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I'm reading this datasheet: http://www.jaycar.co.nz/products_uploaded/ZD0012%20-%20AL-50-30RGBC-C-004.pdf

The Preferred Value Series Resistor on this page recommends 510/470/470 Ohms for 12VDC which to me seems too low because the maximum current is specified as 20mA per channel while Ohms law says

12V / 470 Ohms = 25.53mA

Which is above the recommended 20mA. Why is this?

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Ohm's law applies only to the resistor, so you need to use the voltage across the resistor when calculating current. That means you need to subtract the LED forward voltage from 12V before dividing by the resistance.

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    \$\begingroup\$ So for the green channel for example it is (12V - 3.2V) / 470Ohms = 18.72mA? \$\endgroup\$ – Marko Jan 4 '14 at 21:30
  • \$\begingroup\$ Yes, that's correct. I think the red channel still exceeds 20mA so I can't explain why they recommend 470 ohms. \$\endgroup\$ – Joe Hass Jan 4 '14 at 21:32
  • \$\begingroup\$ Yes. The 12V is across both the resistor and the LED. This tends to render the selection of resistor a somewhat iterative process to get a particular current. Luckily it doesn't usually need to be exact! \$\endgroup\$ – Stephen Collings Jan 4 '14 at 21:32
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    \$\begingroup\$ @Marko: Almost. Anything more than just "19 mA" is wrong since it gives the illusion of more information than is really available. In practise you won't be able to see the difference between 19 mA and 20 mA, even side by side. The normal variation between different parts will be more than the 5% difference from 19 to 20 mA. "18.72 mA" is just absurd. Even you did know the LED voltage that accurately (and you don't), you'd have to use better than a .1% resistor to claim that. \$\endgroup\$ – Olin Lathrop Jan 4 '14 at 21:36
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    \$\begingroup\$ @Marko: The 470 Ohm resistor value seems right. I was commenting on your stating the current to 4 significant figures, which is so much more detail than you can possibly know to the point of it being outright wrong. \$\endgroup\$ – Olin Lathrop Jan 4 '14 at 21:44

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