You assumed (post #4) that each LED was 7V so three in series would be 21V, and a boost converter would be required.In post #7 Ian has already explained the problem with the schematic in post #6. The 7812 is attempting to output 12 volts more positive than it's common terminal which is connected to +5 volts. So it is attempting to output 17 volts (5 + 12) with respect to the negative rail. It cannot do this as it's input terminal is only connected to +12 volts. There is always a voltage drop between the input and output of the regulator and for an LM7812 this would be at least about 2.5 volts. Also as the 7812 is only seeing an input voltage of 7 volts (12 -5) there is no way of knowing how it would behave.
Re post #11 As your input supply is 12 volts you can't use a 7812 to regulate the 12 volt supply as its input voltage needs to be higher than its output voltage. (LDO (low dropout regulators.) regulator can work with a lower input output differential than older regulators like the LM78xx series. Are the brightness of the three LEDs to be controlled independently or all together ?
My suggested method in post #4 was to save power and heat generation but now that you tell us that you want to control the brightness of the LEDs using PWM it is not suitable.
I wrote the sketch (arduino IDE) that controls the PWM (0-255) for fading the LED on, then off. I then flashed the code to the attiny85.The misinformation in this thread is approaching the level of Facebook.
I think we now understand that there is one (not three) LED device that consists of 3 LEDs in series needing somewhere between 6.6 and 8.4V to operate at 450 mA.
We also know that it is driven with PWM by an Arduino.
Naturally, this means our original solutions are no longer operative.
A DIY buck converter powered by the Arduino, as suggested by @Ian0, is the way I would go, but may be beyond the skill of the TS.
The simple method would be using the 12V rail and a resistor.. The disadvantage of that method is wasting about 2.25W of power requiring at least a 3W resistor which will get very hot.
A constant current driver is still in play as well.
It all comes down to the TS’s requirements and skills.
LP2951 will give an even lower quiescent current than a 78L05 (75uA vs. 5mA)To set the LED current to 450 mA Rset on the application circuit in the AL8843 data sheet will need to be 0.22 ohm.
Yo can control the brightness by connecting the PWM output from the ATtiny85 to the control input of the AL8843.
If your total load on the +5 volt rail is only 40 mA you will only waste 280 mW using a linear regulator. You could use an LM78L05 which is smaller than an LM7805. I think designing your own switching regulator based on an Arduino would waste more power than a linear regulator. If you want to use a switching regulator an MC4063 is one possible solution.
@johnaustinkatyThe description of the AL8843 says that the current is externally adjustable, so with the appropriate resistors in the circuit, I should be able to get a constant 450mA?
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by Jake Hertz
by Jake Hertz