| Electronics > Beginners |
| how exactly resistor works |
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| ArthurDent:
tester43 “1. current is limited by resistor to the value described by the Ohm's Law” No, that isn’t what everyone has been telling you over and over again. Ohm’s law is an equation that explains proportional relationships between terms, there is no limit involved and it is baffling why you won’t accept this fact. tester43 “2. but, current above the limit of resistor will be changed into heat” Very wrong! This has also has been explained to you multiple times. Again many others and I have explained this to you. Way back in post #39 I said: “If you use a 4 ohm resistor the current has to be 1A and if you use a 400 ohm resistor the current will be 0.01A. the power that the resistor has to dissipate as heat is given by the formula of P=EI so in the first case you have 4VDC x 1A or 4 watts. In the 2nd case you have 4VDC x 0.01A or 0.04 watts. In the 1st case, dissipating 4W as heat means the resistor will get far hotter than the 2nd case where you will be dissipating 0.04 watts. In the 2nd case you won’t be able to tell by feel that the resistor is ever so slightly warmer than the ambient air temperature.” There is NO LIMIT above which heat is produced in a resistor but below which there is no heat. You are the only one who is misrepresenting what is being said. ALL energy in a resistor is given off as heat. Just because you don’t have the equipment or knowledge to measure the amount of heat produced at lower levels doesn’t mean it isn’t there. In your first post you misrepresented what you claim to have read in books: “is the resistor a way to radiate overflow of energy as heat (wasted energy) - and that's how books are explaining it.” No book would explain heat radiated by a resistor as “overflow” and heat from a resistor is sometimes exactly the product you are looking for as with a toaster, stove or electric heater and not wasted at all. My conclusion is that everyone here has been very patient explaining to you multiple times in multiple ways the correct interpretation of Ohm’s law and you admit reading books which I’m sure said the exact same thing. Because your RESISTANCE to take any sound advice has LIMITED your ability to understand these concepts, I’ll leave you on your own. Good luck. |
| a59d1:
--- Quote from: ArthurDent on September 29, 2018, 07:58:08 pm ---They also have kind of an aura or field emanating from them. --- End quote --- :palm: |
| ArthurDent:
--- Quote from: a59d1 on September 30, 2018, 11:55:05 pm --- --- Quote from: ArthurDent on September 29, 2018, 07:58:08 pm ---They also have kind of an aura or field emanating from them. --- End quote --- :palm: --- End quote --- I assumed you knew that was sarcastic although the glow of a space heater or light from a incandescent light bulb does appear to be an aura. ;) |
| Brumby:
To the OP: You seem to have a fixation about the limit of a current - and I would like to address three "limits" that are associated with resistors... Voltage limit: This is where excessive voltage (more than it was designed to handle) across the resistor is so high, that materials in the resistor break down and sparks can jump. Keep the voltages below this limit for the resistor to perform its job without damage. Power limit: This is where the power being dissipated by the resistor is higher than it is capable of physically withstanding and the materials will overheat. Keep the power dissipation below this limit for the resistor to perform its job without damage. When a resistor is used within its specifications, neither of these limits play any part in how the resistor operates, so please feel free to ignore them. Current limit: This is a simple mathematical calculation where, for a given resistance, the current flowing through the resistor is a function of the voltage available - Ohm's Law. For any circuit with this resistor and voltage, there will be a maximum current that can flow through the resistor. When that circuit contains ONLY the resistor, then the current will be what comes out of Ohm's Law. Where this can change is where you have an additional component in series with the resistor. Take an LED, for example. For an LED with a Vf (forward voltage drop) of 2V, in series with a resistor in a circuit which has 5V available, the resistor has 3V across it. Using that 3V, apply Ohm's Law and you will get the current that will flow. Since you will not get any more current than this flowing, it could be classified as a maximum current, which implies it is a limit - but it is only a limit for this particular circuit. Without a resistor, the above LED connected directly to the 5V supply would draw excessive amounts of current and would soon bake itself into oblivion. IN THIS ROLE, the resistor performs the function of limiting the current to a value defined by Ohm's Law - and is often called a "current limiting resistor". Such "current limiting" is a function of the resistance of the resistor AND the voltage applied. I will also place emphasis on the fact that a resistor is, for all intents and purposes, LINEAR. Put the smallest of currents through it and it WILL dissipate some heat. There is NO magic point where things suddenly change ... that's the sort of thing LEDs will do - but not resistors. |
| CatalinaWOW:
Physical examples seem to speak to you more than even simple mathematics. One example of using resistors to dissipate small amounts of heat is the arrays of resistors used to generate artificial infrared images for testing infrared imaging systems. In order to generate real scenes they have to be able to generate continuously varying temperatures (no large steps in temperature between adjacent resistors) and to test good imagers they need to adjust temperatures by small fractions of a degree Centigrade. The voltages across each resistor are adjusted to give the appropriate current to dissipate the power required to raised that resistors temperature by the appropriate amount. Even this simple (in concept) device requires a fair amount of math to operate properly. The power dissipated by the resistor is not linear with voltage (P=E^2/R) and the temperature rise is not linear with power having terms proportional to temperature difference with nearby objects and other terms proportional to the differences of the fourth power of the temperatures involved. |
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