Questions tagged [ohms-law]

A well known relationship that describes the relationship between Voltage and current through a device's resistance expressed mathematically as V= IR. This formula says that voltage across the device is equal to the current through the device multiplied by the resistance.

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19
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12answers
15k views

Does a diode really follow Ohm's Law?

Does a diode really follow Ohm's Law? Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant ...
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5answers
61k views

accounting for LED resistance

I'm doing a simple lab (I'm a hobby EE) to reinforce my ohm's law math and learn a little about how to do proper measurements with a multimeter. I have simple circuit with a 2.2k ohm resistor ...
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4answers
20k views

Why do LEDs not obey Ohm's law?

In a previous question, it was brought to me that LEDs do not obey the Ohm's law. (See Calculate expected voltage around a resistor) Simply put: how is that? What makes them behave so differently? ...
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4answers
1k views

led resistor calculation, and resistor wattage?

I know this has been asked time and time again but bear with me as I am struggling to understand, so say I have this simple circuit here with 120v (dc for simplicity) as the input voltage and I have ...
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4answers
179 views

How can Ohms law be used to calculate the resistor value for an LED when multiple voltage/current pairings give the same resistance?

Say I have a 9V supply, and I want to power an LED that requires 3V and 0.02A. From what I understand, I need to add a resistor that will drop 6V, and reduce the current to 0.02A, and I can calculate ...
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7answers
67k views

Why do we need resistors in led

I've researched and it says that resistors limit the current flowing through the LED. But this statement confuses me because we know that in a series circuit, the current is constant at every point, ...
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5answers
7k views

Ohm's law of a circuit which has both a voltage source and current source

The voltage source has a specific voltage through it regardless of the circuit's current and its resistance while the current source has a specific current through it regardless of the voltage through ...
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6answers
42k views

Why does an LED have a maximum voltage?

When powering a simple LED circuit (DC power source, LED, resistor), does the supply voltage matter, as long as the correctly calculated current limiting resistor value is used? In other words, is ...
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7answers
31k views

Where does U for voltage come from?

I believe in Europe the letter U is commonly used for voltage in (eg.) Ohm's law \$U = I × R \$. I think I understand where the letter V came from, commonly used in North America. But what's the story ...
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3answers
1k views

Is the electric field in a wire constant?

If \$V=I\cdot R\$ according to Ohm's Law, that implies that \$\frac{\mathrm dV}{ \mathrm dx} = \frac{I \rho}A\$ across an infinitesimal length of conductor, which is constant at all points along the ...
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1answer
12k views

How to choose correct resistor with proper wattage and ohms for 6 leds connected in a serial circuit?

Let's suppose I've six LED lights with each 3.2 Volt and 20mA and all six of them are connected in one serial circuit. I know there is better setup but for this I'm using this circuit setup. I'm ...
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6answers
2k views

How do resistors limit current for an LED? [closed]

If I have an LED with Vf = 2.1V and I = 20mA, Ohm's Law says I should have a resistor in series at a value of R = 145Ω, if the voltage supply is Vs = 5V. However, I believe can put a higher valued ...
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6answers
2k views

General definiton of Ohm's law

There has been some disagreement lately on whether a diode follows Ohm's law or not: Does a diode really follow Ohm's Law? Specifically things like However, the truth is the resistance of a ...
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6answers
7k views

Why are these readings violating ohm's law? (Are they?)

I'm brushing up on my high school electronics knowledge and I decided to experiment with a small acquarium pump I had lying around. I took some measurements with a multimeter and the results confuse ...
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3answers
3k views

Powering a device by USB?

I would like to power a device via the 5V USB port on a computer, because I will also be communicating with the computer via the USB data cables. I know that USB ports provide 5V, but from what I've ...
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7answers
4k views

Why does an infinitely decreasing resistance infinitely increase current?

So I have been mulling over Ohm's law for hours today. It makes sense to me that the current gets bigger as voltage rises, since the voltage is supplying more electrons and current is the number of ...
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3answers
12k views

Max Voltage for Speaker of Given Power Rating

I have been looking into the design of loudspeaker drivers and would like to know how to determine the maximum voltage which can be delivered to a given speaker. From Ohm's law we know that P = V^2 / ...
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3answers
2k views

LED current limiting resistor and Ohm's Law

Yesterday I was trying to figure out how many ohms of resistor should I be using for my LED, and found this post right here in the forum. The formula states that one should subtract the voltage drop ...
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2answers
2k views

High voltage transmission, transformers and Ohm's law

I have difficulties with understanding high voltage transmission and transformers working process. Particularly with this expression: "the higher the transmission line voltage, the smaller the current"...
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3answers
4k views

Calculate expected voltage around a resistor [duplicate]

It's been a few years since I first studied the Ohm's law at school. Now I'm really getting into electronics, and I must admit a part of it still puzzles me. I know this is kind of a strange question,...
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5answers
41k views

How do I calculate the necessary resistance for a voltage divider?

I’m self-taught, and this is a little bit of a thought experiment for me to understand Ohm’s Law better. I have a very simple voltage divider. Given a 15V DC input, each of three 4.7KΩ resistors ...
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8answers
55k views

Power's relationship to Resistance

$$ \frac{V^2}{R} = P = I^2 \cdot R $$ According to this equation, Power is inversely proportional to resistance on the left side and directly proportional to resistance on the right side. How can ...
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6answers
18k views

Ohm's Law confusion — can there be voltage without current?

I know there has been asked a similar question about this before but I still have some issues to understand the whole voltage and current thing. ( Current without Voltage and Voltage without Current? ...
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4answers
700 views

I do not understand what power actually is

I realize this is probably a super common question, but I have to put it in my own words. I try to understand ohm's law with the water analogy. Two tanks with water, one with a higher level than the ...
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8answers
6k views

Why does voltage drop across resistance contradict Ohm's law?

Very basic question here. I have been learning a ton about Ohm's law, and I don’t seem to ever be able to completely understand it. One of which I am getting particularly confused about is voltage ...
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5answers
6k views

Is Ohm's Law violating itself?

Consider the following circuit... simulate this circuit – Schematic created using CircuitLab Now suppose the resistor has infinite resistance. Then obviously the current through the resistor ...
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5answers
1k views

What is the max current from a 9 V battery?

Let's take, for example, a 9 V battery. Forgetting about internal resistance or any temperature restrictions, what is the maximum current I can draw from this? Using Ohm's law with a 1 Ω load, this ...
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7answers
6k views

Ohm's law doesn't seem to be working for this electric motor

I'm a beginner in this field so please forgive me if I'm confusing with my question. There is a component that I can't understand with Ohm's law which is a washing machine drain pump. Washing machine ...
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2answers
5k views

How a DC motor works when connected to a power supply but no resistor?

I connected a 12V DC motor to a battery of 12V only. The motor rotated perfectly fine. But my question is that if you don't have a resistor attached to the circuit, as per theory the current is ...
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3answers
6k views

Simple circuit with transistor using Raspberry Pi

I'm trying to use a Raspberry Pi to control a set of 4 LEDs. I'd like to control the LEDs in software by using a ...
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6answers
148 views

Ohm's law and the voltage drop across a resistor

I'm trying to wrap my head around Ohm's law and there are plenty of similar questions already asked in this forum. But I just still don't understand it. It's about calculating resistor values. Let's ...
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3answers
146 views

Which current should I use to determine the Watt?

I am making a heated blanket and I have some troubles to determine the heat that will be generated. Resistance: Let say, I use this heated cable with a resistance of 0.3 Ohms/m*. If I am using 10m ...
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1answer
766 views

Mesh-Analysis, Ohm's Law & Passive Sign Convention

I am using Fundaments of Electric Circuits 5ed and I have a few questions about Passive Sign Convention with respect to Ohm's Law & PSC FYI passive sign current (PSC) is based on this diagram: ...
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5answers
2k views

What is more accurate definition of Ohm's law?

What is more accurate definition of Ohm's law and resistance? is it $$R=\frac {V}{I}$$ or $$R=\frac{dV}{dI}$$ This is doubt that developed in my mind during a class where professor derived power ...
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1answer
102 views

Using Nodal Analysis Method

Question: I had solved easier circuits problem using nodal analysis but this one it has supernodes on nodes 2 and 3 which I find it difficult solving. thus I need help as I am trying my very best to ...
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4answers
440 views

Why can't I use Ohm's Law on my circuit?

In my circuit, I have 5V power supply and I have 200 ohm resistor. When I measure the current, it's 14.8 mA. Why can't I use the equation I = V/R, which is 5/200 = 25 mA?
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4answers
13k views

Why does current change when adding in an LED?

In the following diagram: The current is 1 mA from: V = IR R = 1 / 1000 = 1 mA However, when I add in an LED with forward voltage 1V: Why does adding in the ...
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1answer
1k views

USB data serial termination resistors. Need them for STM32F2 micro? If so, what wattage?

I'm a little confused. The STM32F2 datasheet says only this, vaguely stated in the features: HNP/SNP/IP inside (no need for any external resistor) Is that referring to the D+/D- termination ...
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1answer
33 views

Can I compute resistive device power consumption at specific voltage knowing its power rated at some other voltage?

Suppose I have a device containing a heating element and no thermostat such as some simple water heater. It's rated "3500 watts 220 volts" which I assume means that if it's plugged into a properly ...
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5answers
262 views

Finding current through LEDs in series-parallel circuit

I'm stuck confirming that the current pulled in a circuit will be 15mA. If I have hypothetical LEDs that pull 15mA each from 1.8V, and they are put in parallel (I know this is not a good idea, but ...
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2answers
556 views

Is it possible for the system of equations of an electric circuit to not have a unique solution?

When using Kirchhoff's circuit laws and Ohm's law to model the system of linear equations corresponding to an electric circuit (so far, circuits involving resistors and sources only), I haven't been ...
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2answers
107 views

Help in understanding a IR repeater schematic

I'm relatively new to electronics projects, as a hobby. So please bear with me. I want to build an Infrared Repeater as a small project. I've found a component which I think will work nicely for the ...
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3answers
9k views

Permanently dimming an incandescent light bulb

I have purchased a nice light bulb with long visible wires/resistors and plugged it in, on the ceiling. It burns too bright without a dimmer and does not look as intended (also the lifespan decreases)....
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2answers
55 views

Calculating the power lost per foot from “\$I^2R\$ losses"

I am currently studying The Art of Electronics, third edition, by Horowitz and Hill. Exercise 1.6 says the following: C. Power in resistors The power dissipated by a resistor (or any other device) is ...
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2answers
2k views

Resistors for transistor led circuit

Im new to electronics and have problems with calculating the resistors needed for driving an IR LED with an transistor. My VDD is 3.3V, the LED needs 1.3V and I want to drive it close to 20mA, so I ...
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3answers
304 views

Why is the calculated current with P=VI and R=V/I not equal?

I'm just starting to learn electronics. I thought I'd start with a simple project of attaching a speaker to a Raspberry Pi (it supplies 3.3V). The speaker I've picked is an 8Ω 0.5W speaker. Now as I ...
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2answers
393 views

Resistor and voltage of a circuit

I'm a beginner in electronics and I was studying when I had a question, how to calculate the voltage after passing by a resistor. Ex: there is a circuit of 10mA with a 5V voltage, what is the voltage ...
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2answers
76 views

How to compute the resistance and voltage of this schematic?

I have the following diagram I've set up in a simulator: However I have no idea how they get 3.75V at that point (the upper left spot is a voltage reader) nor do I really even know how to apply Ohm's ...
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1answer
664 views

Problems with input leakage current in a microcontroller

I'm trying to build a system in which a microcontroller receives (through one of its input pins) a logic 1 if a switch is pressed and a logic 0 if it's not. For that purpose, I want to implement a ...
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3answers
2k views

Determining Resistor for Heating Element

I am attempting to run a Molybdenum heat coil. The best I can tell it is 0.3-0.4 ohms per rack and I would like to run say two racks at 230V at 3.9KW (so it doesn't trip my 230V 20 amp breaker). Does ...