I'm currently "Experimenting" with some LED's. I noticed that some of the LED circuits that I have around the house have different resistor values than I would expect. I created a circuit on the Falstad Simulator http://tinyurl.com/y6ctbjoc. The values that I am seeing are different from my math. Either the website is off or I am. I am betting on the latter rather than the former. By the simulators calculations, I am above the necessary current and below the necessary voltage. Where does the extra current go, heat at the LED? Why does the LED still light up? Can anybody point me in the right direction of a simple explanation? I am familiar with the triangle as well.

I have 4 - White LED's - 3VDC - 150mA, 500mW. I am attempting to work out if its better to power these with 12v or 24. I have looked at constant current, I could go that route. I was trying to avoid the number of components i.e., going from 120VAC -> 12VDC or 24VDC Power Supply -> Constant Current Module -> LED's. I will eventually have 3 different size SMD LED's, 3 Different Colors, and three different quantities. space may or may not be limited, so I wanted to see what the possibilities are, learning opportunity too.

 

LEDs are not resistors.
Resistors are linear devices and obey Ohm's Law.
LEDs and all semiconductor devices are highly non-linear. Use the device specifications of current and voltage as guidelines, not hard and fast rules.

Rather than taking the results of circuit simulators as truth, breadboard the circuit and measure the real current and voltage.

 

By the simulators calculations, I am above the necessary current and below the necessary voltage.

The simulation uses default LEDs (probably red), not real life white LEDs. LED forward voltage (Vd) depends on the LED's chemistry, colour and current, and is also subject to manufacturing tolerances.

 

Also the same diode with the same part number can have a difference even if its from the same manufacturer.

 

LEDs, above a basic threshold, are real crappy diodes (depending on
process) and look largely like resistors. Eg, their exponential I-V relationship
essentially loses its classic semiconductor junction relationships. Their dynamic
resistance essentially becomes a constant.


Regards, Dana.

 

All of the previous responses are excellent. I recommend that you obtain the datasheets for any LED that you use. The datasheets will have graphs showing their behavior over different currents. They also provide two important parameters. Forward voltage and typical/maximum current.

Forward voltage is the voltage drop across the LED. Any voltage less than this amount will not light the LED.

What is important to know is that if the current exceeds the components max rating, the LED will be destroyed. Comparable to what happens in a resistive load when voltage is exceeded. For example, apply 120V to a 12V light bulb.

So current to an LED must be controlled. It could be from an LED driver, a constant current source or in simple cases, with a resistor.

As Ohm’s law tells us, there is a relationship between volts and amps, which is valid for resistive loads.

To use a resistor for current limiting, there is a simple formula. You need to know:

• Supply voltage, Vs
• LED Forward voltage, Vf
• LED current, Il

Then, the desired resistor is calculated as follows:

R = ( Vs - Vf ) / Il​

Of course, the calculated resistor value will likely not be a standard resistor value, so in practice you’ll need to pick the next higher standard value (perhaps this is a contributing factor in the discrepancy between your calculations and the simulation results).

 

I have 4 - White LED's - 3VDC - 150mA, 500mW. I am attempting to work out if its better to power these with 12v or 24. I have looked at constant current, I could go that route. I was trying to avoid the number of components i.e., going from 120VAC -> 12VDC or 24VDC Power Supply -> Constant Current Module -> LED's . I will eventually have 3 different size SMD LED's, 3 Different Colors, and three different quantities. space may or may not be limited, so I wanted to see what the possibilities are, learning opportunity too.

Do not use a Constant Voltage Supply ...
120VAC -> 12VDC or 24VDC Power Supply -> Constant Current Module -> LED's

Use a Constant Current Supply ...
120VAC => Constant Current Supply => LED's

 

Continuing...

Using your example, to light one LED, you’d have the following parameters.

1. Vs=12V
2. Vf=3V
3. Il=0.150A (likely a maximum value. you may want to calculate at a lower value as well)

Then...

R=(12-3)/0.15
=9/0.15
=60Ω​

The power rating would be P=Vr*Il or 1.35W.

Now assume a 24V supply

R=(24-3)/0.15
=21/0.15
=140Ω​

But, the power rating of the resistor becomes 3.15W. The difference is obvious.

I’d also consider a series parallel circuit, if you want to light more than one LED at a time. I’ll provide an example for you if this works in your design. All in all, it uses less power from your supply. You can figure out why.

Let’s say we want to light four LEDs with two in series and two of those strings in parallel. To calculate a resistor for two in series, we modify the above formula slightly.

R = ( Vs - n*Vf ) / Il​

where n is the number of LEDs. Hence,

R=(12-2*3)/0.15
=6/0.15
=40Ω​

The power rating now is 0.9W. Whereas lighting two LEDs individually needs 2.7W, by putting them in series we only need 0.9W.

Hope these worked out examples give you some direction into answering your question.