2 AAA led light, 100 mA, but only 1.00 V?

[viorel00]

I have a Garitty 2AAA LED flashlight like this one

I had to install it into a device and I need to run it off a DC power supply. I measure the current it draws from 2 AAA batteries and I measure about 100 mA. OK, I solder some leads, I grab a power supply, hook it up, turn the voltage on, and at exactly 1.00V, the current is already ~100 mA and the LED is just as bright as it would be with 2 AAAs. I don't get it, is the internal resistance of these AAA cells so high that at only 100 mA load, the voltage sags from 3 V down to 1 V?

the LED appears to running fine from a 1V/100 mA DC supply or 2 AAA.

 

[Mr Happy]

No, it is nothing to do with internal resistance or voltage sag.

Have a bit of a think about it before we tell you the answer.

 

[viorel00]

I haven't taken the LED assembly apart, you think there is some regulation built in? I think those are called drivers? I think the light is pretty cheap (a few bucks), but you never know if they built-in a driver or not.

Please excuse my ignorance, I though it was a simple LED and maybe a resistor in series.

Now tell me the answer, I need to go to bed.

 

[Mr Happy]

There is no LED in existence that can run off a 1 V supply, so if you are taking your measurements accurately it must have a boost driver built in. Even so it does not fully add up as when you increase the supply voltage the current should drop.

Maybe you should take some more measurements tomorrow with the power supply and see how the current varies with the supply voltage?

 

[viorel00]

yes, I was also expecting more than 1.0 V, maybe 1.6 V or so. I will take a V-I curve tomorrow, maybe that will clarify the puzzle. I need to hook it up to a USB port anyway, since I have to use the power supply for something else. I think a simple resistor in series should do the trick, huh? I want to limit the current to 100 mA, so plan to use a 40 Ohm resistor (5.0V - 1.0V)/100 mA

 

[VidPro]

if your going to hook it up to USB 5v, then the "boost" driver in it is going to cause you some problems, as BOOST drivers want the input (battery) voltage to be BELOW the voltage of the LED.
if you were using USB as the power source, just setting it up with resistance of some sort, might be a better choice, without the boost driver getting in the way. ALTHOUGH you can choke off the power going to a boost driver and still limit the total current usually.