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Thread: Help needed with making a pov display for a 3d music visualization development kit

  1. #1
    Junior Member
    Join Date
    Mar 2018

    Help needed with making a pov display for a 3d music visualization development kit

    Hi forum,
    First time posting here
    I'm making a pov display for a 3d music visualization development kit.
    I am going to control 160 rgb leds, split into 10 layers of 16. I will control commonly each rgb leds state, with standard shift registers, common cathode with transistors. Additionally the other side of the leds will be controlled with three common pwm colour channels.
    I thought that this was a simple problem, and all I need is the right transistors and a current limiting resistor. But talking to my mentors work mate, he says this won't work(or maybe just not very well).
    I went to my workmate and fiends for help and checked out many websites and blogs( such as Kynix Semiconductor Electronic Blog I think I found the solution, correct me if i'm wrong.
    I can't simulate it because the specific parts i'm using don't have ltspice files. Plus I'm new to ltspice and I assume its not worth making the parts in the program manually, just for this?
    I will use these individual led transistors to step up for the large pwm lines.
    The gain on this transistor will be about 70 meaning I only need to put in 0.2ma from the micro.
    Putting 0.5ma in the first transistor I should have again of about 60, giving me the ability to switch 30ma. estimation from;
    Ic =0.1ma vce =1v gain = 40
    Ic =1ma vce =1v gain = 70
    Ic =0.5ma vce =10v gain = 100
    The current limiting resistor therefore; 0.5=(3.3-.65)/R ***R = 5.3 = 5 Kohm (giving 31.8 ma to next transistor) *

    Then I will use this transistor for the main switching.
    This will have a gain of 150 min 50*31.8= 4770=4.7Amps

    But I only need 3.2Amps so I will limit the base current to around 25 ma
    25 = (5-.85)/R *R=166 = 160 ohms( gives 26ma)
    26 ma 8 150 gain = capable of switching 3890ma
    more than capable of switching the 3.2amps max I need

    Checking the data sheets I think 50khz is well within the range of the circuit
    Click image for larger version. 

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    This is the transistor I am thinking of using, will it work?
    I guess the 160Ohm is meant to be between the collector of the first transistor and the base of the second, also to ensure I can switch the main transistor off there will be a 4k resistor between 5v and the base of the main transistor?
    Yes I know I need current limiting on the main load, I will have 160 leds in parallel each with their own resistor. I just left it out of the diagram for simplicity.
    I have LTspice, but it doesn't have a pnp with enough power. I tried a darlington set-up but I couldn't get it to switch more than 1.3amps. The circuit I showed did work, but it wasn't with the specific part or same resistor values.
    Why would I need another transistor the drop before the load isn't to large and the dc gains and current limits of two work, as one of my work mate suggested?
    Would I be able to get 50khz+ with a FET, I thought they couldn't handle high speeds. If they can how would I drive them to permit that?
    Can anyone suggest a circuit you know is capable of filling the need I have?

    Many thanks,

  2. #2
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Pretty sure this circuit will not work in practice. Nothing (reasonable) limits the base PNP current.

    R2 would limit the current as you suggested if it were placed between the NPN collector and PNP base. As you've draw this circuit, it won't have any current limiting effect.

    Now that I've found this minor issue for you, I'd like to ask which micros will be actually used in this project? Or is the project just for academic simulation?

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