 Hello,

I have an LED that needs a 125ohm resistor. I do not have that value available but want to get as close to that number as possible. If I used two resistors, a 22ohm+-5% and a 100ohm+-5% would this work? I know that the base value would only be 122 but would the tolerance cover for the difference?

My Logic: 22 + 5% = 23.1 22 - 5% = 20.9 100 + 5% = 105 100 - 5% = 95

Does this mean that my ohm amount is between 115.9 and 128.1? If so, would this damage the LED over time because the tolerance could be lower than the minimum of 125 or would the LED be ok because the tolerance can be higher than 125?

Thanks,

For the average LED that should be plenty good, ±5% will hardly make a difference even with the tolerance stack.

You can always calculate what currents are set by using some other than specified resistors. If you want to be sure without calculation, always go for the next higher value resistor. In this case you could consider using a 27 Ohm resistor, which should get you closer to the desired total value.

Use a 150ohm. Slightly lower current will flow through the LED but I doubt you'll notice. How did you establish your 125ohm value - based on maximum LED current or what

If you just want an LED as an indicator rather than to get a particular light level out of it, an order of magnitude over value is near enough. Just stick a 1K on it, it will light and the current certainly isn't going to damage it. In fact for many digital applications an order of magnitude (10:1) is near enough. Its only comparatively recently that resistors with better than 10% tolerance have become affordable, +/- 20% will work for almost anything no matter how tetchy. I've some 'water clear' LEDs in my box with the almost standard 20mA current rating that light convincingly with a 10k resistor on 5V. Given a forward voltage of 2.2 V it means they light with a mere 280 uA of current.

The 20mA current value is a nominal value commonly used on data sheets when specifying LED parameters such as the light output, and the forward voltage quoted is also a nominal value that may vary within the limits stated on the data sheet...

So there is no need to use a precise resistor value to set the LED current: you can use a lower value if the light output is satisfactory, or a slightly higher value provided the absolute maximum value quoted on the datasheet is not exceeded.

cld_1: So there is no need to use a precise resistor value to set the LED current: you can use a lower value if the light output is satisfactory, or a slightly higher value provided the absolute maximum value quoted on the datasheet is not exceeded.

"Higher and Lower" might be confusing to a beginner. The "lower value" refers to current (achieved by using a higher value resistor) and "higher value" again refers to current (achieved by using a lower value resistor)

A very fair point, so...

"The 20mA current value is a nominal value commonly used on data sheets when specifying LED parameters such as the light output, and the forward voltage quoted is also a nominal value that may vary within the limits stated on the data sheet...

So there is no need to use a precise resistor value to set the LED current: you can use a lower current if the light output is satisfactory, or a slightly higher current provided the absolute maximum value quoted on the datasheet is not exceeded."