- Joined Nov 8, 2022
Hello everyone. This is my first post on the forum, I am currently working on a seemingly simple circuit. I think a constant current driver is the solution; after wiring everything up last night the LED was nice and bright and circuit ran great. I decided to charge the Lipo over night, and when I turned it on this morning the LED was at 10%-50% power. Certainly not as bright as last night. I’m thinking maybe this is due to the Lipo charging up to 4.2 volts? Anyways here is the data. I am running an LED with a 3.3 forward voltage rated at 20 ma off of a 3.7 v rechargeable Lipo battery. Using ohms law (3.7-3.3=.4 divided by .02 Amps) I thought I needed a 20 ohm resistor. This in theory works fine if the Lipo stayed at 3.7 volts. After fully charging to 4.2v ohms law would say that the 20 ohm resistor I put in was not enough and the LED was getting 45 ma of power. Could this have caused it to loose it’s brightness so fast? I would just put a 45 ohm resistor in to drop the led to 20 ohms @ 4.2 volt supply but then when the Lipo dropped to its normal 3.7 volts the LED would only be pushing about 9 ma. If I am correct? I am a noob electrician. For my project I want the LED at max brightness. And it’s datasheet shows a steep drop off in lumens when under 20 ma so under powering it is not an option. I used thin 30 gauge wire and thought maybe a connection got loose. So to validate that it is the LED I was going to re-soldier/ new wire everything. But if the LED is still dim it’s back to the drawing board. I was planning on purchasing a 20 ma constant current driver and replacing the resistor with that. Would this in theory work to constantly regulate the circuit to provide the LED with 20 ma regardless of if the battery is at max charge (4.2) volts or it’s normal 3.7 volts? I am looking at the constant current driver AL5809q by diodes incorporated. It has a minimum input voltage of 2.5 v. My other question is will the constant current driver hog voltage from my circuit? As I am powering a 3.3 v forward led with 3.7 volts I don’t have much voltage to spare to add other components requiring power. Thank you for your help. Will attach a spec sheet for the constant current driver I am looking at.
387.9 KB Views: 9