02GF74
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| posted on 20/10/10 at 02:07 PM |
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LED wizard
noone need ask about wiring LEDs ever again 
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MikeRJ
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| posted on 20/10/10 at 02:14 PM |
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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.
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02GF74
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| posted on 20/10/10 at 03:00 PM |
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^^^ yes, a pretty fundamental error.
The wizard deserves a kick or two in the googlies, no doubt about it.
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v8kid
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| posted on 20/10/10 at 03:57 PM |
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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.
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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MikeRJ
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| posted on 20/10/10 at 04:29 PM |
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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).
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