Re: A useful website for LED array design

iyad.marzouka@xxxxxxxxx wrote:
On Oct 17, 11:33 am, Jasen Betts <ja...@xxxxxxxxxx> wrote:

On 2008-10-16, iyad.marzo...@xxxxxxxxx <iyad.marzo...@xxxxxxxxx> wrote:


But what's wrong with the 1 ohm resistor?

plenty, it might a well not be there.


LEDs do not have internal resistance

yes they do, not enough, but some, but what's worse is the voltage drop they
present is not constant it varies according to temperature (and probably
other factors)


and that's why you should place at least 1 ohm resistor to
limit the current on that line.

1 ohm is not nearly enough.

buit even assuming perfect 3.5 V leds and a perfect 10 V supply
with a 1 ohm resistor there won't be any current flowing.

Bye.
Jasen


Thank you everyone for your feedback.

I will improve the calculator to take into account the Vf and supply
voltage ranges (%).
Good.

One more question. How should the design be if you have a perfect 10.5
power supply voltage and perfect 3 x 3.5 fV LEDs like in this case:
http://ledcalculator.net/default.aspx?values=10.5,3.5,30,3,0

Should the 1 ohm resistor be ommitted? or should we put the third LED
on a new parallel line?

Offer the user a field to enter minimum percentage of the supply voltage to drop across any series resistor. Start another parallel chain of series LEDs when the voltage across the resistor would otherwise be less than the minimum value. Initialize this field to something reasonable like 30% as a default.

Otherwise we'll just give you a hard time for the resistor being too small for safety for cases like:
http://ledcalculator.net/default.aspx?values=10.55,3.5,30,3,0
1.8 ohm!!

Fix the divide by zero error if zero current entered (eg. current less than 0.1 mA should prompt for a value in range like you do for other daft entries.

Its a matter of style, but it would be far more usual to put the resistor between the LEDs and the positive supply, not between the LEDs and ground. (It also reduces the risk of damage if the circuit is in a grounded metal case and one of the LEDs gets shorted to the case.)

Also it would be to force the number of LEDs in each chain to be as near the same as possible instead of putting a single left over LED in the last chain. (Integer divide total LED number by number of chains, then add 1 LED to each chain until you've used up the remainder.) That way the relative brightness of different chains does not change so much when the supply voltage varies. As the aim is to get the same current in each LED, the efficiency and total power dissipated in the LEDs and also in the resistors will remain the same (neglecting small variations in the current in each chain due to rounding the resistors to the next preferred value) as that given by the current version.

I'm keen to see a check box to choose between 5% and 10% resistors (always choose a higher value as the chosen current should never be exceeded).

That should keep you busy for a while . . .
I'll try to remember to check back next week to see your progress if I've got any spare time.




.

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