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You are here: MacNN Forums > Community > MacNN Lounge > Electrial engineer types: why is my battery draining in my gadget?

Electrial engineer types: why is my battery draining in my gadget?
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xMetal
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Feb 5, 2007, 03:35 PM
 
So in a grand experiment, I built myself a one-off laser grip for one of my guns out of carbon fiber, titanium, and LazErz! It's works quite well, but I've run into a baffling issue that I simply can't figure out that's keeping it from being "done". I'm not an engineer or anything, so maybe I've missing something obvious.

Here's a pic of the grip, just for your entertainment.



Here is a pic of the basic wiring setup. It's about as simple as you could get.


The small laser unit takes a voltage (3v in this case, supplied from a lithium watch battery). I connected the negative to one post, and the positive I ran through a very simple switch. The positive and negatives just end up at contacts that touch the battery.

So, with a fresh battery it works great. It turns on when you press the switch (which came out of a cheap laser pointer btw), and turns off when you release. But if you let the unit sit for a few days, it seems to be slowly draining the battery. I tested and the fresh battery had 2.98v. After a few days, it dropped to 2.45v or so, which is not quite enough to drive the laser at full brightness.

What can I check here? I have a rudimentary understanding of these things, and have a multimeter. This is so simple, and since it operates properly I just can't figure out why it would be drawing power while off.

There's got to be some folks on here that are whizzes at this type of thing. Thanks for any tips guys!
     
G4ME
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Feb 5, 2007, 06:37 PM
 
insulate all the contacts, looks like there is a lot of metal around those solder joints, other then that get a new switch.

I GOT WASTED WITH PHIL SHERRY!!!
     
MaxPower
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Feb 5, 2007, 06:59 PM
 
Hey xMetal! I saw that gun in the Arstechnica lounge thread. I have nothing constructive to add except that MaxPower = Boomzilla over on arstechnica, so.. hi!
     
f1000
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Feb 5, 2007, 07:10 PM
 
Originally Posted by xMetal View Post
The small laser unit takes a voltage (3v in this case, supplied from a lithium watch battery). I connected the negative to one post, and the positive I ran through a very simple switch. The positive and negatives just end up at contacts that touch the battery.

So, with a fresh battery it works great. It turns on when you press the switch (which came out of a cheap laser pointer btw), and turns off when you release. But if you let the unit sit for a few days, it seems to be slowly draining the battery. I tested and the fresh battery had 2.98v. After a few days, it dropped to 2.45v or so, which is not quite enough to drive the laser at full brightness.

What can I check here? I have a rudimentary understanding of these things, and have a multimeter. This is so simple, and since it operates properly I just can't figure out why it would be drawing power while off.
As G4ME noted, I see some sloppy soldering there; check for shorts first. Secondly, you may need to add a resistor or, better yet, a voltage regulator to the circuit.
     
G4ME
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Feb 5, 2007, 07:19 PM
 
where did you find this lazer?

is it a simple LED? if so you will need a current limiting resistor, i am sure you could fit one in, you just need to know how much current it needs, then V=i*r

I GOT WASTED WITH PHIL SHERRY!!!
     
xMetal  (op)
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Feb 5, 2007, 11:34 PM
 
the laser is a prepackaged unit, with a power control chip (voltage regulator). It's self-contained and merely need to be fed a voltage between 2.5-3.3v. And if it matters, the metal that you see as the backer plate is titanium, not steel. Does epoxy transmit electricity? I did use a two part epoxy to mount the switch to the grip, and it is covering both contacts.

How would I best go about testing the switch? In the process of building this, I had some other switches that I didn't use, and had another laser unit hooked up to one of those for weeks without any drain that I noticed.

I also did remove the small copper contacts completely and currently have the copper wire, with a thin solder coating directly on the battery.

As I said, I'm not an engineer. I'm a graphic designer, but am juuuuuust smart enough to piece things together. I greatly appreciate any help.
     
xMetal  (op)
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Feb 5, 2007, 11:35 PM
 
Originally Posted by MaxPower View Post
Hey xMetal! I saw that gun in the Arstechnica lounge thread. I have nothing constructive to add except that MaxPower = Boomzilla over on arstechnica, so.. hi!
heh, it's always fun to see the same people in different places. I got lucky and actually have the same user name on almost every forum I visit, which is handy.
     
IceEnclosure
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Feb 6, 2007, 12:43 AM
 
I like the ingenuity. Nothing constructive to add.
ice
     
G4ME
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Feb 6, 2007, 01:17 AM
 
epoxy should be a decent enough insulator, but you have to make sure it is covering every contact/wire in the circuit because i think titanium isn't a great insulator

I GOT WASTED WITH PHIL SHERRY!!!
     
The Godfather
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Feb 6, 2007, 01:18 AM
 
Draw the circuit for us, and give more info about the regulator chip.
     
xMetal  (op)
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Feb 6, 2007, 10:28 AM
 
here's a quick drawing of the wiring. I included the spec chart on the laser. It doesn't say too much about the chip, just "APC (auto power control) IC inside, Low current consumption of the APC circuit."

The switch came out of a laser pointer, and that one had a total 4.5v going through it. As I mentioned before, the unit operates perfectly when the battery is at full charge, so I think the switch is OK.

     
DakarĀ²
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Feb 6, 2007, 10:38 AM
 
"why is my battery draining in my gadget"

sounds dirty.
     
G4ME
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Feb 6, 2007, 12:05 PM
 
see if you can toss in a 100-120 ohm resistor in line with that switch, that will bring the current down a little bit, although i imagine, the APC should limit the current going into the lazer, but they would have mentioned that in the documentation. Also any chance you can get another battery in there?

My guess is you were drawing enough current to drain the battery, even with little use.

I GOT WASTED WITH PHIL SHERRY!!!
     
xMetal  (op)
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Feb 6, 2007, 12:43 PM
 
Originally Posted by G4ME View Post
see if you can toss in a 100-120 ohm resistor in line with that switch, that will bring the current down a little bit, although i imagine, the APC should limit the current going into the lazer, but they would have mentioned that in the documentation. Also any chance you can get another battery in there?

My guess is you were drawing enough current to drain the battery, even with little use.
There really isn't enough room to do two batteries, unfortunately.

I put a completely fresh battery in, and verified that it worked at full brightness. I then let it sit for 2-3 days. At that point, the battery was quite weak, though I had not used it at all.

Is there anything I can test with a multimeter to see what's happening? Before I put it all together I did check for resistance going through the switch, and had various test rigs wired up for a while and don't recall seeing this problem. How would I check for a "short" somewhere?
     
G4ME
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Feb 6, 2007, 12:58 PM
 
short=0 resistance

that switch when "off" should be an "open" R=huge number where "on" shold be a short.

See if you can measure the current (put your DMM inline with the switch on current mode) supplied by the battery, if its >25mA or so you will need a current limiting resistor i mentioned.

I GOT WASTED WITH PHIL SHERRY!!!
     
TheWOAT
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Feb 6, 2007, 01:12 PM
 
I used to be an EE. Besides insulating connections and what not, I would just build the same circuit, but on a breadboard, and see if the setup you have still drains the battery. I would bet that there are some "things" touching that shouldnt be, and it that is draining your battery. Or, the battery you are using isnt suitable for this application.
     
   
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