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I am new to electronics and am tyring to figure out something that is a bit disturbing to me. I am trying to figure out the amount of resistance I need to put in series before an LED, and the equation I keep coming across is:

$$R = \frac{V_S - V_{\text{LED}}}{I_{\text{LED}}}$$

Where \$V_S\$ is the source voltage, \$V_{\text{LED}}\$ is the forward voltage for the LED, and where \$I_{\text{LED}}\$ is the forward current for the LED.

If my \$V_S = 5\$V, \$V_{\text{LED}} = 2\$V and \$I_{\text{LED}} = 15\$mA, then I calculate \$R\$ as follows:

\begin{align} R &= \frac{V_S - V_{\text{LED}}}{I_{\text{LED}}}\\ &= \frac{5\text{V} - 2\text{V}}{15\text{mA}}\\ &= 3\text{V} / .015\text{A}\\ &= 200\Omega \end{align}

However, double checking my math at the LED center, if you punch in 5, 2 and 15 in those fields, it will tell you that you need a \$220\Omega\$ resistor, and this worries me that either:

  1. I've been away from arithmetic for too long, or
  2. there's something else that I'm not considering here.

Is this web tool broken, or am I missing some important info/understanding here? Where did these extra \$20\Omega\$ come from?!?

Null
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smeeb
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    `* This calculator rounds the resistance up to the next standard resistor value. You should actually be able to buy a 5% resistor with the value returned by the calculator. ** Power calculations assume use of the standard value current-limiting resistor shown above. Resistor power ratings are chosen based on operating within 60% of the rated value.` - straight from http://led.linear1.org/1led.wiz –  Jun 08 '15 at 18:10

3 Answers3

27

The calculator is using 5% precision resistors, aka E24 resistors, but in such a way that it is impossible to exceed the given current. With a 200ohm 5% resistor it is possible to have a resistance as low as 190ohm which would result in a current of 15.8mA, thereby violating the 15mA constraint.

Ignacio Vazquez-Abrams
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20

This tool is automatically recommending one of the standard resistor values. 200 ohm resistors aren't a common value, so they recommend a 220 ohm, which is much more widespread. Your math is still sharp :)

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crocboy
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  • well, actually 20-ish values (red-black color code) *are* quite standard and *very* common nowadays, as they are regular E24 series values. I've seen them on many occasions, they allow easy 1:2 R ratio (e.g. for op-amp applications) with just two resistors (10, 20). I'd even say they are *more* common than 22 (red-red) values - Mouser gives me about 1650 different 200 ohm ones & only about 970 of 220 ohm ones... clearly not what I would call "[not] a common value" for 200 ohm & "much more widespread" for 220 ohm. –  Jun 08 '15 at 17:31
  • Not sure what your search criteria were, but typically in through-hole resistor kits the E12 values are the most common. – crocboy Jun 08 '15 at 17:52
  • "my" search critiria were "200 ohm" & "220 ohm" - http://www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1z0x8be , http://www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1z0x89u - also, resistor kits ain't something that talks about the spread of an element in electronics - it's the manufacturer's portfolios that are. Remember that through-hole components are nowadays mostly obsolete. –  Jun 08 '15 at 18:01
  • also - I'm not arguing with the fact that E12 values are more common in *amateur/hobbyist applications* (which is true, because they were ubiquitous since ages ago) - I'm arguing witht the opinion that "200 ohm resistors aren't a common value", which obviously ain't - even for through-hole components, 200 ohm vs 220 ohm is ~ 400/300 on Mouser. tl;dr - the fact *you* seldom seen them doesn't make them *uncommon*. It's the generality of your statement that I find wrong. –  Jun 08 '15 at 18:07
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    Yeah I just saw that he's new and doing things with LED's - so I assumed he's more of a hobbyist. I understand your point though – crocboy Jun 08 '15 at 19:22
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Because of the "variability" in manufacturing resistors, there is a tolerance/precision associated with the resistor's value. Because of this variability, in order to guarantee that the LED current will not be exceeded (with a 10% tolerance), the actual resistor value has to be 220 ohms. This is a real circuit calculation, rather than a theoretical/ideal circuit calculation. As can be seen, each answer is correct, depending on which point of view you use.

Guill
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