliant feature or a rubbish feature.

Construction

I constructed the circuit on Veroboard in two parts - the power convertor and the voltage monitor. The power converter is a tiny board which sits under the 7805 and the whole lot is bolted to the heatsink. The monitor is on a seperate board. All of this is relatively straightforward.

Cake and I went to the Maplin shop to choose a box. This was challenging - we decided not to enclose the entire battery holder because we couldn't make a good lid for that, and we wanted the purpose of the power brick to be explicit (since we aimed to take this through airport security). However the box for the circuitry had to be large enough for the heatsink! We eventually settled on a flanged box which was a little wider than the 8x 'D' cell holder, and about 2/3 the length.

The heatsink was mounted on a polystyrene insert I made from 2mm modelling plastic. The insert is fixed using epoxy. I'd like to say that this directs the heat output to the vent holes on that side of the box, but really it's because I was too stupid to figure out that if I reversed the circuit board layout the heatsink could have been mounted directly to the side of the box.

The tiny power convertor circuit can't easily be seen (it's under the green wires) but the voltage monitor circuit is larger and can be seen side-on. The LED protrudes through the box - this is near the back (left) of the picture. A small polystyrene insert holds the circuit board vertical and foam keeps it up against the box side. Finally the switch is mounted on the left (top) and the power output goes through a grommet at right.

I mounted the flange plate onto the battery pack using epoxy, but also drilled holes to allow the box top to be bolted through the entire assembly (the bolt heads protruded slightly into the battery pack but didn't foul the cells themselves). I then routed a couple of slots out for the power wires. I covered the power connections (solder tags) and the wires with two layers of 2mm black art foam. This conceals the wiring and makes the thing look a lot more professional. I also covered the bottom of the unit with foam - this conceals the exposed metal on the battery holder and provides a neat, non-slip base for the unit.

Once assembled, the resulting power brick looks a bit like a troop transporter. To reinforce this, I added a single LEGO 1x2 plate so that any LEGO "driver" figure can be placed on the unit for aesthetic appeal :-)

And did Airport Security allow it on a plane?

Yes, they did! The PSP Power Brick was allowed onto both of Cake's flights to Australia. By all accounts it performed admirably.

Future Work

The 7805 is wasteful of power and needs a high input voltage. As far as I can tell, the 7805 basically acts as an "active" variable resistor to give 5V across the load but with the same input and output current. In other words, the current drawn from the battery (at 12V) is the same as the current drawn by the PSP (at 5V), so the heat output from the 7805 at 12V is up to about (12 - 5) x 840mA = 5.9W. This gives a large temperature rise in our heatsink (I measured 25 degrees C), and we've got a big heatsink! Note that the PSP in this case is only using 4.2W of power so in fact more than half the energy from the batteries is wasted as heat.

A more efficient power convertor would emit less heat and therefore draw less current from the battery than was used by the PSP. This would therefore approximately halve the current drain and potentially more than double the battery life.

The PIC I am using is probably capable of running a much more complex power supply. For instance, a switch-mode power supply could be used to step down from 12V to 5V, and could in theory work down to a battery voltage of around 6.4V. Once we hit this voltage, it would be feasible to use a voltage-doubler to create 10.4V (and downwards) from which we can still generate 5V until the battery hits 3.2V (0.4V per cell, although the current drain is doubled in this case so it may only add a few more hours). Of course, all this is a lot of work compared to just using a 7805.

It would be nice to provide different output voltages too, and perhaps changable leads for the DS, your mobile phone, camera, PDA, etc. More or less any small device can at least be recharged with a system like this.

The current design contains a simple voltmeter. A simple and attractive additional feature would be an inbuilt ammeter. I've sketched out a design using a PIC - you place a 0.1 ohm wire-wound resistor in series with the PSP and measure the voltage drop across it. For a 0.1ohm, there will be a 1mV voltage drop for every 10mA of drawn current - 100mV for a 1A current. Using the PIC's ADC it would be easy to measure this voltage against, say, a 250mV reference, and light up a 10-LED bargraph display to show currents in 100mA increments up to 1A. It would be fun to watch the power requirements change as you used different PSP features (e.g. WiFi, changing the backlight - even turning the speakers up and firing a gun draws more power!) Also this would give you a battery usage indicator - more bars lit means less battery life. This would allow the user to optimize his settings to get maximum playtime.

Final Specifications

When loaded, the PSP Power Brick weighs about the same as a brick and is about the size of the box that the AirPort Express comes in. It provides 30-40 hours of play time from eight Duracell 'D' cells - enough to travel all the way around the world without any external power. The components cost about 10 (not including batteries). A set of batteries also costs about 10 but this compares very favourably with the cost of a first-class upgrade.

Conclusion

Cake and I hope that this article has been both informing and amusing.

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