Running LEDs on five volts

... 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?

John

--
I see. 

Here you are, a stupid fuck 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.

Fuck you, asshole, you\'ll get no help from me.

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.

--
My previous cat Rocket thought that every time I ate something he
should get some canned cat food.  Marmaduke, the cat I have now, 
thinks that he should get canned food and be allowed to get on the 
table and eat what he wants off my plate.


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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 catheterized).

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 answer.)

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

3.0.)

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 100,000 Newsgroups

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

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

;-)

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.

Brian

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.

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.

Brian

Newsgroups

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

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 ( snipped-for-privacy@misty.com)

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.

--
Al Brennan

"If you only knew the magnificence of the 3, 6 and 9,
 then you would have a key to the universe." Nicola Tesla

MPF-102 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, John

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.

-- My previous cat Rocket thought that every time I ate something he should get some canned cat food. Marmaduke, the cat I have now, thinks that he should get canned food and be allowed to get on the table and eat what he wants off my plate.

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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.

It's like you just heard from John and Don. The results will be unpredictable and likely ineffective and non-repeatable.

--
Al Brennan

"If you only knew the magnificence of the 3, 6 and 9,
 then you would have a key to the universe." Nicola Tesla

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" current.

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 ( snipped-for-privacy@misty.com)

voltage

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.

=---

probably

will

will

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

8.2mA.

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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