H-Bridge + LEDs

Discussion in 'General Electronics Chat' started by msr, Aug 17, 2010.

  1. msr

    Thread Starter Active Member

    Jul 8, 2008
    62
    1
    Hello,

    http://lh3.ggpht.com/_ZJFezM7weLA/TGsXtrDNT6I/AAAAAAAAA2c/dnaBzcJlXGA/digitalizar0002.jpg

    I tried to understand this circuit before I have mouted it on a breadboard. I thougth I understood it but now that is mounted I have some doubts.

    I have just an half of this circuit on breadboard. The two leds are a RED led and an IR led, Im just testing the red led and the necessary circuit (transistors 1 and 2) to make it work.

    The 555 is used to light up leds alternatively. So when the pulse is HIGH transistors 1 and 2 must be conducting and the RED led will be ON. Right? In case I had IR included on breadboard, when pulse is LOW transistors 3 and 4 will be conducting and the IR led will be ON. Thats an h-bridge. Am I right?

    I haven't a DG303 chip but only a DG333AL from Vishay. Im using this circuit:
    [​IMG]
    Vin is the voltage applied to NPN transistors base in order to control current that flows through LEDs. "pulses" are the 555 pulses.
    I connected Vout_1 to transistor 2 base. However the red led doesn't lights up. Otherwise, when I connect Vout_2 to transistor 2 base it lights up. Why? Shouldn't be the opposite? Am I reading this circuit wrong?

    Also, Vin is a DC value, adjustable with a potenciometer, between 0-5V. However when I get about 800mV (+- the value LED starts to conduct) it starts to have a square "shape". Why? From DG333AL Vout I can get only about 1.3V maximum.

    Another question: what is the criteria to select the transistors? BC179 and BC108 are being used. Maybe maximum collector-emitter current (50mA for BC179 and 100mA for BC108)?


    Thanks for your patience!
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    When T1 & 3 are on nothing's going to happen, likewise with 2 & 4. An H-bridge operates as sort of an X switch, 1 & 3 conduct or 2 & 4 conduct when properly biased.

    The IC that's only going so low, well, that's probably only as low as it will go.

    Are you just wanting to alternately flash the two LEDs? If so, you can do that right from the output of the 555 and forget everything else.
     
  3. msr

    Thread Starter Active Member

    Jul 8, 2008
    62
    1
    I think you contradicted yourself. Besides not drawn in a "X" way, this h-bridge works when 1 & 2 conduct and when 3 & 4 conduct. Right?

    The purpose is flash the two LEDs but I also have to control current flowing through LEDs. Next step is implement an AGC.

    I didn't get the DG333 part, sorry... Why LED is ON when I connect transistor 2 base to Vout_2 and not the oposite (Vout_1)? And why Vout starts to have a "square" shape once LED starts conducting?

    Thanks!
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Not the way they're designed, in essence they're an electronically controlled DPDT switch. Yours isn't even drawn as one, look at the second simplified circuit here:

    http://en.wikipedia.org/wiki/H-bridge
     
  5. msr

    Thread Starter Active Member

    Jul 8, 2008
    62
    1
    That's not what matters. It works like an h-bridge, the purpose is to flow current through RED and IR led. Once they are inversely placed (in terms of polarization) the sources and sinks of h-bridge can't be side by side, but an half of the circuit is "inverted". I got this circuit from a TI app note, so I think there's no mistake in here...

    Thanks for your reply. I will keep analysing this circuit. However if anyone have more ideas to share, I will appreciate.
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    I'm speaking of your hand drawn circuit which I can now see how it would function, it just isn't drawn the way I'm used to seeing things. Perhaps sme of that stuff you're using to feed it with threw me off as well.
     
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