Forward voltage of an LED to use?

Question

I'm making an LED array with the help of a online LED wizard ( http://led.linear1.org/led.wiz ) and it asks for the forward voltage number of the LED's. My problem is the LED's I have say 1.9-2.3V forward voltage. Do I use the high number, low number, or in the middle at 2.1V? Changing the number by just .1V changes the array big time.

Answer 1

You have to design for worse case. And that depends on your other parameters. Is the current you selected close to the maximum? then you have to assume the lowest voltage, otherwise the current could go over the maximum on some LEDs.



That means most of the LEDs will be operating at less than optimal brightness, but that is all you can do.



If you are using a current well away from the max, then I'd use the typical of 2.1. That means some will draw more than your selected current, some less. Check that the current, for a unit with the lowest Vf is below the max current.



edit: good question, most users don't bother with this detail, but a good designer must.

You also have to consider the tolerance on your power supply voltage. For example, if this is for an automobile, you can't use 12 volts, as the voltage when the battery is being charged can be as high as 15 volts. And don't forget the tolerance on the resistors.



EDIT2. Those online calculators are usually bogus. First of all they don't account for variations as you mention. And second, ohms law is all you need to find resistor values.

Answer 2

It doesn't rather subject which value you select. The ahead voltage range is small adequate compared to your deliver voltage that you just would use either worth with out appreciably altering the calculated resistor worth. After getting a calculated worth, you can nonetheless ought to prefer a real world value. A resistor that's rather bigger than calculated will outcomes in rather scale down current. There isn't any reason why you have to decide on 20 ma as the current for your utility. It's simply used as a benchmark for probably the most different requirements. Your battery will last 20 times as lengthy in the event you come to a decision to opt for 1 ma rather of 20. Of path the brightness can even be much less however it won't show up to be 20 occasions less. You would also go within the other direction as much as a special factor. Doubling or tripling the current will outcome in some further brightness on the fee of battery lifestyles. Too much present and you end up with serious mark downs in lifetime. Consider free to scan and even burn out a few. I've one LED that has been continually lit for over 33 years. I've visible others that were pushed well beyond there reliable limits last now not than a short flash. Don

Answer 3

Yes it is true that the range is wide. This is to allow for variations in the production process as well as different conditions of operation like current and temperature. The typical value is usually specified too, and there is a bell shaped curve that a larger number of leds fits around this value. This allows for production spreads. It is likely the production spread is less than allowed for, because th eproduction process is usually improved over time. On average most are closer to the typical value.



For design though, you will need to consider the maximum and minimum values for a worst case situation. The voltage to use is the voltage for the current you chose.



The higher value determines the minimum supply voltage for the number of leds in series, allowing extra voltage about 20% for the ballast resistor. WIth a battery, this would be for a low battery. A regulated supply is best as that removes one of the variables.



The lower value has an effect on the ballast resistor chosen, as the current can be too high if the led string has a low voltage or the supply voltage is high.



In the end it is a compromise. Use less leds and a bigger resistor value in each series string will solve the problem, as the current is more constant. The tradeoff is more power wasted in the ballast resistor. This can be overcome by an active regulator using switchmode techniques, but that is a step too far for you I think. There is also a matter of the power dissipated by the leds and the resistors. When all bunched up they get hotter than intended, and a derating is needed (less current). The 5mm type are rated for single items in free air.



The calculator in the link seems to come up with too many leds in series. Think of the number of leds in series is for 75% of the supply for the typical Vf and supply and things will work better.. A higher supply voltage up to about 32V is more efficient, if you have a big array.