Variable power supply DC questions

[edy]

Hi folks,

I picked up this variable DC supply from China eBay for under $3 with free shipping. Took about 3 weeks to get here, but I'm excited to be able to use it on my point-and-shoot camera which I programmed for interval shots (for time lapse photos).

I have a few questions though before I blow up my camera. I am using a small Canon Powershot which has no external power hookup. It relies on a battery 3.7V or you can make a battery hack which essentially they use in stores (when on demo displays) which supplies a DC source through a fake battery with wires which come out the back and pop out of the bottom of the camera.

The DC variable supply seems to take a broad range of inputs. I want to be efficient to ensure nothing heats up too much, but at the same time need 3.7 V output. Current I believe is quite high rated on this device but I assume my camera should not draw more than it needs.

Once I have it attached, I can set up the Canon with CHDK and set it up for long time lapse sequences and only be limited by the memory card. Before I would drain the battery quite fast and the camera would get hot.

Any suggestions? The schematic could be derived hopefully easily from the picture as the PCB layout and parts are easily visible. Hopefully someone can decode what is going on in this relatively simple variable DC voltage supply. Thanks!

[edy]

Sorry... Forgot to mention, it apparently takes 5V-35V in and outputs 3V-33V out. It is a "Buck" type. Specs say for stable output should have minimum 1.5V differential between in and out. I assume that means if I want 3.7V out I should give it at least 5.2V in.

I will likely find an AC mains wall transformer from some other gear that pushes out 7-12V to use, as long as it has sufficient amp. The device can safely push out about 2A it says. So ultimately limited by my camera on what it draws...  as long as the wall transformer is high enough should be ok. Or is there also an effect on current supplied in versus max current able to ouput?

Parts look like 2 caps with an inductor in between, a variable resistor and a few fixed ones, and finally an LM2596S which is a step-down switching regulator.   


I just found this document which I'm trying to learn from:

http://www.jaycar.com.au/images_uploaded/dcdcconv.pdf

[edy]

I hooked up an AC wall adapter rated for 8.5 V at 1.7A and measured on my multimeter at 8.68 V to the Buck converter. At first the output on the Buck was still close to the input... Around 8.58 V or so, even after I turned the variable resistor screw a lot! I thought maybe it was broken!

But I just kept turning the screw counter-clockwise several more turns and eventually the output started to budge! It started to go down and I was able to tune it to 3.7 V, as you can see by the picture attached. I didn't know the variable resistor needed so many turns, but I can control it within a tenth of a volt easily.

I do notice some "drift" every time I power on and off, the voltage is out slightly even without any adjustment of the screw. But for my purposes and tolerances I assume it should not affect the camera much.

[peter.mitchell]

While it is stated for a 3A total power output, it will not meet that current without over heating at any significant voltage. It should be able to supply about 7-10W continuously just fine though, which is easily more than enough for what you need.

I assume you're using a canon DSLR, from your description, and these all use lithium technology batteries, and im sure you know, when a battery is fully charged and when a battery is empty, the voltages are different, so the camera a should work with the input range of approximately 3.3v-4.2v, I would keep the input voltage (to the camera) towards the higher side, as cameras use power in a fairly non-continuous manner, so a sudden load may cause the input voltage (to the camera) to drop.

You should be able to find a very cheap replacement battery for your camera from ebay or the like, you can gut the battery and use it for the contacts, so you can nicely attach the external supply. I would suggest you attach the supply to the small PCB inside the battery pack where the battery terminals attach, and remove the battery, as the cameras have some circuitry that monitors battery health, and you may need this PCB to "fool" the camera into thinking it is OK.

I assume from the battery life of my DSLRs that they use a very small amount of current when idle, without the LCD on, somewhere in the order of 10ma, however I do not know their power consumption when using alternative firmwares, but I would assume it is no more than 500ma.

Assume that the buck converter is terribly inefficient (say, 50%) and that works out at something like 4W of required power for your 4v at 500ma, your 8.5v 1.7A "wall wart" is well capable of supplying this.

The variable resistor is a 10-turn trim pot, used to make fine adjustments, over the range of 3-33v that means there is about 1 full turn per 3 volts and since your desired voltage is at the far end of the scale, I am not surprised it took a large number of turns.

[edy]

Thanks so much for your help!

I've attached photos of my camera battery... It has 3 contacts and I made a "dummy" battery out of acrylic which I poured into a putty mold I made of the battery (I have access to these materials for free). The acrylic hardened and after a bit of sanding it down, it fits perfectly into the camera. That is the yellowish thing you see. My camera is in the background.

So as you can see I taped a flat-flex cable (which I gutted from an old printer) on the end of the battery and slipped it into my camera and was able to power it on with just using the 2 contacts at each end labelled + and - on the original battery, supplying it the 3.7V from the Buck converter mentioned above. I am not sure what the middle contact does on the battery.

[edy]

I've attached a couple more photos showing the setup.

Next I have to solder all the bits together so it is easy to use and won't fall apart in transport. I am still using this camera as my "daily driver" with battery reliance, but it is nice to have this option for special shots like interval timing a full day or even weeks of photos for time lapse movies (I use ffmpeg to stitch them together automatically and step away from my computer for a few hours and when I return the MP4 is ready)!