Simple LED circuit Grade 6 Project

[madfish100]

Hi. I am doing a grade 6 science project building a simple circuit. I want to build a LED board that spells my name so there will have to be a LOT of LEDs. I’m just starting to learn about circuits and electricity. I was wondering what the best way to arrange it would be, parallel or in series. Also wondering how to calculate exactly what battery I need or if I need to add resisters. Do the LEDs use up current as the current goes through them, and also do they have resistance? If they do that means the voltage drops as current goes across them right? Anyway, I was wondering if you have any design help you could give me. I’m learning about Ohm’s Law and just wondering how to start using it so thank you for any help.

 

[thannara123]

What do you mean parallel or serial ?
Normal LED consumes about 20 mA ,calculate according LED ratings .http://www.csgnetwork.com/ledsingresistancecalc.html

 

[D.A.(Tony)Stewart]

Dear Madfish
Since LEDs are like batteries and nonlinear, unlike resistors, in order to use Ohms Law you must treat the LEDs as a fixed voltage with a series resistance. Then when voltage is applied the current will depend on the voltage difference across the series resistor.

But when putting many in a string such as in letters for your name.
1st count how many LEDs in total you want to use. Say 50, 100, 150.. you decide
If using Red then remember RED is like a 1.5V battery. 1.2 when low or very dim and 1.6V when high and bright when current should not exceed 20mA depending on the series R and Voltage you apply. You can split the strings and letters into parallel strings of 10, 12 or 20, 30 if you want. I would use a laptop charge of 19~24V.. Measure the voltage. then figure how many LEDs can fit inside that voltage. So if using many strings with 100 LEDs that would be 150Vdc in one string ( too high) and if using 19V then 150V/19V = 7 strings x 14 per string = 98 LEDs each string has its own resistor.

Now apply OHms Law on that resistor../.. If dim is 1.2V then 14LEDs x 1.2V = 16.8V so the Resistor shall drop 2.2V and 20mA means you need V/I=R 2.2/20mA = 110 Ohms The other trick to know is every LED has an internal resistance. Small 5mm LEDs are 15 Ohms +/-5 Bigger 1W LEDs are < 1Ohm. so that means you already have 15 x14 LEDs in series = 210 Ohms so the current with NO RESISTOR will be half of 20mA. So we need to use 13 LEDs if using 19V but 14 LEDs would work OK if using 24V.

So the trick is to remember 5mm LEDs are like a 1.2V battery with 15 oHms in series. then add up all the voltages and resistance and see what voltage you need to be just 1 to 2 Voltage higher and choose a small resistor using Ohms law to make up the difference voltage.

The other trick is that not all 5mm LEDs are the same. so look for the brightest like 10000 mcd then these will have the lowest and more consistent internal resistance we call Effective Series Resistance or ESR. since this tolerance is wide.. so is the Vf or forward rated voltage at 20mA.

Exactly the same principle of using Threshold voltage and ESR can be used on 100W LED arrays to drive many off the same power supply.
8mm reels of LEDS often have 4 to 6 per string with SMT resistors so they can be driven from 9 to 14 V. This would be the easiest way for you to build a NAME SIGN.

RECOMMENDATION
Buy one 5m Reel of LEDs to run off 12V and string the LEDs in the shape of your name. Each string has a cut mark for 12V string of 6 ( 9V) with resistors on the flex board 8mm wide tape on a reel. You can buy these off EBAY for $28. They are not as efficient or bright as my idea. But easy to build. and drive off a PC power supply. or 12V battery wall XFMR with 1.5 to 2A rating.minimum. Since they come with peel off adhesive strip, you can build your name sign in 1 hour if you have a soldering iron and magnet wire or telephone wire or any light gauge wire. If you build with 5mm LEDs it will take 10 to 20 hrs to solder and assemble the 1st time... then a few hrs the second time after you learn the shortcuts to solder 250 solder joints.

Afterthought
===========
Because of this "complex" Use of Ohm's Law and Threshold voltage. many recommend use of a constant higher voltage supply designed just for LEDs.
But if you want to learn how they work. My advice tells you how and why they work like batteries. So if you wanted 19V battery you need at least 12 cells. or 12 x 1.5V = 18V. Except like LEDs the real voltage when new is 1.6V so 12x1.6=19.2V which is close enough. Like LEDs batteries also have ESR but since current is drained from the battery its voltage drops according to internal resistance we also call ESR in the battery. So a car battery is < 1 milliOhms while a rechargeable drill battery would be < 1Ohm. And since LEDs drain current rather than supply current the voltage rises with more current, ( until it gets too hot, then it drops)

- - - Updated - - -

If you look at the 8mm x 5m x 200 LED reel specs in my previous post.
Color: Yellow

o Working Input Voltage: 12VDC
o LED Quantity: 300 LEDS/5M
o View angle:120
o Working Current/meter: 1.2A
o Long life span 50,000+ hours
o Output power: 75W /5 Meter

Let me assure you the 12V is +/2.5V and this will affect brightness from 30% to 150% but 12V is nominal but designed for a truck battery.
The current will increase above 12V so 1.5 to 2A is a better supply.
75W is not the INPUT POWER rather what the seller equates to OUTPUT power of a filament light.
The input is 12V x 1.2A = 14.4 Watts.