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Running LEDs on five volts

Discussion in 'Electronic Basics' started by Daniel Prince, Jun 12, 2005.

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  1. I want to run some LEDs on five volts from one of the USB ports on
    my computer. Can I just use three of them in series or do I need to
    use a current limiting resister.

    I want to keep it as simple as possible. The LEDs I have are a
    package of 20 assorted (RS cat no. 276-1622). They do not have any
    specifications with them. What is my best option? Thank you in
    advance for all replies.
  2. John Fields

    John Fields Guest

    I see.

    Here you are, a stupid **** who has to ask for help on how to wire up
    LEDs and yet, startlingly intelligent enough to be able to impugn the
    education and motives of the medical community.

    **** you, asshole, you'll get no help from me.
  3. John Smith

    John Smith Guest

    .... might just as well not help me either...

    MD's which are worth a chit are few and far between--from my
    observations, most have mental problems... your post suggests you might
    have those qualities--have you considered becoming an MD?

  4. Bob Eldred

    Bob Eldred Guest

    You've got something against resistors? Maybe they're too expensive at a
    couple of bucks per hundred?
  5. Lord Garth

    Lord Garth Guest

    Is that a Magnetic Disc or an M.D.?

  6. Are you claiming that all intelligent people automatically know how
    to wire LEDs?
    I have been disabled with ME (Myalgic Encephalomyelitis): IDC 10
    (International Disease Classification 10) code of G93.3 for more
    than 32 years. Articles about ME have been published in the peer
    reviewed medical literature since 1959. ME has been in the IDC
    since 1969. The IDC classifies ME as a neurological illness.

    I have probably been to more than 100 MDs. None of them knew much
    about my illness. Most of them knew NOTHING about my illness. None
    of them were willing to learn much about my illness. Most of them
    were unwilling to learn ANYTHING about my illness.

    I have been to numerous doctors who "diagnosed" me with depression
    without even asking me about my mood, appetite, or enjoyment of
    activities. One infectious doctor "diagnosed" me with depression
    without even seeing me.

    I have tried eight different antidepressants. Most of them made me
    WORSE. One of them, Zoloft helped my neurally mediated hypotension
    for a few months but it stopped working.

    In the spring of 1993 I agreed to take the antidepressant Imipramine
    because I was told that I wouldn't be allowed to see the infectious
    disease specialist unless I did and I wanted to prove to the doctors
    that it wouldn't help me. The drug didn't make me feel any better.
    It made me feel MUCH worse. I had insomnia (especially the first
    few days) and I wasn't able to do much of anything. I wasn't able
    to exercise AT ALL. My libido went down substantially and I had
    difficulty reaching orgasm. It also caused such severe night sweats
    that I was SURE my waterbed had sprung a leak and made me feel
    awful. I was on Imipramine for a total of seven weeks (two weeks at
    200 mg) and I gained seven unwanted pounds which I haven't been able
    to lose since.

    The psychiatrist originally told me that it usually took three to
    four weeks to take effect but that he would try it for a maximum of
    five weeks and if it had not helped me by that time he would take me
    off it. After I had been on it for more than six weeks, I called
    the psychiatrist and asked him how much longer I would have to be on
    this drug. He said I would have to be on it for five weeks at the
    maximum dose of 300 mg. When I told him that the PDR says that the
    maximum recommended outpatient dose is 200 mg and that optimum
    results take one to three weeks, he said that his recommendations
    were based on newer research. I thought about this for a while and
    called his service and left a message asking where I could find this
    research. He never returned my call even though I left him another
    message several days later.

    I have been in long term psychotherapy with three different
    psychiatrists. I went to one for nine months and two for six months
    each. This psychotherapy did not help me with my neurological
    illness Myalgic Encephalomyelitis.

    I once collapsed with VERY low blood pressure (78/undetectable) and
    was taken to the county hospital. They kept me in the hospital for
    two weeks trying to reach a diagnosis. The Internal medicine
    resident called in the psychiatrists three times. He was convinced
    that it MUST be mental but the psychiatrists didn't find any
    evidence of mental illness. They finally decided that there was
    nothing wrong with me and that my symptoms were caused by
    dehydration. They didn't explain why a supposedly healthy 25 year
    old would become THAT dehydrated without becoming thirsty and have
    750 ml of urine in his bladder (I was unable to urinate and was

    One doctor I went to said that he had read several journal articles
    on ME and they were all "bad science" (When I asked him if it was
    because they disagreed with his preconceived notions he had no

    Two different doctors have told me that I couldn't possibly have a
    chronic viral infection because there was no such thing. There are
    several chronic viral infections including HIV and hepatitis C.

    I had one MD tell me at least three times in one office visit:
    "You're much healthier than you think you are." When I asked him
    how many medical journal articles he had read about my illness he
    replied: "none". (There are more than 2,000.)

    Doctors at one clinic told me that I had a lipid myopathy and that a
    special diet would cure me. The diet did not cure me. I have since
    learned that children with lipid myopathy all die in their teens. I
    was in my late twenties at the time. The diet that was supposed to
    cure me caused me to go from about 194 pounds to 116 pounds (I am
    just under 68 inches tall). I developed a hernia because of the
    diet and had to have surgery.

    One MD told me that I couldn't possibly have a chronic infection
    because my sedimentation rate was normal. All the medical text
    books I have looked at say that the sedimentation rate is only
    sometimes elevated in infections.

    The first time I took my elderly, very obese mother who was cold all
    the time to her MD because she was too disabled to go by herself, I
    had to ask four times before the MD agreed to run a TSH test. The
    MD said that she was doing it just to humor me and that there was NO
    possibility of it being abnormal. About a week later we got a call
    from the lab. Mother's TSH was 12.58. (At the time, normal was
    less than 5.5. Since then normal has been changed to less than

    Once I had an ENT insist that I could not have a sinus infection
    because my sinus X-rays (taken 11 months earlier) were negative. I
    have since learned that sinus X-rays are not a very good way to
    detect sinus infections. Obviously X-rays taken eleven months
    earlier could not rule out an infection on that day.

    Recently a MD was ready to have my thyroid removed even though the
    thyroid ultrasound and thyroid CAT scan results were not yet
    available. She was sure I had thyroid cancer because I had
    antithyroid antibodies and a CEA of 16.3. (CEA is a test that is
    usually elevated in cancer and normal is <5.0.) I refused the
    operation and insisted on a thyroid biopsy. When I got to the
    hospital for the biopsy, they refused to do one because the other
    thyroid tests were normal and there were no masses in my thyroid to

    Based on these FACTS, what conclusions do you draw about the medical
    profession? If these were your experiences, what would you think
    about MDs?

    I will change my signature to one that is less offensive to you.
  7. I don't have any on hand and my VERY poor health makes it difficult
    for me to go get some. If I really need some resistors, I will go
    to RS and get some the next time I drag myself out of the house if I
    know which ones to get.
  8. Lord Garth

    Lord Garth Guest

    <snip medical report>

    Okay, so what color are the LEDs you wish to run?
    How many do you wish to have on?

    The USB port supplies 5 volts at a maximum of 500mA
    and I wouldn't push that limit since the fuse is microscopic
    if you can get into the laptop and find its location.
  9. Brian

    Brian Guest

    You could put 2 sets of 2 LED's in series with a 56 ohm resistor (1/4
    watt) and a 150 ohm resistor (1/4 watt) in series with one LED.

  10. Brian

    Brian Guest

    That is, you will need two 56 ohm resistors and one 150 ohm resistor.

    One string with two LED's and a 56 ohm resistor.

    Another string with two LED's and a 56 ohm resistor.

    Another string with one LED and a 150 ohm resistor.

  11. That was something I was curious about but not badly enough to dig this
    up... Thanks!!!

    Meanwhile, his original post, if I remember correctly, mentioned having
    a Radio Shack 20-piece grab bag of LEDs and suggested using three of them,
    at least in a proposed example.

    I would not put them in series, but use a resistor in series with each
    one, and put the LED-resistor series combos in parallel wth each other.

    In my experience, Radio Shack 20-piece LED grab bags have LEDs that sure
    look like ones mostly with typical current 20 mA maximum current 30 mA,
    maybe some with maximum as high as 50 mA.
    I have always been able to use these LEDs at 40 mA, although with no
    good idea what their life would be like. My experience with doing this is
    more than a few hours but no further data, and I am guessing most likely
    merely falling well short of the typically-claimed 100,000 hours.

    These LEDs have voltage drop at 20 mA in the 1.6 to 2.3 volt range in my
    experience, with less than 1.8 volts only for one red chemistry.
    Last month I looked at the LED grab bags at Radio Shack and they looked
    like the same ones of a decade or more ago, lacking blue LEDs and
    apparently lacking the other colors with voltage drop closer to (usually
    above) above 3 volts.

    I would say design for 20 mA if you want a good chance of 100,000 hour
    "typical LED life", 30 mA for a more aggressive but probably quite
    tolerable design.

    With 2 volts across the LED, this means 3 volts across the resistor.
    For 20 mA (.02 amp), use a resistor of (3 volts /.02 amps), which is 150
    ohms. For 30 mA (.03 amp), use (3/.03), which is 100 ohms.

    - Don Klipstein ()
  12. NRen2k5

    NRen2k5 Guest

    So then find yourself an online electronics vendor and order resistors or
    resistor packages. Surely the mailbox isn't too far for you to move in a day?
    You're already parked in front of the computer, so I know that making the
    order won't be any trouble for you...

    - NRen2k5
  13. Kitchen Man

    Kitchen Man Guest

    Pretty much anything can be had online via mail order. Proper LED
    design requires a resistor and an LED in series. Browse the web for
    design ideas and equations, and order up an assortment of resistors.
    It is a very rare electronic design that omits resistors.
  14. John Smith

    John Smith Guest

    LED 1K Resistor
    POS -------|>---| |-------\/\/\/\/\---- NEG
    | | |
    ------- |
    | |

    The above circuit will only pass 10 ma in though the FET over a voltage
    range of 5v to 25, should work well for you--the fet functions as a
    current limiter...

    Warmest regards,
  15. Just out of curiosity, what would happen if I connected three in
    series without a resistor? Would they draw too much current even
    though the voltage across each one would be about 1.67 volts?

    What would happen if I used four in series (1.25 volts)? Thank you
    in advance for all replies.
  16. LEDs have a very nonlinear current response to applied voltage (as all
    PN junction diodes do). Something like a 60 millivolt increase will
    cause about 10 times the current to pass through them. Conversely,
    dropping the voltage by 60 mV will cut the current to 1/10th. The
    problem is that exactly what voltage produces the rated current varies
    batch to batch and with temperature.

    So there is certainly some safe voltage that can be applied to an LED,
    but it is safe, only because the current is certainly so low that it
    is not important exactly what it is or how much it will vary, unit to
    unit or versus temperature. Establishing a useful current that is
    also stable and predictable unit to unit is the reason resistance (or
    other current stabilizing means) is added to LED circuits.
  17. See John Popelish's response - current varies a lot if voltage across an
    LED varies little, and voltage across an LED varies little when current
    varies a lot. Although in my experience most LEDs aren't quite as extreme
    as 10-1 change in current for only .06 volt change in voltage reported by
    John Popelish - I think more usual is .1 volt change is
    typically associated with current changing by a factor of 10.

    But watch out for voltage drop varying with temperature (or current
    varying significantly, sometimes wildly and in the same direction as
    temperature when voltage is constant). Also watch for minor voltage
    variations from one piece to another, which can (but does not always) make
    for major variations in current from one piece to another if the voltage
    is fixed.

    As for 3 in series? I think generally safe, but with very weak
    possibly essentially no light output, for green, yellow, orange, and red
    other than "original forumla red" (GaAsP on GaAs substrate,
    non-high-brightness 660 nm peak wavelength). The "original formula red"
    may draw close to typical or close to maximum current at this voltage,
    possibly excessive current but then again maybe no more than "normal"

    As for 4 in series? I expect with 1.25 volts across each, most likely
    there wil be essentially no visible output, although GaAsP on GaAs
    substrate may visibly glow very dimly as in uselessly dim.

    - Don Klipstein ()
  18. Kitchen Man

    Kitchen Man Guest

    It's like you just heard from John and Don. The results will be
    unpredictable and likely ineffective and non-repeatable.
  19. I can't figure out where you came up with that lollapalooza of a
    whopper. The datasheet for the MPF102 calls for an Idss of anywhere
    from 2 to 20mA. That's with the gate shorted to the source. In my
    experience this value varies wildly between the min and max, so probably
    half of them will _never_ be able to pass 10mA. And that's with _no_
    resistor; with your value of resistor, none will be able to. Many will
    give only a few mA. The MPF102 varies so wildly tht you really need a
    half dozen or more to find one that's about right. Use the cheaper
    single resistor per LED, and you won't despair. A 150 ohm resistor will
    work okay with the red LED, depending on what current you want.
  20. As an experiment, I pulled out two dozen MPF102 JFETs from a pile of
    several makers. I put them on a 5.0V power supply. With the gate
    shorted to the source, all of the JFETs had Idss of between 3.9 and

    I'm sure if I tried several dozen more, I would hit a few of them that
    would have a higher Idss. But this experiment shows me that the MPF102
    makes a very poor choice for currents of more than 8mA.
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