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diode series or array?

Discussion in 'Electronic Basics' started by [email protected], Dec 29, 2005.

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  1. Guest

    I need a series of led's (about 7) to use as a voltage reference. I
    want to use led's because I need a certain tempco and using a series of
    led's for the voltage reference will give me the right tempco. Ideally
    I'd like a part that I can get cheap (naturally), maybe from one of the
    surplus houses like allelectronics, goldmine or bgmicro. But I haven't
    found anything from them, so I might have to go to a regular supply
    house. I used digikey's excellent web site to search for diodes in
    series or array but didn't get anywhere.
    To make the prototype I soldered led's in series by hand, but I may end
    up making a bunch more of these for friends and I'd like to find a part
    that has led's (or for a distant second choice, ordinary silicon
    diodes) in series, or perhaps array. For reasons that I don't want to
    go into here, I won't use a single led and divide the voltage up
    resistively.
    Any info on availability of a part like this? Nothing fancy, just a
    bunch of led's in series, and the smaller the better.
     
  2. Maybe use some SMT LEDs. But there are many kinds of LEDs with
    different forward voltages at a given current, different curves of
    voltage vs. current and different tempcos. 7 x 3mm leds will probably
    be pretty cheap and small if you have the height.


    Best regards,
    Spehro Pefhany
     
  3. kell

    kell Guest

    You know, I'll probably just buy a cheap bag of surplus red led's,
    which
    will probably have a Vf a bit over 2 volts. They won't come with a
    data sheet.
    I'm wondering how much tempco varies among plain-vanilla red led's?
    I thought it would be about 3 mv/degC.
     
  4. Franc Zabkar

    Franc Zabkar Guest

    What about an LED bargraph?

    http://rocky.digikey.com/WebLib/Fairchild/Web Photos/New Photos/MV53164,MV54164.jpg

    - Franc Zabkar
     
  5. Pooh Bear

    Pooh Bear Guest

    How did you establish this tempco ?

    If you've selected a part why not simply say what the part number is ?

    Graham
     
  6. Pooh Bear

    Pooh Bear Guest

    Any reason even to expect it's consistent ?

    Graham
     
  7. Jasen Betts

    Jasen Betts Guest

    ["Followup-To:" header set to sci.electronics.basics.]
    a bunch of SMD leds soldered end-to-end ?
    a bargraph module?

    Bye.
    Jasen
     
  8. Rich Grise

    Rich Grise Guest

    Well, aren't they generally made out of the same kind of stuff?

    And isn't "band gap potential", or whatever you call it, kinda related
    to the type of material itself?

    I've never done any tests like this, I just sort of ass-u-me-d that
    a bag of LEDs would have come from the same run, and be fairly consistent
    from one to the other. I wouldn't be so confident from batch to batch,
    but you get the idea.

    Thanks!
    Rich
     
  9. Jamie

    Jamie Guest

    http://cache.national.com/ds/LM/LM3914.pdf
    there chip. just google for a dealer to sell them..
     
  10. kell

    kell Guest

    I haven't selected a part. That 3 mv figure is just something I saw
    somewhere on the net.
    It looks like I am going to have to do something ridiculous like dunk
    led's in boiling water or something to measure their tempco, or just
    give up on it, because I find basically no info about it.
     
  11. budgie

    budgie Guest

    if it's just the tempco that you need, have a look at zeners. Their tempcos are
    usually documented, and varies with the breakdown voltage. From memory, the 5v1
    units have a nearly zero tempco. IIRC it's negative below 5v1 rating and
    increasingly positive as you go up in voltage above.
     
  12. Pooh Bear

    Pooh Bear Guest

    Since the OP won't reveal his real requirements I hardly know what to suggest.
    I know I wouldn't trust the results from a few leds out of a bag.

    Graham
     
  13. kell

    kell Guest

    Don't worry about it. I'll adopt a more effectual approach.
     
  14. carl0s

    carl0s Guest

    Oooh, that's interesting. I have been wanting a dash-mounted LED RPM & Speed
    display for my car, since the needles are obstructed by the steering wheel
    at my driving position. This looks like it would let me create a rev-counter
    bargraph off of the back of the analog needle, somehow, although my
    electronics knowledge is very poor. Do you have any tips for something that
    would let me drive a three-digit speed (MPH) display off of the back of the
    speedo (analog needle) voltage? I think calibrating the speed might be
    difficult, whereas I could do the rev-counter on my driveway, but something
    which would let me adjust for a scale of 0 - 200 over a voltage change of
    0 - 5v or whatever the car uses would be handy.

    thanks,
    Carl
     
  15. Jasen Betts

    Jasen Betts Guest

    ["Followup-To:" header set to sci.electronics.basics.]
    not quite that simply, a typical analogue tacho measures the "pulse rate" of
    the cars ignition system externally.... I guess somewhere deep inside that
    could be convertted to a voltage (but it might not be - typical moving coil
    meters only give 90 degrees of motion, not the 300 degres seen on automotive
    dials) sop there's probably some other drive mechanism....

    there are bargraph tacho kits available.
    if there's a voltage there get a three digit milivoltmeter unit and scale
    the voltage apropriately.
    Lift the driving wheels off the ground, calibrate to your existing
    instrumentation. or buy some time on a chassis dynomometer(sp?)
     
  16. Guest

    Mechanical car speedos are usually driven by a rotating cable from the
    gearbox, and are all mechanical.

    Probably the simplest way to convert to digital would be to glue a pot
    track on the instrument face, with a pot wiper on the needle. Run it
    off a regulated 10v or whatever, and feed to a multimeter or voltmeter.
    Bit crude, but youre going to have fun trying to do it other ways.


    NT
     
  17. Guest


    yes bargraphs are available, but pricey. Same goes for high power led
    arrays, with series paralell elements. If you want an array of leds,
    buy the leds... what else is there to say


    NT
     
  18. Hal Murray

    Hal Murray Guest

    Mechanical car speedos are usually driven by a rotating cable from the
    "All mechanical" is an interesting term. The one I took apart as a kid
    (many years ago) had a magnet rotating on the end of the cable. That
    was near a conducting disk on a spring loaded needle. Eddy currents
    and such.
    The one I played with wasn't very strong. I don't think a scheme
    like that would work. Maybe newer ones are different.
     
  19. Jasen Betts

    Jasen Betts Guest

    ["Followup-To:" header set to sci.electronics.basics.]

    If there's enough torque behind the needle to move the wiper that could work.
    but, most mechanical speedos work bt a rotating magnet driving an aluminium
    disc (or cup) by induction, opposed by the force of a very weak spring.

    a carefully placed hall effect sensor could detect the passes of the magnet

    the rate of these pulses would need to be translated into the speed...
    the sort of thing a small microcontroller excells at.

    Bye.
    Jasen
     
  20. Rich Grise

    Rich Grise Guest

    So, you tear apart the speedo, and mount a coil or Hall thing next to
    the rotating magnet, and voila! PFM!

    Cheers!
    Rich
     
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