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Circuit design help... - basic_schematic.jpg (0/1)

Discussion in 'Electronic Design' started by Martin, May 30, 2004.

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  1. Martin

    Martin Guest


    Sorry to be posting what to some (most ;) ) of you guys will seem a
    really stupid question.
    Here is a link to a cct

    What I would like to know is, what is the function of the diode (D1)
    in this cct.
    I am making a similar circuit, Instead of using transitors I am using
    a LM7812 to regulate the voltage to the LED's, then a resistor to each
    LED array to limit the curent. Should I also be placing a diode before
    the current limiting resistor? Or do I not need a diode at all?

    I have attached a jpg of the basic schematic I am using.

    Any and all help gratefully received.
  2. Yoiu need no diode and no opamps!
    Switch and resistor will do.

    Someone's gone get pedantic about this :)
  3. Tim Wescott

    Tim Wescott Guest

    The diode is there so that there won't be _too_ much difference between
    the brightness when S2 is closed vs. when S1 _and_ S2 are closed.

    Your LM7812 idea won't need the diodes, probably, depending on what
    you're doing.
  4. Gareth

    Gareth Guest

    In the notes it says:

    "Please note that the brake current is obtained by paralleling R2 and R3

    so I think that the diode is there so that when SW2 is on current can
    flow through both resistors, but when only SW1 is on current can only
    flow through R2.
    That should work.

    Should I also be placing a diode before
    You don't need a diode.

  5. Martin

    Martin Guest


    Thanks for that.

    My circuit is also a stop and tail lamp unit. I am making it for my

  6. Martin

    Martin Guest


    I am using the LM7812 to regulate the voltage.
    This is being used on my motorbike where the voltage whilst on the
    move is in the region of 14V.
    I currently am using the cct without the LM7812, but would like to
    keep the voltage constant.


    Should I not have posted the jpg ? Feels guilty now !!
  7. (snip)

    I think the designer was after two brightness levels, one controlled
    by SW1 and one by SW2. But if the brightest level is near the limits
    of the LEDs, and you close both switches, you get a current that is
    the sum of what you get by closing each of the switches, and this may
    put you over the maximum current limit. D1 reduces the peak current
    to something less than a sum. (the current from SW2 plus about half of
    the current from SW1, equivalent to about 12 ohms total.

    There is a solution that eliminates the summing, totally. Move R2 to
    the position of D1 and lower its value to 18 ohms. When SW1 closes,
    it will have 15 + 18 = 33 ohms in series with it, but when SW1 closes
    it will have 10 ohms in series with it. Closing SW1 and SW2 at the
    same time is no different than closing just SW2.

    However, if you want there to be a significant brightness change when
    both switches close, (turn signals slightly more evident when the
    brakes are on) just eliminate the diode and use an 18 and 36 ohm pair
    so that the maximum current with both in parallel is about the same as
    the this circuit provides with the diode, but with a slight dimming of
    brakes only situation. The three effective resistances being 36, 18
    and 12. for a 1:2:3 brightness ratio.
  8. Fred Bloggs

    Fred Bloggs Guest

    The most reasonable explanation is that the diode is intended to achieve
    roughly the same brake light brightness regardless of whether S1 is
    closed or not. So if S1 is closed the LED current becomes
    0.8/(33||15)=75mA at S2 closed, and when S1 is open when S2 closes, you
    have Iled= (0.8-0.25)/33+ 0.8/15=70mA- about the same- and explains why
    it is essential to use a relatively high current Schottky. Without the
    diode, the difference would be more like 20mA.
  9. Activ8

    Activ8 Guest

    You mean regulator and right. You don't need that either. In fact a
    7812 won't work too well at 13.8 V or less, IIRC, will it?
    No shit. Autozone sells the LED lights with a 10 yr warranty. You
    just replace the old incandescent bulb.
  10. Martin

    Martin Guest

    I must admit I am a little concerned about this, I have read various
    specs and some say the 7812 will work from 13.5 volts upwards. This
    'should' be O.K.

    I have tried these and to be honest didn't like them, I am making a
    custom unit that fills the whole of the rear lamp unit.

    See http:// for my prototypes.
  11. Rich Grise

    Rich Grise Guest

    When I looked, I thought, "Hm. A diode OR." It's so that the brake/signal
    lamp will come on full brightness whether the running(parking) light is on
    or not.

    And hasn't anybody noticed the two parallel strings of LEDs with no
    current sharing?

  12. Ahm, but for LEDS to be constant, you need constant current!
    (Not a constant voltage), so the LEDS in teh collector of a NPN,
    with a small resistor in the emittor, and say a volt or so on the base.
    You can make a 1.5V wit ha LED on the base, and a high value resistor.
    So, anyways : CONSTANT CURRENT.
  13. All in good time. This circuit is very simple to arrange for parallel
    current regulation. Just add 1 transistor and another pair of
  14. Doubling the resistances, of course.
  15. Activ8

    Activ8 Guest

    Sorry. I'm not going to register with btinternet to view a page.

    I never had to worry about the dropout voltage on a 12v linear reg
    before, but the reason I brought it up is because that schematic you
    were asking about had a 12V source and that of course got me
    thinking automotive. Now *assuming* that you get 13.8 volt with the
    engine on, the inspection mechanics check brake and tail lights with
    the engine off and you might not quite have enough voltage to do all

    So, out of curiosity, what didn't you like about those lights from
    the parts store.
  16. Activ8

    Activ8 Guest

    Hmmm... Let's think about that. *If* he wants that effect. Noemally,
    one filament lights for brake or signal. If the brakes are on,
    signaling what? turns it off? I can't recall. I think the tail light
    is always a separate filament from the brake/signal, though.

    Now OP has come forth with info that this is for his scooter. Don't
    some bikes have a center tail and brake light and outrigger signals?

    Hey, isn't management/customer supposed to figure all these
    requirements out? Why do I find myself doing systems analysis before
    I get to design anything? :(
  17. Martin

    Martin Guest

    Sorry. I'm not going to register with btinternet to view a page.

    No need to register I made the folder public, I guess really that I
    should have given the correct address though ;)
    Didn't think of that, very good point well made :)
    The high intensity LED's used have a very narrow viewing angle and as
    such, even with the diffusing properties of the red lens cover do not
    provide a wide pattern of light, also the ones that I bought did not
    provide a white light to shine through the clear bottom part of the
    lens to illuminate the licence plate. For a car these bulbs are fine,
    but a bike, well in my opinion they are not really suited.
    I know that there are newer bulbs which have a white LED section at
    90degrees to the red to provide the illumination, but the viewing
    angle is not to my liking.
  18. Martin

    Martin Guest


    You are indeed correct, if you look into a stop / tail bulb you will
    indeed see 2 seperate filaments. The tail filament is around 5watts
    and the Stop filament is around 21watts.
    My basic schematic has the tail section being current limitied by a
    suitable resistor (to provide a current of say 0.01 amps) then when
    the brake is applied another resistor is applied to increase the
    current to 0.02.
    I wanted a voltage regulator to limit the voltage to a stable 12v so
    that the current would be constant.
    Now I am pretty poor with all this electronics mallarky but surely
    even if you are providing a resistor to limit the current, this
    current will not be constant if the voltage fluctuates?
    So if my bike runs between say 11.7 and 13.7 volts and I am using 5
    LEDs with I would need a variable resistance betwwen 320 and 520 ohms.
    That's why I wanted a constant voltage.
  19. Martin wrote:

    An active current regulator (not just a resistor) holds the current
    nearly constant in spite of both supply voltage changes and load
    voltage drop (and LEDs drop less voltage as they get warmer). Since
    it corrects for both effects but the voltage regulator corrects of
    only one of those effects, the current regulator is better.
  20. Activ8

    Activ8 Guest

    Here's a bit of malarky that might help. LM317 voltage regulator.
    Check out the spec sheet for constant current config. Just switch in
    the appropriate resistor(s).
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