Timeline for Understanding this LM317 LED Driver circuit
Current License: CC BY-SA 3.0
15 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jul 25, 2015 at 22:19 | comment | added | vicatcu | @DallasCarter Brightness of an LED is related to the current. Constant current == constant brightness. LM317 in this case keeps current constant while voltage varies / decays. | |
| Jul 25, 2015 at 22:13 | comment | added | Ignacio Vazquez-Abrams | The battery's voltage decreases as it drains. The CC driver compensates for that. | |
| Jul 25, 2015 at 22:12 | comment | added | Ignacio Vazquez-Abrams | That is in fact the voltage that the device outputs. Anything that isn't output is burned up as heat in the device (which is why it is usually strapped to a decent heatsink). | |
| Jul 25, 2015 at 22:12 | comment | added | Dallas Carter | Just one more question which leads from the main question. The reason we don't directly just use a battery + resistor + LED is to regulate the voltage first through the LM317 first . Correct ? | |
| Jul 25, 2015 at 22:08 | comment | added | Dallas Carter | Yup got it. That was the main part which confused me at first as I had thought the output is the voltage across R + LED . | |
| Jul 25, 2015 at 22:07 | vote | accept | Dallas Carter | ||
| Jul 25, 2015 at 22:05 | comment | added | Ignacio Vazquez-Abrams | @DallasCarter: Vout-Vadj must be 1.25V. This is an invariant specified by the part itself. The ground connection completes the voltage divider. | |
| Jul 25, 2015 at 22:04 | comment | added | Dallas Carter | Thanks for clarifying, just so as to not misunderstand this. Q1 . Looking at Image 2 , LM317 regulated Output voltage is set to = Vout-Vadj , correct ? | |
| Jul 25, 2015 at 22:02 | comment | added | Ignacio Vazquez-Abrams | @vicatcu: Helped by the fact that the LM317T has decent power handling, which makes it suitable for >350mA LEDs. | |
| Jul 25, 2015 at 22:01 | comment | added | vicatcu | Incidentally the only reason I can think to use this circuit is to make insulate your LEDs perceived brightness from a varying source voltage. | |
| Jul 25, 2015 at 21:57 | comment | added | vicatcu | @DallasCarter, no the LM317 regulates such that VOUT - ADJ = 1.25V. | |
| Jul 25, 2015 at 21:57 | comment | added | Ignacio Vazquez-Abrams | @DallasCarter: It's not a divider at all. The voltage is measured between the output and ADJ, therefore only R is taken into account. | |
| Jul 25, 2015 at 21:55 | comment | added | Dallas Carter | So basically its just a voltage divider at the output with LED being the lower resistor in the divider, and the Wire to the ADJ pin doesn't get affected as its already constant . ? Also in the first sentence do you mean voltage across R+LEDR ? | |
| Jul 25, 2015 at 21:55 | comment | added | vicatcu | yup there's always a fixed 1.25V drop between VOUT and ADJ. The size of the resistor determines the amount of current delivered to the load (in this case an LED). | |
| Jul 25, 2015 at 21:52 | history | answered | Ignacio Vazquez-Abrams | CC BY-SA 3.0 |