Basic resistor question for parallel LED circuit

Question

I'm planning on building an LED circuit to place inside a model miniature for wargaming. The circuit would be in parallel as I'd need a really big battery to power up all the LEDs.

I wanted to check my understanding of setting up the resistors so things work properly. I'm going to have different types of LEDs with different voltages but here's my basic question.

Let's say I have 10 blue lights at 3.4V 20mA and a 9V supply. Based on my calculations each lamp would need a resistor of only 28 ohms? My math is

$$\frac{9-3.4}{\frac{10\cdot 20}{1000}} = 28$$

Is that right? The final circuit will have close to 20 LEDs, and it seems like at that point the ohms of the resistors will get much smaller?

Once I figure out all the different types of lights (all at 20mA), I will need to calculate the resistors individually for each as they have different voltage requirements?

Answer 1

No, your calculation is not right.

Since your 9V source is a voltage source, and all LEDs are in parallel, the number of LEDs can't have any influence on the resistor. One LED can't "know" of its neighbors!

So, it's simply \$\frac{(9-3.4)\,\text{V}}{20\,\text{mA}}=280\,\text{Ω}\$.

Answer 2

Using a resistor for each LED would waste most of the battery energy you have, especially if you want to use a 9V cell. I would not recommend it.

A better way to provide a solution is to run as many LEDs as possible in series. This means having a higher voltage, but you get to use the battery energy more efficiently.

You could design a boost/CC provider but this may be too complex for you.

I'd suggest you consider a simple linear constant current device like the AL5809-20. This is a simple two terminal device that sets a constant current through a string of LEDs.

The device comes in various current levels designated by the last two digits of the part number:

To use them you simply connect as you would a resistor, and it sets the current for the string.

The recommended minimum voltage is 2.5V but the device will work down to a minimum of 1.75V.

You would then be able to use a higher voltage such as two or three LiPo cells to provide the power (and rechargeable). For example if you used 3S than you have a minimum of 3*3.6=10.8V and a maximum of 3*4.2=12.6V. Which LED forward voltages in the 2.2 - 3.4V range depending on color you could get at least 2 or as many as 4 LEDs in each string. If you could even mix lower and higher Vf LEDs together, they all still get only the 20mA.

If you want to be really creative you could use a single LiPo cell (3.6 - 4.2V) and a boost DC-DC convertor (there are many on Ebay etc) to get say 20-24V and connect 5 to 8 LEDs in each string.

Answer 3

Consider Eneloop rechargeable NiMH AA cells.

Generally you want the minimum cell voltage to match the minimum acceptable current in the LEDs. This means the battery array voltage matches the LED's as close as possible or slightly higher then the loss of power in the selected series R is much less.

^{simulate this circuit – Schematic created using CircuitLab}