Amperage Drop?

[Wolfman29]

Hello there,

I built a relatively small but simple LM1117 drive with a set current at (supposedly) around 378mA (3.3Ohm resistor). Now, I set it up in the host and everything and when I turn it on, the current starts out at 378mA, as it should. But after about 3-5 seconds, it drops to around 220mA.

This can't be normal. What's going on? Why won't it stabilize?

 

[Wolfman29]

I think I found the problem: when I blow on it while exposed to the air, it raises back to 380 or so mA, so I think it's a heat issue. I suppose I should sink it, or at least coat it in thermal compound, neh?

 

[LarryDFW]

Wolfman;

The LM1117 does need some heatsinking.

It is usually mounted on a PC board like the one JIB made:

http://laserpointerforums.com/f39/fs-lm1117-based-linear-driver-1-3a-fixed-60213.html

The amount depends on the voltage you are dropping across it.

 

[Wolfman29]

A PC board like what one?

And yeah - I just cut a small piece of aluminum from an old computer heatsink and thermal-compounded it onto the back of the through-hole board. While I can no longer run it off of 2x 14500s, I can run it off of one 14500 li-ion and one alkaline 1.5V (which, incidentally, is enough voltage, apparently). It still overheats if I use two 14500s... think it would be possible to reduce it so I *could* use two of those?

 

[anselm]

I can run it off of one 14500 li-ion and one alkaline 1.5V

LOL, didn't your mommy (and EVERY instruction manual I have EVER seen) tell
you never to combine different kinds of batteries, be it in series or parallel?

 

[Wolfman29]

No? What's the problem with it? And is there any other solution to decrease my voltage? Because right now I am sitting on far too much voltage for my tiny little LM1117 to dissipate as heat.

EDIT: Because I have been running it with those two batteries for a while now and things *seem* to be going fine....

 

[anselm]

Well to tell you the truth, I don't really know, because I never actually tried it out.
I DO know people advise against it all the time.

One simple issue is that one battery will run out of juice before the other.

There might be a situation in which the Li-Ion would try to "charge" the alkaline,
which could be dangerous.
I'm not sure though if this can happen when they in series as well, or only when they
are in parallel....

A simple and safe way to reduce voltage is simply wiring a rectifier diode (1N4xx) in
series, each diode will reduce voltage by 0.7V
I used this method to drive a 3V green module with a 4.2V LiIon.

 

[Cyparagon]

Yes, it is a bad idea, but not too much worse than running your laser until it dies. The alkaline has more current capacity than the lithium, so the lithium will die sooner. As long as you monitor the level of charge on them both, they will be fine.