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 = (Vs - Vled) / Iled
$$R = \frac{V_S - V_{\text{LED}}}{I_{\text{LED}}}$$
Where Vs\$V_S\$ is the source voltage, Vled\$V_{\text{LED}}\$ is the forward voltage for the LED, and where Iled\$I_{\text{LED}}\$ is the forward current for the LED.
If my Vs = 5V\$V_S = 5\$V, Vled = 2V\$V_{\text{LED}} = 2\$V and Iled = 15mA\$I_{\text{LED}} = 15\$mA, then I calculate R\$R\$ as follows:
R = (Vs - Vled) / Iled
R = (5V - 2V) / 15mA
R = 3V / .015A
R = 200 ohms
\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 ohm\$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. So I ask: is
- I've been away from arithmetic for too long, or
- 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 ohms\$20\Omega\$ come from?!?
