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LED wizard

02GF74 - 20/10/10 at 02:07 PM

noone need ask about wiring LEDs ever again

MikeRJ - 20/10/10 at 02:14 PM

Be very wary of the results this gives, it's not a very "clever" wizard. In fact it's a bit thick.

e.g. ask for 4 LEDs with a Vf=3.0v to be powered from a 12v source and it will suggest putting 4 LED's in series with a 1 ohm current limiting resistor. That's great if you live in a perfect world where your 12v source is always exactly 12v, and the Vf of your LEDs never varies from 3.0v. In reality a small change in supply voltage or Vf (e.g. due to temperature) would see a large change in current.

02GF74 - 20/10/10 at 03:00 PM

^^^ yes, a pretty fundamental error.

The wizard deserves a kick or two in the googlies, no doubt about it.

v8kid - 20/10/10 at 03:57 PM

Isn't that down to inputting the right values in the first place?

If the voltage is nominally 12v but in practice is say 14.4 it don't take a lot of brains to guess which is the right value to input.

MikeRJ - 20/10/10 at 04:29 PM

quote:
Originally posted by v8kid
Isn't that down to inputting the right values in the first place?

If the voltage is nominally 12v but in practice is say 14.4 it don't take a lot of brains to guess which is the right value to input.

No, it still produces results that won't work well in an automotive environment. It assumes the supply and LED voltages are fixed and it tries to minimise the voltage drop across the resistor by putting as many LEDs in series as possible. This is good for efficiency, but no good if you have a voltage source that varies considerably.

LEDs are current driven devices, if you want them to work at constant brightness then you need a constant current. A constant current source has infinite impedance - to emulate this behaviour with a resistor the resistance needs to be as high as possible.

Ideally the web page needs to ask for the possible range of input voltages and then ask if you want to design for either highest efficiency (smallest resistor) or the most constant current (highest resistor).