Toy flashlight LED conversion

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My 5 year old daughter brought me her little toy flashlight and asked me to make it into an LED light like the ones I build. I broke the glass out of the bulb and soldered in the filed down 5600 white I made a while ago. I used a spring and a 6 volt battery. This thing actually turned out pretty good for such an inexpensive flashlight. Here are some pics.

 

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Correction. I ment to say that I used a 12 volt battery in this light. I wish I knew where to buy a bunch of these little cheap lights. With the 6 volt battery and no resistor this think could be really bright. No reflector or lense. The LED is just deeply recessed into the plastic head. Probably no rings even if I didn't file the LED.

 

[axolotls]

Brad or others: question about overdriving the leds.
At what point (what's the tolerance of these whites?). I assume without putting in series a 400+ Ohm resistor you are overheating the LED. But based on experience, what is the threshold before the LED melts or cracks. Discounting burn-out (longevity). It seems like you are going for brightness (but who the heck isn't?)

 

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I think it depends on what batteries you are using. I burned out an LED running it with straight six volts from 4 AA batteries. I'm sure 6 volts from 4 C or D batteries would fry it too. The little 6 volt and even the little 12 volt batteries work fine though. The LEDs don't even get hot. I am kind of curious as to why this is. Maybe someone with an electrical background could explain this for us?

It would seem that voltage isn't the whole thing when considering how hot to run the LED.

Also. Does anyone know the formula to figure out how many mA you get from how many volts.
I know how to figure for the resistor size but what if I have 6 volts. How do you know how many mA it will supply? Thanks.

 

[axolotls]

From what I think (someone correct if I am wrong):

AA = 2850 mAH (regardless of how many you stack in series, the mAH remains the same)

L544 (6 V) = 190 - 200 mAH (200 if a button stack)
3 V Lithium = 750 -1300 mAH

The button batteries are probably around 50 mAH.

 

[Brock]

>So if a battery is 200 mAh and takes 20mA >while running, the battery will last 10 >hours?

Yes, well sort of, the problem is as the battery dies it will drop the voltage and then the load (led) will draw less power. So say you start at 20mA, when the battery is half empty (100mA left in the battery) you will probably be drawing about 15mA, and half of that (50mA left) you might be drawing 10mA. I believe this is how you get those really long run times on some LED lights.

Also you can check the voltage of the battery under load. 4 AA in series will supply 6v at 2750 mA. While 2 2016's lithium coin cells will produce 6v at 80mA. Now put them both under load, say a single 5600mcd white and the AA will drop to 5.9v supplying 200mA and the lithium's will drop to 4.5v and supply about 50mA. This is due to the internal resistance of each battery. Every battery type is different and has different characteristics.

So this is why I usually tell people it is more important to watch the mA draw rather than the voltage. Really both are important, but I can tell you if you have 50mA at the LED you have 4.5v. Of course the reverse is true also. If you have 4.5v at the LED you will also have 50mA. The thing is you can't rely on the battery itself for the voltage in your circuit. It's voltage will change a lot depending on load and how dead the battery is.

Now every LED has a different voltage / mA draw, but once you know what it is you are set. The best thing to really do is measure both under load and then you know for sure.

Hope this helps

Brock

 

[Mike]

All batteries have an internal resistance which is why some don't burn up LEDs even though the voltage is high. The batteries themselves limit how much current can be supplied.

 

[axolotls]

OK. I just found this out:

AA = Internal resistance = 150 - 750 Ohms (the higher number obviously when the battery is about 90% discharged)

Don't know what it is for the lithiums or buttons.

$0.02

 

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So if a battery is 200 mAh and takes 20mA while running, the battery will last 10 hours?