Help with resistor calculation for LED

[vltowell]

Hi,
I have made a simple circuit with a 3V battery, and 3 blue LEDs. A guy at work worked out the resistors I needed to get the LEDs in parallel. This all works fine, but He is a little hard to understand, so I was wondering if someone could explain simply how he got these results. Here is the data

3x Blue 10mm LED:
Voltage drop of each LED = 3.5
30mA

each resistor is 84Ω

2x AA 1.5V batteries.


I tried using the formula R = (Volts supplied - Volts drop) / current. This obviously won't work as I get a negative number.

Please don't get too technical. I have a hard enough time as it is. Is there another formula to use?
Thanks
Vincent

 

[bychon]

In un-technical terms, something stinks here.

In technical terms, you can't develop 3.5 volts across an LED with 3 volts worth of batteries. The normal method is like, a 9 volt battery minus 3.5 volts for an LED = 5.5 volts.
5.5 volts divided by .03 amps is 183 ohms.
Install a 220 ohm resistor because you can't buy a 183 ohm resistor.

The fact that your LEDs work means they do not require 3.5 volts to light up, but I bet they look a lot brighter if you feed them properly.

 

[JCLrd]

The only way to really understand how he got that, would be to build the circuit and measure the voltage drop across one of the resistors, then you would know the branch currents, then from there you could determine what voltage drop he chose to be across each LED.

He probably interpolated using the 3.5v / 30mA. and then chose a value for the LED voltage.

 

[vltowell]

Hmmm. I think there is something fishy in what is happening. The specs, if I am reading them properly does say 3.5V drop. This might be where I am going wrong!! I know this doesn't make sense, and that is the whole reason for this post. But I have made the circuit and it works fine.

I will measure the drop as as suggested. Hopefully that will make more sense of it all.

Thanks for the replies. I gives me something to think about, and a way to work out what is happening
Cheers
Vincent

 

[mneary]

The LEDs probably have a maximum drop of 3.5 volts when there is 30mA flowing in them. If the voltage is less, the current might not be 30mA.

 

[audioguru]

LEDs do not have a fixed voltage like a light bulb, they have a range of voltages even if they have the same part number.
Some blue LEDs will be 3.0V and others will be 3.8V. Maybe yours are close to 3.0V. Maybe your "3V" battery is actually 3.2V.

The current-limiting resistor uses up some voltage so that the current is limited. But your battery voltage is too low for your LEDs to have their current limited properly.
Your LEDs will not light when the battery voltage runs down a little.