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Help needed to choose right transistors — Parallax Forums

Help needed to choose right transistors

AndreaAndrea Posts: 17
edited 2010-04-12 20:09 in General Discussion
In my projects I have 8 rows x 8 columns LED grid.

Each cell contains 4 LEDs (5mm, Super-Red, If=30mA, Vf=2V) connected in parallel and powered by 12V trough 100 ohms resistor.

SX28 pins on RB used to control the LED cathodes - COLUMNS and the pins on RC to control the LED anodes - ROWS.
Now, I need some PNP and NPN transistors to control ROWS and COLUMNS, am I right? If so what you suggest?

I try to find ZTX605 or ZTX689B (suggested by Nuts &Volts column #6) with PNP complementary. Unfortunately in my area suppliers do not deal with ZTX series, instead supplier recommended BC639 and BC640 but datasheet says base current is 100mA. My opinion is that SX28 can’t handle it.
So, any advice is welcome.

Thank you

Comments

  • LeonLeon Posts: 7,620
    edited 2010-04-07 10:05
    That's the maximum base current! You won't be using anything like that if your circuit was designed properly.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Leon Heller
    Amateur radio callsign: G1HSM
  • AndreaAndrea Posts: 17
    edited 2010-04-07 10:26
    Thanks Leon.

    Other question is what is base-resistor-value, SX pin > BC639/640 and how to calculate it?

    www.datasheetcatalog.org/datasheet/siemens/BC639.pdf datasheet.

    Thanks
  • sylvie369sylvie369 Posts: 1,622
    edited 2010-04-07 11:02
    Someone just sent me this page in response to a similar question I had:

    http://www.kpsec.freeuk.com/trancirc.htm

    It looks pretty useful.
  • Martin HodgeMartin Hodge Posts: 1,246
    edited 2010-04-07 15:49
    Nice page! Thanks.
  • AndreaAndrea Posts: 17
    edited 2010-04-07 19:24
    Thanks, that's what I looking for.

    And for the end last question:

    Is this connected in correct way?...for test purpose I cut off matrix and took just one cell, instead SX pin VDD from SX-Teach board used for base of transistors...and guess what, nothing happening. What is wrong?
    Help plz!

    Post Edited (Andrea) : 4/7/2010 8:04:51 PM GMT
    img1.jpg
    1143 x 457 - 59K
  • TonyWaiteTonyWaite Posts: 219
    edited 2010-04-07 20:38
    Hi Andrea,

    The circuit as printed looks fine.

    For your testing, remember to use a logic one to turn on the 'npn' type and logic zero to turn on the 'pnp'.

    You did use base resistors too?

    Regards,

    T o n y
  • kwinnkwinn Posts: 8,697
    edited 2010-04-08 03:54
    I would suggest putting the four leds in series rather than parallel if you are running them from 12V. Uses 1/4 the current.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-04-08 04:15
    Andrea,

    I'm afraid your circuit will not work. The base voltage on the PNP can never be higher than 5V, which is 7V less than the PNP's 12V emitter supply. What this means is that the PNP will always be conducting, regardless of the SX's output. You will need to drive the PNP's base via an NPN transistor, configured open-collector and pulled up to 12V for your circuit to function as you want it to.

    -Phil
  • AndreaAndrea Posts: 17
    edited 2010-04-08 17:25
    Thanks guys.

    Finally I got some results...problem was wrong datasheet.
    -correct link: www.datasheetcatalog.org/datasheet/fairchild/BC639.pdf

    NPN works! But with PNP happened exactly as Phil said.
    Phil said...
    PNP will always be conducting

    Now I got new headache...how to configure open-collector and pull up to 12V? what should be resistor value?

    Phil, can you post some drawing for me, pleas?

    Thanks
  • LeonLeon Posts: 7,620
    edited 2010-04-08 18:48
    You need something like this:

    www.leonheller.com/images/Driver.pdf

    I simulated the circuit using Pulsonix SPICE. The current through the 200R resistor (representing your R1 and the LEDs) is 59.6 mA and the voltage across it is about 11.9V. It cuts off OK when either input is reduced to 0.1V. The resistor values were just an educated guess, I didn't bother to calculate them.

    Try calculating your own resistor values and using SPICE, it's very instructive. Or, I'll check them for you. Simulation can save a lot of work, and avoid damaged components.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Leon Heller
    Amateur radio callsign: G1HSM

    Post Edited (Leon) : 4/8/2010 7:20:21 PM GMT
  • AndreaAndrea Posts: 17
    edited 2010-04-12 19:36
    Leon thank you once more.
    Several days ago I sign up for SPICE, but never got a link for trial download from Pulsonix.
    Thanks to Leon's schematic and Wikipedia now I understand (or hope so) what is open collector configuration and what is a pull up resistor .
    But, I still struggling to find formula to calculate right value of pull up resistor. In attempt to do that this is what I found:
    VIH Vcc [noparse][[/noparse]min] - {VResistor [noparse][[/noparse]min] x IIH [noparse][[/noparse]max]} freaked.gif

    On this link:
    www.interfacebus.com/IC_Output_Input_Pullup_Resistor_Values.html (I don't understand any damned word.)

    So, could you or anybody show me a "Parallax style(easy to Swa[noparse][[/noparse]fo]llow)" example or as last option to calculate the value for me.
    As I mentioned before NPNs are the BC639 and the PNPs are BC640.

    Thanks

    PS"To make things worst for me, English is not my native language and my background is a software not electronics."

    Post Edited (Andrea) : 4/12/2010 7:57:40 PM GMT
  • LeonLeon Posts: 7,620
    edited 2010-04-12 20:09
    I'll ask them about it tomorrow if you let me have your email address.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Leon Heller
    Amateur radio callsign: G1HSM

    Post Edited (Leon) : 4/12/2010 8:30:58 PM GMT
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