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Custom external power supply - plans and ideas

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Offline gtrecordings

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Custom external power supply - plans and ideas
« on: 19 / November / 2008, 22:07:45 »
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I have an sd550 it uses a 3.7v battery and has no external power option.  I was looking at the bottom of the camera and noticed a rubber cover on the battery door. 

My plan was to make a wooden block the size of the battery and run wires from the power contacts in the camera out the whole in the door.

I noticed that the battery has 3 contacts, +, T, and -.  My idea is to make two of these blocks, one for the camera and one for the charger. Then I would put connectors on either end for cable extensions.

Idealy, i would just make a bigger battery pack, but i don't know how to wire up the contact with the T symbol.

Is that the ground?
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Offline ewavr

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Re: Custom external power supply - plans and ideas
« Reply #1 on: 20 / November / 2008, 05:44:09 »
Quote from: gtrecordings on 19 / November / 2008, 22:07:45
Idealy, i would just make a bigger battery pack, but i don't know how to wire up the contact with the T symbol.
Is that the ground?

T=thermistor (usually) to measure battery temperature:
http://www.electronics-lab.com/articles/Li_Ion_reconstruct/
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Offline PhyrePhoX

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Re: Custom external power supply - plans and ideas
« Reply #2 on: 20 / November / 2008, 06:06:07 »
Quote
I have an sd550 it uses a 3.7v battery and has no external power option.  I was looking at the bottom of the camera and noticed a rubber cover on the battery door.

well, this rubber cover IS for external power adapters. for the small ixuses you get an AC together with a "fake battery". the cable then is led through this rubber cover. the canon acs are very expensive though.
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Offline gtrecordings

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Re: Custom external power supply - plans and ideas
« Reply #3 on: 20 / November / 2008, 20:50:31 »
thanks for the diagram.. i am wondering if target/bestbuy/walmart booths has tons of those things laying around... maybe getting used one that way could be an option.

I wonder if i ran 3.5v to the camera and bypassed the temp circuit altogether.  My camera is pretty trashed already, so I am willing to experiment. If this is reasonably safe to do, i think i might try it.

I wonder how i could recreate the temp fuse...

I thank you guys for responding, you have both been very helpful.

gt
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Offline gtrecordings

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Re: Custom external power supply - plans and ideas
« Reply #4 on: 06 / December / 2008, 23:28:07 »



I just completely bypassed the temp circuit and it powered up with 4.5v and shot a timelapse for about an hour.
It go kinda warm, but nothing harmful.  I plan to lower the voltage with a volume pot, or a resistor if i can find the right one.

The fake battery has two halves.. the picture shown is the middle.

more updates soon.
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Offline fudgey

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Re: Custom external power supply - plans and ideas
« Reply #5 on: 07 / December / 2008, 05:20:41 »
You may know this, but you can enable battery temperature reading visibility on CHDK OSD menu to check your resistor choices. Many cameras don't have all 3 sensors, though (I believe in that case they give equal readings).
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Offline fbonomi

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Re: Custom external power supply - plans and ideas
« Reply #6 on: 07 / December / 2008, 05:35:52 »
Quote
I plan to lower the voltage with a volume pot, or a resistor if i can find the right one
I think that's a bad idea!
I am not an electronics guru, but a resistor will heat, and decrease its resistance. This might go on and on until it "melts"

Use a voltage regulator instead.

there are a few threads about this, for example:
external Battery - connected to DC-IN

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Offline pelrun

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Re: Custom external power supply - plans and ideas
« Reply #7 on: 07 / December / 2008, 18:05:44 »
Resistance goes *up* with temperature, not down, so you're safe :)

But you definitely want a regulator instead of a single resistor or pot, as the voltage drop across the resistor will vary with the current draw from the camera.

But since the camera hasn't died with a 4.7v input, it's probably already got some voltage regulation hardware already.
« Last Edit: 07 / December / 2008, 18:10:09 by pelrun »
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Offline fbonomi

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Re: Custom external power supply - plans and ideas
« Reply #8 on: 08 / December / 2008, 01:35:30 »
oops, I stand corrected: I told I wasn't a guru :-)

All I remembered was along the lines of "regulating voltages with a resistor makes Santa Claus cry" ...
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Offline hunter99

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Re: Custom external power supply - plans and ideas
« Reply #9 on: 08 / December / 2008, 13:14:51 »
Use this

Zener diode:
 Also sometimes called avalanche
diode or Zener. A semiconductor diode (usually
silicon) specially fabricated to take advantage of
the effects of avalanche breakdown. It is available
in a wide variety of configurations and ratings; it
is commonly used as a voltage regulating device
in low-voltage power supplies.

George
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