I've seen a lot on LEDs and series vs parallel resistor usage, but found nothing regarding the combination of both, so here goes.
I am working on a watch of sorts, that will work similar to a 7-segment display, where different LEDs are lit simultaneously depending on the driver's output (sometimes 2, sometimes all 7), so I will use that as an example.
In order to conserve battery, I am looking to have a low brightness during "normal" operation, with a momentary high brightness option. Since this needs to be rather small to be wearable I am, of course, looking to minimize component count. Cost is relevant, but much less of a factor than attaining a small size that can be hand soldered.
It is widely understood that running parallel LEDs on a single resistor is not the best approach, and I have allowed for a resistor-per-LED arrangement in my design, but I would like to be able to bypass SOME of the resistance temporarily.
What I figured I could do is put the Resistor-LED series pairs in a parallel array that feeds to a single resistor with a bypass, as in this schematic:
So, the question: Would placing a single resistor in series with a parallel LED array be as bad if the LEDs also had dedicated series resistors, and if so, are there other alternatives for the dim/bright outcome I am looking for?
- I am planning on a 3v (2xAAA) battery source running LEDs with a Vf of 2-2.2.
- Once I have my LEDs, I intend on testing different resistance values for brightness yield, but am hoping for around 2-4 and 7-10 mA for the low & high respectively.
- The actual array will have 12 LEDs, with a maximum of 9 lit at one time.
- There will be 4 "digit" arrays in total.
- I am comfortable working with SOIC and 0805 (and some 0603) packages.
- If there is a risk of (slightly) uneven brightness from one LED to the next, that will be ok, as it will give a "battle worn" look that will work with the design.