[HN Gopher] Voltage, Amps, Resistance and LEDs
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Voltage, Amps, Resistance and LEDs
Author : ingve
Score : 57 points
Date : 2021-06-16 06:49 UTC (3 days ago)
(HTM) web link (blog.demofox.org)
(TXT) w3m dump (blog.demofox.org)
| readingnews wrote:
| I really mean no offense, but for the love of electrons, please
| stop saying things like: "Amps are measured in amperes". Current
| is measured in Amperes. It is like being a cook and instead of
| saying you need one cup of water, you ask for one cup of one cup.
| You would not do that. Ergo, your title should read Voltage,
| Current, Resistance and LEDs.
| tyingq wrote:
| Perhaps a more accurate analogy would be saying "kilos are
| measured in kilograms".
| Atrix256 wrote:
| Fixed this typo and i get what you mean how "one is not like
| the other" in the title.
| wl wrote:
| I'm not measuring anything but current in amperes. There's
| plenty of things I can measure in cups besides water.
| sobriquet9 wrote:
| Fixed voltage drop on an LED is too much of a simplification for
| this application. A better mental model is to imagine a piecewise
| linear approximation of current-voltage characteristic aka I-V
| curve.
|
| I-V curve of a resistor is a straight line that goes through
| (0,0) point, with slope inversely proportional to resistance. The
| curve for LED has a bend somewhere between 1.5V and 2V, where its
| slope changes from low to high.
| compiler-guy wrote:
| And yet it is exactly how many hobbyists approach the topic,
| and quite successfully. Not that he does hint at these curves
| when he talks about how running it at lower current will make
| it less bright.
| sobriquet9 wrote:
| My problem with the way the topic is being presented here is
| not just that the mental model is too simple, but that it is
| so much simplified that it becomes internally inconsistent.
|
| Resistance is voltage over current, here's LED, it does not
| have resistance. Or maybe it has more than one resistance,
| depending on voltage.
|
| Battery is a voltage source, wait, no it's not, it has
| internal resistance.
| dekhn wrote:
| a lot of electronics tutorials start with resistors and
| capacitors and have you build RC circuits and measure their
| decay, but I found that a diode-first class makes a lot more
| sense.
|
| Once I discovered constant current controllers, I started to have
| real fun with LEDs. Sure, you can use a resistor to drop voltage
| but that's just spewing heat. Switched Constant current
| controllers are much more elegant not very hot at all and have
| excellent control over intensity.
|
| These days I typically run LEDs at _half_ their rated current;
| they are still plenty bright and run FAR cooler than max current.
| carapace wrote:
| This is almost physically painful to read.
|
| Before I rant off, let me link to a _good_ resource for learning
| about electricity: http://amasci.com/miscon/whatis.html -or-
| http://amasci.com/miscon/elect.html
|
| First, don't buy electronics from Amazon (or anything from Amazon
| really) buy from a reputable source like Digikey or Mouser.
|
| Second, please don't try to explain electricity if you don't know
| what you're talking about. There are enough misconceptions about
| electricity (see amasci links above) without adding more noise
| and half-baked understanding.
| JKCalhoun wrote:
| Mouser, Digikey are great but, wow, the cost of shipping and
| time to get parts delivered is painful. For the hobbyist that
| just wants a shift register to play with their Arduino they're
| a tough sell.
|
| Maybe SparkFun or Adafruit are better recommendations?
| boredprograming wrote:
| SparkFun and Adafruit are great, but they're for casuals. If
| you're building breadboard prototypes there's nothing better
| than Digikey and Mouser.
| compiler-guy wrote:
| The disdain for casual hobbyists you show here implies that
| you are probably not the target audience for the article,
| which is targeted even lower than hobbyists: people who
| need an early introduction. Professionals have very
| different needs than hobbyists and therefore need different
| materials and suppliers.
| boredprograming wrote:
| Eh true. And it's not disdain, those companies are great
| for what they do. The guy above just implied they sucked,
| which isn't the case. They're great for their target
| market
| amichal wrote:
| I tried to read your links above. The first spends over 80% of
| the article telling everyone how stupid they are and then
| presents 5 different definitions of "electricity" and then
| ANOTHER rant about how they are all wrong and finally a list of
| other articles about various related concepts. If I were a
| trained EE I would be turned off by the unnecessary ranting. As
| a grade school level hobbyist I have learned nothing that will
| help me connect some batteries and LEDs in a useful way.
|
| Same thing for Digikey and Mouser. I've tried to use them to
| buy a educational kit, set of parts for screwing around with on
| my desk. Can't figure it out. Here is what i get looking for
| "kits" at digikey
| (https://www.digikey.com/en/products/filter/miscellaneous/657).
|
| Amazon has sadly replaced Radio Shack in this space. I have
| half a dozen FUN kits on my shelf I ordered there from a coin
| cell battery "toothbrush" robot up to one with bluetooth, USB,
| ultrasonic, "AI" cameras and all the other fun bit. I even have
| a SMD solder practice board that cost me 5 dollars and after
| stressing like mad that I needed to buy some special solder,
| flux, iron etc I decided I'd give me old butane powered iron
| and the "High-tech Silver-bearing" solid I got 20 years ago
| from RS a try and use my cellphone as a microscope. Works fine.
| 49 of 50 0603 resistors tested fine on the first try.
|
| It's tempting for folks with lots of experience and
| understanding to look down on beginners or fear they will miss
| some deep understanding if they learn the wrong metaphors or
| tools early on. Its fine, my daughter at age 5 thought
| variables where 'bags' that could hold numbers or letters (some
| of them only worked with one thing or the other). It doesn't
| describe the memory model of a modern computer correctly at all
| but it works well enough that she can write some games for
| herself a few years later. If she continues to have an interest
| she can go read "data sheets".
| tyingq wrote:
| The US Navy's "NEETS" material is pretty good as well.
|
| http://www.compatt.com/Tutorials/NEETS/NEETS.html
| midnightclubbed wrote:
| Why the hate on Amazon? I can understand why you may want to
| discourage shopping from Amazon in general but is there a
| reason why they are particularly bad for electronics?
|
| As a hobbyist (with an EEE background) I seldom have an order
| that is large enough to make the mouser/digikey shipping costs
| palatable. If I need something quick (forgot a particular
| component) then Amazon often works great, although the 'free'
| shipping means everything is going to be at least $10. If you
| area business then Mouser etc are the place to go, but you'll
| also have a business account with friendlier credit and
| shipping terms.
|
| For hobbyists it is really difficult to beat Aliexpress. Long
| shipping times but you can order a whole slew of components and
| stock up on things you might want to try out in the future. Yes
| you have to pick through potential counterfeits and you may get
| the odd broken board/component but I'm not sure that is very
| different from mouser, digikey, spark fun etc?
| boredprograming wrote:
| I've had bad experiences with counterfeit components on
| AliExpress. It's rare to get anything real and unused.
| omreaderhn wrote:
| I'm a software engineer that dabbles in electronics. I have
| been looking for explanations like that for about 20 years.
| croo wrote:
| Best resource I found so far on the subject is the
| allaboutcircuits webpage:
| https://www.allaboutcircuits.com/textbook/direct-current/
| CapitalistCartr wrote:
| I do this professionally, and his description is exactly how we
| do it, except I have a $400 Fluke meter. A bit of testing and
| basic arithmetic, some searching on Amazon, and I have some
| LEDs and resistors next day. Because 64 volt electronics are
| hard to come by.
| dekhn wrote:
| I work with hobbyist electronics and I don't see the problem
| with Amazon. Can you be more specific?
| willis936 wrote:
| Counterfeits, mostly. You will never find a legitimately
| sourced component on amazon.
|
| Maybe that doesn't matter to some people, but it guarantees
| GIGO.
| rytis wrote:
| For people like me who can't remember all 100k acronyms:
| "GIGO" is "Garbage In, Garbage Out".
|
| I agree with the sentiment though - a lot of stuff on
| Amazon are not exactly what is written on the tin, and so
| when you read official spec of a component you may get
| stuff that's way off - noisy opamps, that sort of thing.
| sobriquet9 wrote:
| Writing an article about a subject that you are studying and
| getting other people to point out the mistakes is actually a
| pretty good way to learn.
| guenthert wrote:
| But does it have to be on the Internet for all to see?
| EricE wrote:
| I see - the author stood behind you and forced you to not
| only click on the article, but this hacker news thread and
| also forced you to comment too. Yes, that is horrible!
| sobriquet9 wrote:
| How else can you get other people to comment on it, if they
| can't see it?
|
| Once you fix the mistakes, what's wrong with leaving it up?
| Atrix256 wrote:
| Plus notice no mistakes were called out. It was just
| "don't buy parts from amazon".
| Dzugaru wrote:
| I'd add the MIT 6.002 Circuits and Electronics course available
| on Youtube. It's been immensely helpful for me starting
| experiments with Raspberry PI GPIO.
| fallingfrog wrote:
| Fun fact: When you accelerate an electron across a potential of
| 4.5 volts, and then stop it suddenly, you get a photon with 4.5
| electron volts of energy. That's why different colors have
| different voltage drops.
|
| So a blue LED has about a 4.5 volt voltage drop, and a red LED
| has about a 2.2 volt drop. And a blue photon has about 4.5
| electron volts of energy, at a wavelength of around 350nm, and a
| red photon has about 2.2 electron volts, at a wavelength of about
| 600nm.
| JKCalhoun wrote:
| Thank you. I had understood that the wavelength of an LED
| correlated with the voltage drop but did not know why.
|
| (I had just assumed it was a characteristic of the physical
| materials needed to obtain the various colors.)
| tyingq wrote:
| Expanding on that a bit, generally, the shorter the wavelength
| the higher the voltage drop. So UV leds have the the highest,
| followed by violet, blue, green, yellow, orange, red, then
| infrared. Then white being a special case since it's typically
| 3 leds (r/g/b) and not one.
| jpmattia wrote:
| > _Fun fact: When you accelerate an electron across a potential
| of 4.5 volts, and then stop it suddenly, you get a photon with
| 4.5 electron volts of energy. That's why different colors have
| different voltage drops._
|
| This explanation does violence to conservation of momentum,
| since \hbar / \lambda is miniscule for the photon compared to
| the electron. The correct explanation involves recombination
| with a hole across the bandgap, which is how momentum is
| conserved in the interaction, and the motion of the electron is
| not involved other than transport for electrons to find holes
| for recombining.
| tyingq wrote:
| It might be a good idea to talk about PWM in the article, since
| so many devices drive LEDs with PWM. With PWM, you typically run
| higher current than the 20ma mentioned in the article...just not
| continuously.
| zokier wrote:
| One thing to note that LEDs do not really have constant voltage
| drop across them, i.e. they very much are not ideal diodes.
| Typically LED datasheets have two curves that are of interest:
| current vs voltage curve (IV curve, typically roughly
| exponential) and light output vs current curve.
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