This is an old revision of the document!
I'd like to share just another idea of how to modify the Panasonic DMW-DCC8. Inspired by this thread and a thread next door as well as encouraged by @feha's tutorial I started my modification.
My post is divided in two parts: Part 1 is about the concept. Part 2 is about the implementation.
Part 1: Concept
I will try to give you a straight forward description of my doing but it actually was a very iterative process with countless loops.
My goals have been the following ones:
- build an adapter which can be used with almost every battery, preferably one of those Lithium-Polymer-Batteries for RC Cars, Helicopters & Co. - adapter must deliver the same voltage which is delivered by the original adapter in combination with the original power supply - usage of a highly flexible cable to be able to get small bending radii and to unload the adapter and therefor the camera when mounted on a support rig - modification of high quality and durability, it must look nice and you don't have to care about it in everyday life
I picked up the idea to integrate a step-down converter right into the adapter (http://forums.planet5d.com/threads/119993-Panasonic-DMC-GH2-Power-Mod).
I've decided to start with the original coupler by Panasonic because it has this little circuit board with two resistors inside. I've got no idea what these resistors are for, but I thought it's better to keep their functionality. The original coupler is also a good starting point for the housing because it is basically an empty case.
Then I searched the web for a step-down converter which is small enough to fit into the housing. Later I saw that Vitaly has already found the same small converter (http://www.personal-view.com/talks/discussion/3957/voltage-step-down-module/p1). The dimension is declared with 33 mm x 33 mm x 12 mm. But is it small enough? Unfortunately not! The adapter housing is 32.2 mm wide inside at the narrowest position. Damn! But if you look at the pictures of the step-down converter there is still a little bit room for reducing the size of the circuit board without damaging the electrical parts on the board, hopefully. The hight should be enough even if the 12 mm out of the converter description don't include the pins for soldering. I gave it a try and ordered such a converter. The exact dimension of the converter turned out to be 33.2 mm x 33.2 mm x 15.4 mm (from top of potentiometer to bottom of pins for soldering) and may vary from item to item. So it's going to be hard to integrate this circuit board into the adapter housing. Even the height is critical. See picture “adapter inner dimension.png” for the inner dimensions of the adapter housing (not to scale).
The heart of the converter is an integrated circuit named LM2596. See file “LM2596.PDF” for some detailed technical data.
A lot of discussions existing about the efficiency of such a converter. You should have a look into the data sheet on page 4. There is a nice chart called “Efficiency”. This chart is measured based on a layout shown on page 7. I can't recognize if the converter which I have chosen has the same layout. But I guess there is not much difference. Interestingly the efficiency goes up with higher input voltages up to around 25 V. So if you go like me with a Lithium-Polymer-Battery pick up rather a 5S-LIPO than a 3S-LIPO even if the minimum voltage of a 3S-LIPO is still around 1 V above the voltage we need (when you shut off at 3.3 V per cell), see also chart called “Dropout Voltage” on page 4.
To make sure that the converter becomes not a heater just a rough consideration: The GH2 average power consumption is about 3.4 W at 8.4 V according to the manual which means it draws about 400 mA. This current draw could be validated by the guys at dvxuser. Additionally it was measured by the guys at dvxuser that the GH2 draws up to around 800 mA in some situations for a short time (camera turn on, after using flash light). That means for me that the converter is perfectly designed for this application and that there is no risk of overheating because this little piece is designed for up to 3 A.
The maximum current is also the key data to find the right wires / cable. I've used two approaches to calculate the minimum wire cross-section:
1) Current density
The maximum current density for copper is estimated with 4-6 A/mm2 (empirical or practical value I guess). The formula to calculate the cross-section is: cross-section = current intensity / current density. For this application it means: 0.8 A / 5 A/mm2 = 0.16 mm2.
2) Voltage drop
The maximum voltage drop should be small and is generally considered with 3-5 %. But that is too much! 5 % of 8.8 V is 0.44 V. The voltage would drop below 8.4 V and the camera will display the message “This battery cannot be used”. By the way, 8.8 V is the measured output of the original Panasonic power supply for the original Panasonic DC coupler. The voltage may drop from 8.8 V to 8.4 V. That is about 4.5 % voltage drop. Let's take 1% and the following formula to calculate the cross-section: cross-section = (current intensity x specific resistance of copper x 2 x length of wire) / voltage drop. For this application it means: (0.8 A x 0.0172 Ω mm2 / m x 2 x 0.05 m) / 0.086 V = 0.016 mm2. So the voltage drop is no issue because the wires are just too short.
At the end I've decided to use a wire with a cross-section of 0.25 mm2! The wires of the original Panasonic adapter are rated AWG20 = 0.5 mm2. But I think the decision for 0.5 mm2 was not only driven by the current intensity but also by the cable thickness. It's a dual cable which means it's a cable with two wires without an additional cable coating. If Panasonic would have chosen a 0.25 mm2 wire then the cable would be simply too tiny to withstand everyday life.
Now I had to look for a cable which has two 0.25 mm2 wires. I've found a cable, see bill of material in part two. And the coating of this cable is PVC which means it is highly flexible. It almost feels like rubber or silicon cable coating. The outer diameter is 3.9 mm. Finding the right plug was quite easy, see bill of material in part two. The plug is rated for 4 A. Enough! And the sleeve support has a diameter of 4.0 mm. So the selected cable fits perfectly into the cable sleeve.
The most difficult part was to find a solution to attach the cable on the adapter housing. There is already a big hole in the adapter housing. Much too big for a cable with 4 mm outer diameter. My first idea was to use a grommet to fill the hole nicely. But there is no space. To solve this problem I desoldered the sockets from the step-down converter. Then the next problem. How to secure the cable so it does not become loose or even worse the wires tear off the circuit board when you pull accidentally on the cable? I developed several ideas with a clamp, with glue and with shrink tubings but nothing met my goal of high quality. Finally I decided to design my own parts, see picture of concept drawing “sheet metal and cable junction.png”. This solution basically consists of a sheet metal and a specially designed cable junction. The sheet metal will be placed inside the adapter housing similar to the circuit board. And the cable junction will be the bridge between the inside and the outside of the adapter. With this cable junction I'm able to bend my cable by 90 degrees in a small area because inside this cable junction only the two little wires are routed without the cable coating.