Re: how to control LED array? (follow-up)

Michael Noone wrote:
>>
> https://netfiles.uiuc.edu/mnoone/www/Electronics/LED-Array.jpg
>
> This is essentially identical to what John Fields was suggesting,
> except I've modified some resistor values as I have now chosen LEDs:
>
> http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7513499883 (3.3v
> forward voltage, 20-30ma typical current)
>
> The design of this array calls for only one column to be on at any
> given time. The columns are active high, and the rows are active low.
> The goal is to saturate the transistors. The value for R1-R16 was
> chosen by dividing the typical base-emitter saturation voltage by the
> base current, thus 0.85/.015 = 56.67, or about 56 ohms. These values
> were found on page two of both the 2N4401 and 2N4403 datasheets:
>
> http://www.fairchildsemi.com/ds/2N/2N4401.pdf
> http://www.fairchildsemi.com/ds/2N/2N4403.pdf
>
> The value for R16-R24 was chosen by taking (V+ - 2*VCesat -
> Vled)/ILED = (5 - 2*0.4 - 3.3)/.03 = .9/.03 = 30 ohms. A current of
> 30ma was chosen because only one led on each row will be on at any
> given time, and as each column will only be on for 1/8 of the time -
> I feel it is best to use the maximum current allowed.
>
> Oh - lastly, if anyone is curious as to the odd placement of gnd on
> the row connector and +5v on the column connector (as the layout would
> probabaly be nicer the other way around) - I did this as a reminder
> that the rows are active low and the columns are active high.
>
> So how does everything look? Some of my calculations and final numbers
> differ from the original thread - thus I thought I should check back
> to see if I'm doing something wrong. Thanks so much,
>
> -Michael J. Noone

Did Mr. Fields also advise to take these 56 ohms base resistors? They will
consume more current than the LEDs. You can use 470 ohms for them. with 8mA
you can drive easily 100mA collector current in case you multiplex. You have
to calculate the voltage across the resistor, not the transistor. when you
have 4.5V control signal (typical PIC) and 0.75V Ube, then R= 3.75V/8mA =
468 ohm. You can even take 1k if you use transistors with beta of 150 or
more.
Uce sat will be a bit lower than 0.4V, maybe 0.15V, so it is better to use
47 ohm for 30mA. But if you always multiplex them and the program doesn't
get stuck with one LED always on, you can probably even put 100mA through
the LED with a duty cycle of 1/8. It would be better to make a current
source for the Leds then you do not need these resistors at all and can
easily adjust the brightness.

--
ciao Ban
Bordighera, Italy


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