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Author Topic: MrWell's pit dream  (Read 1620 times)

Offline mrwell

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Re: MrWell's pit dream
« Reply #15 on: June 16, 2015, 04:05:13 PM »
I don't know what I'm doing wrong. I use this calculator:

http://www.shoptronica.com/img/images/calculo%20resistor%20led.htm

where it says 'Leds en paralelo' (in parallel). I use 12V, then 1.9V, then 20mA (as you said) and 26 leds. The result is 22 Ohm.
I have them connected that way but I haven't checked yet because I don't have the correct resistor. I used the same calculator for my ATD and it seems to works good. I used only one resistor for the whole circuit, not one for each led.
Cheers!

Offline henkie

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Re: MrWell's pit dream
« Reply #16 on: June 16, 2015, 04:56:53 PM »
Ahhh, now I understand  ;D
I hate to be the messenger of bad news ... this method is not the best way  :whistle:
It will work (I assume), but from an engineering view it is "not-done" to do it this way.
It would only be okay *IF* all LEDs have exactly the same voltage/current curve.
Even when the LEDs are from the same batch, that will not be the case.

Note after the calculation the power rating of the resistor (Watts). The sum of all LED current (that is 26 times 20 mA) flows through the resistor. Power rating for this resistor is the product of resistor value times the square of the current. To be on the safe side over-dimension the resistor, so take a 10 Watt type. It will generate some heat!

Offline mrwell

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Re: MrWell's pit dream
« Reply #17 on: June 17, 2015, 02:04:20 AM »
Understood. Many thanks. I didn't know about that bad side of leds parallel wiring  :-[

Offline henkie

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Re: MrWell's pit dream
« Reply #18 on: June 17, 2015, 06:41:11 AM »
If you still have to do wiring for the LEDs you may do one of the following approaches.
  • Connect each LED with its own series current limiting resistor to the power supply.
    Calculation of the resistor value is easy, and the resistor can be small (no significant heat generated).
  • Connect several LEDs in series with one appropriate series current limiting resistor.
    Given a power supply voltage of 12V you can connect 3 or 4 LEDs in series. 5 might be possible, but I personally would not do that. Note that the forward voltage (specified 1.9V) has certain tolerances, and also depends on the forward current. Resistor can be small (no significant heat generated).
- Henk

Offline mrwell

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Re: MrWell's pit dream
« Reply #19 on: June 17, 2015, 07:36:58 AM »
I'm afraid I have it already done  :brickwall: I'll be conservative and I'll add bigger and better resistor for the parallel approach. I put a lot of work on the circuit (I made strips by hand with a Dremel!). I can go for 5V or 3.3V and avoid too much heat...
Thanks again