Incorrect LED-Resistor Calculations

To find the resistor in hm: needed to protect a led i use the following equation:

R = (Supply Voltage - Led Voltage drop ) / I.

This seems to give an accurate value, so i use the next biggest resistor, about 50hm: more. However when i measure the voltage drop over the led, it does not come even close to what i calculated. Nor does the current.

The multimeter appears to be working correctly, as it will measure accurately ( it agrees with my calculations) the voltage and current, in a simple voltage divider circuit.

Could these differences in calculated values be down to invalid specs given to me by supplier, or am i doing something wrong?

Also, if i know the current and resistance of a simple led and resistor circuit,
how can I find the voltage that runs through the LED?

thanks very much.

You can't calculate the LED voltage drop, you have to measure it. The LED voltage drop depends on the color, I gathered some data from catalogs which will give you an idea of what to look for.

LED manufacturers say a wide range of the voltage, and say the maximum voltage for their LEDs. "Our blue LED has a forward voltage of 2.5V to 4.5V at 20mA, and its max voltage is 4.5V.
How many volts are the LEDs that you got?

Hi, thanks for the replies.

I have tried many different types of LED's, and have enclosed details of them. The data that i Have compiled comes from the ebay shop that i bought them from.

Using these figures (SEE PIC) and the equation i mentioned before, i calculated the nessercary resistor for each one of these leds.

My source voltage is 5.87V.

For example, my calculation for the resistor for the SuperBright Blue led (3.8V @ 30mA ) is as follows:

( SupplyVoltage [5.87v] - LedDrop [3.8v] ) / 0.03A
= Resistance = 69hm:.


I then chose the nearest bigger resistor to this value (100hm, and tested its resistance. The actual resistance as measured by my multimeter turned out to be 99hm:.

with this resistor i measure 3.15v over the led, 2.6v over the resistor but only 20.1mA of current!

These values are no where near the calculated values.

Thanks

bitem2k said:

my calculation for the resistor for the SuperBright Blue led (3.8V @ 30mA ) is as follows:

( SupplyVoltage [5.87v] - LedDrop [3.8v] ) / 0.03A
= Resistance = 69hm.

Click to expand...

The 3.8V is probably the max voltage of some LEDs at the max current of 30mA. The typical voltage is probably less and will be less at a lower current.

I then chose the nearest bigger resistor to this value (100hm, and tested its resistance. The actual resistance as measured by my multimeter turned out to be 99hm.

Click to expand...

Your 100 ohm resistor reduces the current. With reduced current then the voltage of the LED will be less.

The next 5% resistor value is 75 ohms, then the next is 82 ohms, then the next is 91 ohms, then the next is 100 ohms. Your nearest value was way off.

The absolute max current for the LED is probably 30mA so it is good that you reduced the current.

with this resistor i measure 3.15v over the led, 2.6v over the resistor but only 20.1mA of current!

Click to expand...

2.6V over 99 ohms is 26.3mA. If you used your multimeter to measure the current then its resistance reduced the circuit's voltage and current.

Your LED is 3.15V at 26.3mA, much lower than the spec'd 3.8V max.

It shouldn't have. Even the cheapest multimeters voltage measuring impedances is over 1meg, definitly something going wrong with the measurment.

Ok,

thanks very much, that helped.