# 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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11k 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 ...
56k 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 ...
18k 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? ...
55k 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, ...
4k 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 ...
35k 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 ...
11k 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 ...
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 ...
6k 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 ...
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 ...
3k 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 ...
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 ...
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,...
24k 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 ...
40k 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 ...
35k 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 ...
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 ...
5k 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 ...
5k 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 ...
140 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 ...
4k 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 / ...
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 ...
1k 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 ...
323 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?
639 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 ...
943 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 ...
254 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 ...
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 ...
209 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 ...
176 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 ...
671 views

### Current Flow into MCU pin

If I have an MCU, e.g. ATTiny85, and I connect a 5V source to a pin on the ...
1k 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 ...