I must admit, I haven't been keeping up with advances in electronic components.
I just bought a ( small ) LED flashlight. It uses 2 - AA cells, has 3 - LEDS, and the light output is amazing.
As I remember, LEDs needed a current limiting device, usually a resistor.
I've peeked in the flashlight case, and can't find any other components.
Has the industry gone to a ( micro ) current limiting chip for use in these flashlights ?
No. Resistor is used only if the source voltage is higher than LED's required voltage. In this case, 2 AA equals 3v and typical blue and white LED requires 3v. No resistor is needed.
-- When you hear the toilet flush, and hear the words "uh oh", it's already too late. - by anonymous Mother in Austin, TX To reply, replace digi.mon with phreaker.net
I think it still takes over 3 volts to power white LEDS
What has changed is the size and efficiency of inverters used to boost the voltage. There's lots of small one and two transistor circuits if you want to google for them.
Current limiting could be built into the LED (there are 12 volt leds on the market with current limiting built in) Don't know of any white ones with limiters . . .
This is about the simplest, cheapest, smallest, inverter circuit I've seen:
Three parts, not including the LED
----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Blue LED typical 3.6 maximum 4.0 volts Green typical 3.5 maximum 4.0 White typical 3.6 maximum 4.0
For 20 milliamps current and full rated output luminosity
Most flashlights use three 1.5 volt cells or a boost regulator I've got one with eight LEDs that runs on a single AA battery.
----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
Using resistors to limit LED current is suited for when you don't care about the power wasted in the resistors. For example in indicator lights.
In battery-powered flashlights you DO care about wasting power, since that shortens battery life. Instead, some kind of active current control which requires minimal power is used. Others in here know better than I about what specific circuits are best for this.
Don't believe for an instant that LED's are ever connected directly to any voltage source, such as betteries. Small changes in voltage will result in relatively large variation in current. Even if the LED is lucky enough not to get fried when connected to a fresh battery, it would become uselessly dim after the battery drops down one or two tenths of a volt.
Mark
It depends on your needs of course, but two cheap dry cells directly connected to some white LEDs can put out usable light for ever. For example, LEDs nominally rated for 20mA can handle
40-50mA . which is what some will draw from two fresh cells. That shortens their life but the resulting life loss is insignicant for practical purposes. Many cheap flashlites run the LEDs at these high rates anyway. The flash will be lost etc., well before they die.This low current draw with a direct connection, about 40 to 5 mA as the cells age does not load down the output through the increasing (as the cells age) cell internal resistance. As the cells age the current drops due to the resistance but their open circuit voltage still usually stays at 1.4+ volts, high enough for two cells to light an LED, albeit only just enough to be useful for finding something dropped or reading a map etc..
I've modified several old maglites by merely replacing the original 0.3-0.5A filament lamp with a white LED. They last for days 24/7 so are great for camping, or having in a bag or briefcase.etc. And a pair of any old AAs are ubiquitous.
I donno . . . the fact that the white/blue/UV LEDs are rated at 20 ma and they have a higher dropping voltage says they are using more power than red LEDs. More power means more heat; Heat kills semiconductors.
I guess one really has to know how much luminosity is lost by operating the LED over-current, and how much overcurrent ,for how long . . .
LEDs may last forever - given that they don't overcurrent. HOWEVER, they do lose their light output over time. They lose their light output faster as heat/current is increased.
I know - my LED "night light" has been great for the first year - and essentially dead for the next 4 years. It still puts out light; but I can't read by it, or even find the right doorway. It gets 20 ma at `1.4 volts for the last 6 years.
My feeling/guess is that any "flashlight" that uses no limiting resistor or current control and is connected to any low impendance source, no matter how well matched, voltage-wise, is a poor design.
It may work for a time . . . but it ain't good design practice.
Unless white LED's have some special indestructibility clause?
----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.