need your opinion^^ - Page 1
Forum section: electronics / beginners

author: kenichi
author_rank: Newbie
timestamp: October 29, 2013, 04:37:33 pm
content: exam question from paper 1.based on picture A and B:question 1: what will happen if the supply voltage in the picture is 220v is it ok to use 0.1ohm resistor?question 2: what if the resistor is 1Mohm , is it ok to use 0.1v as the supply voltage?this is the some of the question that appear in a quiz given by my lecturer...me : sir go and open your laptop and google ohms law use V=IR.lecturer : no , i want you to explain what will happen if the supply voltage in the picture is 220v is it ok to use 0.1ohm resistor?me : hmm i dont know how to explain.. because i only know V=IR.maybe lets say if the battery(supply voltage) is 9v and the LED is lets say live 2v then the voltage drop across the resistor should be 7volts...so if we use ohm law.. then i think the current is 7v=(I)(0.1) we get I=70ampere..lecturer : that only in theory but you didnt really answer the question. next monday i expect u to have the answer...i ask all of my senior who is at their final year Diploma E&E eng ... sadly most of them dont know the how to answer the question. they say "as long that there is a transistor no mater how many ohm your led will not get burn even if you use 500000v".hmmm

author: Kremmen
author_rank: Super Contributor
timestamp: October 29, 2013, 04:50:15 pm
content: Please familiarize first with something called the Ohm's Law (Google will help you here). And stay away from voltages higher than a 9V battery until further notice. Especially 220V.Once Ohm is your friend, find out what is different between a LED and a resistor.

author: ablacon64
author_rank: Frequent Contributor
timestamp: October 29, 2013, 05:14:12 pm
content: I second that! Electricity is very dangerous. This is a good place to start:http://en.wikipedia.org/wiki/Ohm%27s_lawI recomend reading the book Electronics All-in-One For Dummies, it has lots of tips on safety and a very clear and concise explanation of the basics, very easy to understand.

author: FoXX
author_rank: Contributor
timestamp: October 29, 2013, 05:37:15 pm
content: Sniff sniff...This smells a bit like homework

author: Kremmen
author_rank: Super Contributor
timestamp: October 29, 2013, 05:50:46 pm
content: Hence the wording of my answer

author: kenichi
author_rank: Newbie
timestamp: October 29, 2013, 06:00:36 pm
content: Quote from: Kremmen on October 29, 2013, 04:50:15 pmi have google ohms law and still after hours of reading...still no explanation to answer my question posted... hmm would you mind answering the question posted... say if those were the question in exam..how is your approach to answer it?

author: c4757p
author_rank: Super Contributor
timestamp: October 29, 2013, 06:03:50 pm
content: Forget the hours of reading, Ohm's Law is just an equation.V = I RIn other words, the voltage V appearing across a resistance is equal to the product of the current I through the resistance and the value R of the resistance itself.You have numbers, so put them into the equation.

author: Zero999
author_rank: Super Contributor
timestamp: October 29, 2013, 06:07:35 pm
content: You need to know the voltage drop across the LED but it's fairly constant, regardless of the current. If it's a red, orange, yellow or yellowish-green LED, assume about 2V, if it's bluish green, blue or white, assume 3.5V.

author: kenichi
author_rank: Newbie
timestamp: October 29, 2013, 06:12:15 pm
content: Quote from: c4757p on October 29, 2013, 06:03:50 pmeverybody know that... i just wanna know how is your approach on answering those two question posted..

author: c4757p
author_rank: Super Contributor
timestamp: October 29, 2013, 06:14:01 pm
content: Quote from: Hero999 on October 29, 2013, 06:07:35 pmUnless the voltage drop across the LED is 219.999V, you do not need that number at all to answer at least the first question.Quote from: kenichi on October 29, 2013, 06:12:15 pmBy solving the equation and explaining what the numbers mean. Dude, stop trying to get people to do your homework.

author: G7PSK
author_rank: Super Contributor
timestamp: October 29, 2013, 06:19:48 pm
content: I would tell the examiners to write a proper question, the answer to that one is NO thrice NO

author: Crazy Ape
author_rank: Regular Contributor
timestamp: October 29, 2013, 06:21:29 pm
content: I'm still wondering where he's going to find the 220v single cell.

author: Maturola
author_rank: Contributor
timestamp: October 29, 2013, 06:42:21 pm
content: Quote from: kenichi on October 29, 2013, 04:37:33 pm#1 - What would happen on that case is that the current on the diode would be about 2000+ Amps (so most likely it the Diode will burn or the Resistor will blow). If you want to dissipate all that power, the 0.1ohm resistor would have to be at least a 2000 Watts resistor.so no is NO ok to use a 0.1ohm RES#2 - with a 0.1v Supply there is not enough voltage to forward bias the Diode so it deosn't really matter what value of the resistor you have on the load.So NO is not ok to use a 0.1v on that case either.

author: kenichi
author_rank: Newbie
timestamp: October 29, 2013, 06:50:54 pm
content: Quote from: Maturola on October 29, 2013, 06:42:21 pmi am really happy and thank you very much.... now i can deepen my understanding about how current and voltage affect a component in a particular circuit....thank you for helping me sir

author: Bored@Work
author_rank: Super Contributor
timestamp: October 29, 2013, 07:45:48 pm
content: Quote from: Maturola on October 29, 2013, 06:42:21 pmCongratulations, you have been successfully gamed in doing the guys home work.

author: Maturola
author_rank: Contributor
timestamp: October 29, 2013, 07:51:04 pm
content: Quote from: Bored@Work on October 29, 2013, 07:45:48 pmIf his homework was just 2 questions, then ok, he got away not doing his work, I doubt that. He needed some help, asked a simple question, not reason why not to support him, it is not going to completely damage his entire career or future because someone help him a bit with two simple questions. We all need help at some point of another one and more often than not, we need that help on the simplest things.

author: Stonent
author_rank: Super Contributor
timestamp: October 29, 2013, 08:21:29 pm
content: And after graduation he'll be designing and selling 200v LED power supplies to the masses!

author: Maturola
author_rank: Contributor
timestamp: October 29, 2013, 08:27:38 pm
content: Quote from: Stonent on October 29, 2013, 08:21:29 pmI am sure that everyone in here never got any help from anyone at any point.

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 29, 2013, 08:41:41 pm
content: To a first approximation, a forward biased diode is a constant voltage source, as long as it remains forward biased (you might call it a constant voltage sink, since it always consumes energy instead of sourcing it, but the important point is that its voltage is relatively fixed).To a first approximation, a battery is a constant voltage source.If you connect two different voltage sources in parallel, with no resistance between them, to a first approximation, infinite current will flow.When the first approximation theoretical equations predict infinite current, the real world result is very likely to be that some magic smoke will escape. Sometimes, it's worth going to second and third approximations to get a more detailed model of what really happens. Look up an "LED Throwie" for a case where the internal resistance of the cell saves the day. But usually, if the first approximation says it won't work right, it's time to back up and try something different.If you want to make the circuit work properly, use a resistor to connect the two constant voltages, and use ohms law to choose a resistance value that will produce a reasonable current, given the differences in voltages.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: October 29, 2013, 09:53:29 pm
content: QuoteCheck out this videoHe demonstrates a methodology for analyzing simple diode/resistor circuits in various configurations.Also, some of the *bullshit* answers you received to your inquiry really are not the normal responses that beginners usually get with legitimate questions. I can only assume that some of these folks were having a bad day and took it out on you.

author: Stonent
author_rank: Super Contributor
timestamp: October 29, 2013, 11:38:03 pm
content: Quote from: Maturola on October 29, 2013, 08:27:38 pm

author: Bored@Work
author_rank: Super Contributor
timestamp: October 30, 2013, 02:42:54 am
content: Quote from: Maturola on October 29, 2013, 07:51:04 pmLetting others do ones homework under some disguise has become a way of life for some students. You will change your mind about them when you will meet one who cheated his way through university in your job and that colleague piles his work on you.

author: Nerull
author_rank: Frequent Contributor
timestamp: October 30, 2013, 03:05:32 am
content: If he's asking questions like this in an actual EE course he's either going to flunk out or kill someone. The world will be a safer place if he has to retake the course from the beginning.

author: Alex
author_rank: Regular Contributor
timestamp: October 30, 2013, 08:10:32 pm
content: Quote from: Bored@Work on October 30, 2013, 02:42:54 amThis is unfortunately too true to pass by without reiterating. Some people are very good at it too - taking advantage of others' benevolent nature that is. Doctorate in an electronics field, electrician background, doesn't know that a relay contains a coil - true story.Alex.

author: chrisbrown
author_rank: Contributor
timestamp: October 30, 2013, 08:35:29 pm
content: Holy vitriol, Batman! Am I the only person who read this?QuoteThe kid's not stupid, he just didn't know how to interpret the question. It's not his fault he doesn't intuitively know what typical LED currents are.

author: c4757p
author_rank: Super Contributor
timestamp: October 30, 2013, 08:45:48 pm
content: Quote from: chrisbrown on October 30, 2013, 08:35:29 pmNo, he's not stupid, and I agree he probably didn't know how to interpret the question. But he also refused to participate in finding the answer. I gave him the equation, all he had to do was plug in the numbers and get a result.Then, once he knows how much current is flowing in each case, it's time to ask how much current is typical for an LED. But he stood still, refusing to take one step just because he knows he'll need help again before the second step.

author: ampdoctor
author_rank: Frequent Contributor
timestamp: October 30, 2013, 09:53:19 pm
content: You're all wrong! The RIGHT answer to the question is "prof, you're a dick and you've got a shitty attitude! There's too many degrees of freedom and the questions are too poorly written to give any reasonable answer!"

author: G7PSK
author_rank: Super Contributor
timestamp: October 30, 2013, 10:00:56 pm
content: The questions have been edited since first being placed, originally they were along the lines is OK to put 240 volts on an LED with a 0.1 ohm resistor, surely this is either a trick question or not one from an EE course at uni.

author: c4757p
author_rank: Super Contributor
timestamp: October 30, 2013, 10:10:13 pm
content: Quote from: kenichi on October 29, 2013, 04:37:33 pmI do not know what this means, but whoever said it is an idiot.Quote from: G7PSK on October 30, 2013, 10:00:56 pmNah, the original is still there, he just added more.

author: Jon86
author_rank: Frequent Contributor
timestamp: October 31, 2013, 08:07:56 pm
content: That is literally the most confusing post I have ever read. I think we can be pretty sure that this isn't some high-level EE course, otherwise the poor guy would have some real trouble...