# Is this circuit possible? 3 V - one coin cell CR2032, one 0603 surface mount LED that has forward voltage of 2.7 and 30 mA and slide switch

I am making this for a tiny pendant, where I want the light to be kind of dim, hence I used these tiny low power consumption LEDs.

Even if the LED goes off (in which case the battery should be replaced-ideally the light should last a whole day or two) the pendant would still look good as it would be made very artistically. The LED light offers extra dimension to the art piece pendant.

One coin cell battery that is 3 V - CR2032, one 0603 surface mount LED that has forward voltage of 2.7 and 30 mA and a slide switch with contact resistance of 30 Ω.

Resistor value - subtract the LED forward voltage from the battery voltage (3 V - 2.7 V = 0.3 V). Then, divide the result by the LED current (0.3 V / 0.03 A = 10 Ω). To account for the contact resistance of the slide switch, it's recommended to use a slightly higher resistor value, such as 15 Ω. This will ensure that the LED receives the appropriate amount of current and doesn't burn out too quickly.

Should I use 10 Ω resistor or 15 Ω? Is this circuit OK?

• A contact resistance of 0.03 Ohms is insignificant compared to the 10 Ohm resistor. In any case, to compensate for that you would reduce the resistor value, not increase it. May 25, 2023 at 10:34
• Please check the battery datasheet for its ESR (series resistance). It's probably in the 30 ohm range by itself. May 25, 2023 at 10:35
• Turn on the switch and watch the LED get dimmer and dimmer as the overloaded battery runs down. The Duracell datasheet for a CR2032 battery says Standard continuous current= 0.3mA, Max continuous current= 3mA but you are trying to blind people for a few seconds with the LED max allowed current of 30mA when the battery is brand new. May 26, 2023 at 16:24

I have several LED keychain lights that use coin batteries. No resistors are used, the batteries have some internal resistance.

The lights with a red LED use one CR2032 cells. The brighness will drop noticably as the battery depletes. It will last about 5-10 hours, depending on where you decide the LED is too dim to be useful.

The lights with a white LED use two CR2016 cells in series. I have never tracked how long these last, I suspect they won't last as long as the red lights because CR2016 cells have less capacity.

When the voltage drop accross the resistor is small compared to the LED voltage drop, you can't use a fixed drop for the LED in your calculations, you need to use a V-I curve. Your battery internal resistance is difficult to determine, and it may change as the battery discharges.

One cell with a white LED will be rather dim, but it might be satisfactory for your use. You will need to experiment.

– AshJ
May 31, 2023 at 11:09

According to this battery datasheet, CR2032 internal resistance is between 8-10 ohms when fresh, and over 100 ohms when almost discharged. The datasheet also shows useful discharge curves.

If you use a low resistor value for high current, or no resistor at all, the LED should work fine but it will drain the battery rather quickly. So it will be bright at first, then progressively dim.

Using a higher value resistor, for a current of a few mA on a fresh battery, will make it less bright at first but it should stay brighter for a lot longer.

You don't need a 5W resistor, a small SMD resistor should be fine.

I would recommend using a LED with a low Vf (so it stays lit at a lower voltage) and high lumen/W efficacy (so it requires less current) to get the most out of your battery. Since most of the cost of using this device will be in the cost of the battery, using a more expensive LED that requires less current makes sense.

You could also use a rechargeable coin cell battery. Some are available with pre mounted pins to solder on your PCB. These are LiIon so they have higher voltage (3.6V nominal) and higher output current so they should provide a usable voltage for a white LED for a longer time. However you will need a BMS chip for charging and over-discharge protection, and a convenient USB port for charging.

• Hi Bob, high lm/W is beneficial of course, but low Vf is making it worse actually, because for the same watts drained from the battery, you end up with fewer watts in the LED. Blue is not very well visible; Best is usually green. May 26, 2023 at 17:49