# led circuit

Discussion in 'The Projects Forum' started by WILLIAM RYAN, May 15, 2010.

1. ### WILLIAM RYAN Thread Starter New Member

May 15, 2010
3
0
What circuit do I need to light 5 x Superbright 3mm LEDs ?

2. ### tracecom AAC Fanatic!

Apr 16, 2010
3,869
1,393
If you will post the current required by each LED and the forward voltage drop, as well as the voltage of the power supply you plan to use, designing a circuit will be easy.

3. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Bill Marsden has quite a bit of info in his blog, here: http://forum.allaboutcircuits.com/blog.php?b=378

Basically, you need to start off by finding out what their typical forward voltage (Vf) @ current rating is. This is usually supplied on datasheets.

If you don't know what the current rating is, you can assume 20mA for super-bright LEDs.

You can measure the Vf using a multimeter, a 12V DC supply, and a 470 or 510 Ohm 1/2 Watt resistor in series with your LED.

4. ### WILLIAM RYAN Thread Starter New Member

May 15, 2010
3
0

Each Led current is 30 mA.

TypMax Forward voltage:Vf3.2V3.6V forward voltage drop 3.2V Min to 3.6V Max

Power Supply 5 V from 3 x 1.5V batteries.

5. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
If you use three 1.5v batteries in series, you will have 4.5v available.

If the batteries are NiMH or NiCD, you will only have 3.6v available, which will not be sufficient.

You will need to wire your LEDs with separate resistors for each.

I suggest that you use a typical Vf of 3.4v @30mA.
The generic formula for LED current limiting resistors (Rlimit) is:
Rlimit >= (Vsupply - Vf_LED) / DesiredCurrent
Rlimit >= (4.5v - 3.4v)/30mA
Rlimit >= 0.9v/0.03A
Rlimit >= 30 Ohms
Next, we consult a table of standard resistance values. One is here:
http://www.logwell.com/tech/components/resistor_values.html
Refer to the E24 (green) columns. We can see that 30 Ohms is a standard value (300/10)

You will need five 30 Ohm resistors, one per LED.

I suggest also using a switch so that you can turn them on and off easily.

File size:
8.6 KB
Views:
76
6. ### Wendy Moderator

Mar 24, 2008
20,764
2,534
What Wookie has shown is the simple way, which I would go for. There are more complex ways to do the same thing, some circuitry required. If you need to use the battery voltage shown and want to drive the LEDs let us know, there is a very simple 555 circuit (the LM555 is a type of integrated circuit) that can do it.

7. ### WILLIAM RYAN Thread Starter New Member

May 15, 2010
3
0
Thanks,very much appreciated

8. ### Audioguru New Member

Dec 20, 2007
9,411
896
Maybe the absolute max current allowed for the LEDs is 30mA. Then they will last much longer at 20mA or 25mA.

Three AAA alkaline cells produce 4.8V when brand new. With a max current of 25mA for 5 LEDs then the battery voltage will be only 3.6V and the LEDs will be extremely dim in 4.5 hours. The LEDs will dim slowly over the entire 4.5 hours, most dimming in the first 2 hours.