# Multiple LED Circuit

Discussion in 'The Projects Forum' started by Evie, Mar 11, 2015.

1. ### Evie Thread Starter New Member

Mar 11, 2015
9
0
I want to make a circuit with 20 LEDs here are the specs of the LEDs that I will be using:
Forward voltage:2.5V
Forward current max.:25mA
Reverse voltage max.:5V

I want to use two AA batteries to power the LEDs I have entered all the details into LED Center calculator. Here is what they said I would need to make the circuit:

Solution 0: 1 x 10 array uses 10 LEDs exactly

+3V
R = 22 ohms
R = 22 ohms
R = 22 ohms
R = 22 ohms
R = 22 ohms
R = 22 ohms
R = 22 ohms
R = 22 ohms
R = 22 ohms
R = 22 ohms
The wizard says: In solution 0:
• each 22 ohm resistor dissipates 13.75 mW
• the wizard thinks 1/4W resistors are fine for your application
• together, all resistors dissipate 137.5 mW
• together, the diodes dissipate 625 mW
• total power dissipated by the array is 762.5 mW
• the array draws current of 250 mA from the source.
Will I have to use that many resistors? will the two AA batteries be enough?

Many Thanks.

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
9,618
1,088
It is advised to use resistors with leds.
But 2 AA batteries can be used with certain leds without resistors.
Better to use resistors

3. ### DickCappels Moderator

Aug 21, 2008
3,937
1,098
An important but omitted parameter is the forward voltage. The calculator seems to "think" or was told to expect a forward voltage of about 2.5 volts, and allocates 2.5 of the 3 volts from the 2 x AA cells to the LED and another .5 volts for the resistor.

If you want to use half the resistors, you would need to increase the voltage, for example, 4 x AA = 6 volts so you could make five parallel strings of two LEDs (total of 5 volts) and one resistor (1 volt), thus cutting the number of resistors in half. You can still use 1/4watt resistors.

If you increase the voltage further, you can get down to one resistor when all of the LEDs are in series.

An alternative is to use a voltage converter that would boost the voltage from your battery pack to a higher voltage, but then it starts getting complicated.

Evie likes this.