Hello,

I am not an electronic engineer, so analog stuff is not my strong point. Hence please excuse the beginner question.

I need to design a simple circuit for a light box (A box with 5 small LEDs which, in case you are wondering is used in astrophotography to take calibration images)...

Anyhow, I need 5 LED which will be driven with a 3.3V CR2032 battery (unless I decide to use 5V USB)
Should I put my 5 LED in series or parallel? Where/how do I place my resistor.
Anything that will help reduce power consumption.

Thanks for your help.
Cyrille

 

First you need to find the forward voltage of the LED. Red is 1.8 to 2.2 volts. Green 2.0 to 2.4 volts. This is very dependent on how the part is made, and temperature.
Lets say you have a 2.0V LED and a 3.3V battery. 3.3-2.0=1.3V Place a resistor in series. The resistor will have 1.3V across it. Simple math. 1.3V across a 1k resistor causes 1.3mA (100 ohm = 13mA) Pick the right resistor for about 1/2 of the max LED current. I would use 5 LEDs and 5 resistors. It is not good to parallel 5 LEDs with only one series resistor.

A CR2032 battery is rated for 4mA by some companies and 0.2mA by others. There is a resistor inside the battery of about 200 ohms. (depends on who made the battery) This limits the max current. If you have 5 x 10mA LEDs in parallel = 50mA which is too much for that little battery. You can get 15mA for a short time.

 

The color of an LED determines how much voltage it needs. A red LED needs about 2V and a white or blue LED needs about 3.4V.
Your 3.3V battery voltage drops with a load and drops as it is used. Then your LEDs cannot be in series.

 

Welcome to AAC!

I am not an electronic engineer

You should still be able to draw a block diagram showing what you want. We can help you with the details.

Should I put my 5 LED in series or parallel?

Depending on color, the forward voltage is going to be in the 2-4V range, so you can't even put two in series if you expect to be able to control the brightness.

LEDs will turn on when placed directly across a single CR2032 batteries with no current limiting resistor.

When using LEDs, brightness will be determined by LED current. You use Ohm's Law to calculate resistor values:
\( \large R=\frac{V_{SUPPLY}-V_{LED}}{I_{LED}} \)

 

Hello,

My LEDs are white BTW. Sorry for not specifying, I did not think that it was that important!

Ok, I did a couple of tests...

Anyhow, I did some tests here and ended up using 7 LED in parallel.
For the power side of thing, I ended up using a 1K resistor.
The end result seems to work. The LED is appropriately light (not too bright, but still emiting light).

So, the schematics would be something like:

5V ----1K------------------------------
D1 D2 D3 D4 D5 D6 D7
0V ------------------------------------

Thanks for your pointers and for the discovery that various color LED have different characteristics!

Cyrille

 

Depending on how the white LED was made but it probably needs 3V to turn on.
Some LEDs change color a little when not driven at their rated current. Maybe that does not matter.
It is likely the battery will not last long.
Why do you need 7 LEDs?

 

So, the schematics would be something like:

5V ----1K------------------------------
D1 D2 D3 D4 D5 D6 D7
0V ------------------------------------

This isn't a proper schematic. Even something hand drawn would be better.

This is what you drew and it's not a generally accepted design practice:

You can only do this with LEDs that have a matched forward voltage. Otherwise, the one with the lowest forward voltage will hog current and be brighter than the rest.

To avoid that, we use separate current limiting resistors for each LED.

It would be bad if you didn't include the current limiting resistor. That could cause a cascading failure where the LED with the lowest forward voltage would burn out. Then the one with the next lowest voltage would burn out. That would continue until all were burned out.

For the power side of thing, I ended up using a 1K resistor.
The end result seems to work. The LED is appropriately light (not too bright, but still emiting light).

Do you have a datasheet for the LEDs?

 

You can only do this with LEDs that have a matched forward voltage. Otherwise, the one with the lowest forward voltage will hog current and be brighter than the rest.

To avoid that, we use separate current limiting resistors for each LED.

100% agree. It's NOT the preferred method. In keeping with DL's comment about "the one with the lowest forward voltage will hog current" here's a video I did; a test of putting parallel LED's on a single resistor. It's doable, but it's just not the right way to do it. Pay attention to how one color of LED's will go out when a different color LED is plugged it. That's the result of different forward voltages (Vf)'s.