LED's in computer case

[carlco9020]

I am tring to hook up some LED's (5mm) 3.7 volt typ. into my computer case. I have 5.15 volts coming out of the power supply. What size resistor would I need to drop the voltage to 3.3 volts? I am not to good with figuring these things out could someone help???


Thanks in advance ... Carl

 

[matk95]

Formula

This is the formula for working out the right value is:

R = (VS - VL) / I

 

[carlco9020]

Ok so r = resistance VS = voltage source VL = Voltage loss and I = ??? amps??? If it is amps I do not know the amps coming out. The source is a 550 watt power supply in a computer????

 

[mvs sarma]

Ok so r = resistance VS = voltage source VL = Voltage loss and I = ??? amps??? If it is amps I do not know the amps coming out. The source is a 550 watt power supply in a computer????

Simple ,
Even if you have a million gallon tank, the jug you have will take only a jugful, and not a drop more.
Likewise, even if the power supply is capable of delivering 550watts, your circuit element has 'r' as a limitation.

BTW, Vl appears Voltage drop across LED but not Vloss, i fear.
the lamp drops 3.4 V or so across it as it is white LED. so balance is 5.15-3.7=1.45V
divide this by the 'r' value you get the current.(=1.45/r) and same current flows thro' the LED.

 

[pixnum]

Here it is my experience(sometimes mathematical equations confused me as this result comes out from trial & errors and it may differ man to man):

Red LED: Resistor 100K
White LED: Resistor 150K

It is approximation !!!!

 

[mvs sarma]

Here it is my experience(sometimes mathematical equations confused me as this result comes out from trial & errors and it may differ man to man):

Red LED: Resistor 100K
White LED: Resistor 150K

It is approximation !!!!

Infact calculations are better done converting all units into their basic form
like inductor in Henrys, resistance in ohms and capacitor in farads, at the end, one can convert into the acceptable form. Decimal place fixing causes confusion at times in the bigining. Later you get corrected and follow a standard notation.

 

[Hero999]

Here it is my experience(sometimes mathematical equations confused me as this result comes out from trial & errors and it may differ man to man):

Red LED: Resistor 100K
White LED: Resistor 150K

It is approximation !!!!

Where did you get that from?

Surely you mean 100R and 150R?

 

[audioguru]

A 2V red LED will light perfectly with 20mA if the current-limiting resistor is 100k and the supply voltage is two thousand volts.
The current with such high values is the same for any LED.

 

[Mickster]

I am tring to hook up some LED's (5mm) 3.7 volt typ. into my computer case. I have 5.15 volts coming out of the power supply. What size resistor would I need to drop the voltage to 3.3 volts? I am not to good with figuring these things out could someone help???


Thanks in advance ... Carl

It depends upon how bright you want the LED to be.

A general guide to powering general LED's is to supply them with 20ma. Some "Superbright" LED's require more than that...maybe 30ma upwards.

The best option is to look at the datasheet for the LED you have and use the recommended value in the formula.

Say you have an LED which is required to be driven at 20ma. The formula is:

R = (Vs - Vl)/I

R is the resistance required,
Vs is the Voltage - Source,
Vl is the Voltage - LED,
I is the current required.

so,

Vs = 5.15V
Vl = 3.7V
I = 0.02

5.15 - 3.7 = 1.45
1.45 / 0.02 = 72.5Ω

The nearest higher value standard resistor to this, would be 82Ω, (Grey, Red, Black), which would actually supply the LED with 17.7ma.

Now, if you wanted to have the LED a little dimmer, you could supply it with less current using a higher value resistor. Let's use 10ma:

5.15 - 3.7 = 1.45
1.45 / 0.01 = 145Ω

The nearest higher value standard resistor to this, would be 150Ω, (Brown, Green, Brown), which would actually supply the LED with 9.7ma.


Does this help?

 

[audioguru]

Do not connect an LED directly to the 5.15V without a current-limiting resistor. Some people who dood dat said pieces of red hot LED sprayed around the room.

 

[Hero999]

5.15 - 3.7 = 1.45
1.45 / 0.02 = 72.5Ω

The nearest higher value standard resistor to this, would be 82Ω, (Grey, Red, Black), which would actually supply the LED with 17.7ma.

The nearest standard value is 75Ω which would be violet, green, black or violet green, black, gold.

There's a list of standard values on Wikipedia.
Preferred number - Wikipedia, the free encyclopedia

The lower E-numbers (i.e. E6 is more popular than E12) are generally more common but E24 pretty popular.

True, is you don't have 75Ω then use 82Ω.

 

[Mickster]

Hero,

good catch, thanks for the correction.

 

[Sceadwian]

Only problem with that audio is the resistor is gonna burn off 40 watts, might as well use a lightbulb =) If you don't know what you're doing buy LEDS with integrated resistors. E-bay sells tonnes of them and the ones rated for 12 volt use are great for PCs and cars alike. (PCs have 12 volt lines as well as 5, try the yellow line) As the voltage increases above what the LED needs the resistor has to drop more voltage wasted as heat, but with large voltage and resistors it acts more like a current source (which diodes like) and suffers from under/over voltages less because the resistor drops proportionalty more of whatever is applied to the series setup.