LED arrays

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LED arrays

Postby Radio_Dave » Sat Jul 13, 2013 10:53 pm

I've made three Klaas Robers single transistor LED drivers for a 2 colour televisor. I notice that the two example LED arrays draw 100mA. Is this 100mA important to the circuit?

I want to use five red 2v 20mA LED's and five of each green and blue 3.5v 20mA LED's. I could make arrays that draw 100mA but I'd be dropping a lot of voltage!

Thanks
David
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Postby Klaas Robers » Wed Aug 07, 2013 9:45 pm

Yes, the 100mA is important for the circuit. The gamma of the pure BUZ11 isn't correct. That is the reason of the resistor from source to ground. The value is choosen to approximate a gamma of 2 as close as possible. For a different current for white a different resistor value must be used.
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Postby Radio_Dave » Thu Aug 08, 2013 4:25 am

Thanks for the reply :) .

Unfortunately I went ahead and made my array before your reply. I used six of each colour, which made 100mA unobtainable. The six red LED's draw 40mA and the six green and blue ones, each, draw 60mA. How do I calculate the correct resistors to use?

Thanks
David
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Postby Klaas Robers » Fri Aug 09, 2013 5:48 am

I expect that you routed the LEDs in series, so the resulting current for the RED chain is 40 mA and the current for the Green and the Blue chain is 60 mA. That is the optimal configuration. You need a somewhat higher voltage for the LED chains, see that you have about 10 volts left for the FET. It is not needed that this voltage is stabilised.

The first aproximation is for the 60 mA RED Led:
- the source resistor is 5,6 ohm / 0,6 = 9,3 ohm (try 9,1 ohm)

For the 40 mA Green and Blue LED chains:
- the source resistor is 5,6 ohm / 0,4 = 14 ohm (try 15 ohm)

It should be such that for a voltage increase of 1 volt that the current increases from 1 mA to 60 (or 40) mA.

If your LED configuration is different, please let me know.
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Postby Radio_Dave » Fri Aug 09, 2013 11:06 pm

Thanks Klaas :)

Best Regards
David
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