# 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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### Claulating the resistance for the IR Flame Photodiode sensor

Hi I wish to make a IR Flame sensor. I have made a schematic as below. But I could not determine the values for R(x) & Rpot(y). I have tried x = 10K, y=10K , x=1K , y=10K,etc (randomly). But I ...
724 views

### led lights resistor

I recently installed an H7 Led Headlight kit 12V/25W in place of a standard halogen 55w bulb. Lights are great, but they flash periodically during can-bus Cold/Warm checks to see if a bulb is out. I ...
107 views

### why my power calculation for this resistor is wrong?

i encountered a very simple question in a test and i easily calculated the answer but somehow i am making a mistake because i can not find my answer between the available answers, what am i doing ...
4k views

### Calculating sum resistance of multiple series and parallel resistors

! I know how to find resistance, current, and voltage through series/parallel circuits. The problem I'm having is figuring which part of this circuit to simplify first. What is the correct order of ...
1k views

### Multimeter giving wrong current readings

I just got my first multimeter (TekPower TP8268) and I've been try to test it out, but I have been getting some strange readings with the current measurement. I have an AC adaptor to DC 12v and 5A. ...
84 views

### Parallel circuit with LED on one path and resistor on the other

I'm a beginner to circuits. Unlike many other questions, my circuit involves a parallel circuit with a LED on one path but a resistor on the other, as shown in the picture. Q1: assuming the LED has ...
241 views

### How should I chose a resistor for a home-made electromagnet?

I have a magnetic wire and I want to use it to make an electromagnet. The electromagnet will be powered by a 5V, 2A power supply. How do I know what resistor should I use in series in order to avoid ...
418 views

### How to calculate induced current?

Suppose we have a closed loop with two resistors in series and at particular moment in time the induced emf was two volts .i want to know why do we use ohm's law in calculating the induced current ?
56 views

### Creating a small range, small baseline varying resistor

I am doing a design project on making a passive varying resistor device. I have a flex sensor that increases in resistance when you bend it. It has a 10k ohm nominal resistance and increase to 110k ...
94 views

### Why would venting an engine create a negative amperage on a generator?

I'm writing simulation software for a civilian helicopter. The helicopter has two engines and two generators. Normally, we calculate the amperage reading of a generator through Ohms. I = W / V. ...
342 views

### Calculating Necessary Resistance In Current Source Circuit

I have a problem where I must calculate the resistance required in R3 to have 1A flowing through the two 9.7V sources in the image below (i.e. Charging the batteries at a rate of 1A). I have ...
770 views

### Arduino powering an external circuit with higher voltage

First of all, I'm sorry for the newbie question. I'm a software engineer and only a hobbyist in electronics. I'm trying to power up an external circuit using an Arduino digital pin. The external ...
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 ...
8k views

### Darn you V=IR, you are wrong (does V really equal IR?)

Here is my simple reasoning. We apply a potential difference across a resistor. All the electrons begin responding. Since it takes time for electrons to respond, our current is not yet fully ...
1k views

### Can a battery produce more amperes as per Ohm's law?

Ohm's law says I = V/R. So let's say I take a 9V battery and attach 1Ω resistance across the terminals. So the current running across will be 9A. But the battery rating says only 500mAh. So are ...
352 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?
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 ...
306 views

### Smallest possible resistance (highest current) [duplicate]

I'm new to electronics and I've been familiarizing myself with some basic formulas, specifically Ohm's law. It seems to imply that if resistance is smaller than 1, let's say 0.00001 Ohm, then I can ...
113 views

### How to read a multimeter (digital) — should I get 2A readings from a couple of AA batteries?

I have been using some old multimeters in a high school science lab. We did a current an voltage reading from a AA supply (two batteries) with a variety of resistors. In some cases a 1M resistor. I ...
276 views

### What is voltage exactly? [duplicate]

First, let me start off by saying that I have read the answers to this question (which is the same as mine), but the answers basically explain the different ways voltage is created or compare it to ...
6k views

### Pull-down resistor ; why is it crucial

These 3 diagrams represent 3 test cases. Dia 1: When I perform an analogRead of analog port 0, then I get a high value obviously. According to the manual, the <...
176 views

### Confusion about Ohm's law and electrical power (basics of electrical engineering)..?

I am studying electrical engineering, but I still have some doubts about Ohm's law and electrical power. According to Ohm's law: $I=\dfrac{V}{R}$ So that means voltage is proportional to current. ...
2k views

### What is the Danger in Plugging an 4 Ohm Speaker into an 8 Ohm Amplifier?

I know it is a commonplace warning not to plug in a 4 ohm speaker to an amplifier rated for 8 ohms. I am wondering what is the actual danger that is presented by doing this in an electrical sense. ...
797 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 ...
54 views

### Current flowing in a short circuit with a voltage source

I want to calculate the current flowing in the node A of the following circuit: simulate this circuit – Schematic created using CircuitLab This can be approached in several ways. The tension ...
259 views

### Transistor driving LED

I have already asked a question regarding this problem but it was about a different thing, slightly. Here is the drawing of a LED driving circuit. The problem I have encountered with traditional ...
162 views

### Why doesn't voltage drop across this resistor when transistor is off?

The following excerpt from a book explains the functioning of the circuit below: When V_in < V_th (device threshold voltage), the supply voltage (V_dd) is measured at the outlet. When V_in is ...
2k views

### Switch mode PSU minimum load

Alright, I know I am going to get a lot of flak for this, but let me reassure you that I have already spent the last few days reading on Google while going back and forth from the bench to test ...
344 views

### How do transformer obey Ohm's law?

I am honestly confused, A transformer "Steps up the voltage and steps down the current" or vice versa. So how does it obey Ohm's law? for example, a transformer with a winding relation of 2:3, should ...
147 views

### Why can't you just reduce the current instead of the voltage?

Why does voltage even matter, if it's the current that goes through a circuit? I understand the relationship V = IR (I think) and that increasing the voltage (pressure) will increase the current. ...
164 views

### Finding the value of a specific resistor

Let say you have a network of resistors. All resistors have values but one of them is labeled "R" Given the value of a total voltage and a total current, find the value of R. My head start would be ...
175 views

### Calculating resistance when ohm law has no response

I have a 3v DC source and a 3v light emitted diode, i want power this led with just 5 mA . There is a formula : R = V/I But in my case V is 0 V[source] - V[LED] = 0 So R is 0 always!!! How can i ...
3k views

### Does this RGB LED really need 220 ohm resistors per color?

I have an arduino kit (5v) and instead of just following their simple instructions I want to know why they are using certain resistors for their RGB LED's. The RGB LED that comes with this is ...
117 views

### Can V=IR be applied to a short circuit?

Because on a short the voltage goes down while the current goes up, and a resistance of the line would be nill and remain unchanged. Does the formula fail or am I misunderstanding something?
134 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 ...
264 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 ...
353 views

### Ohm's Law for voltage divider

I'm currently working my way through electronics for dummies, where I am shown a simple voltage divider circuit comprising of a 6V battery and 2 resistors, R1 (220 ohm) and R2 (1000 ohm) in series. ...
100 views

### Where did I miscalculate ohms law in my experiment?

I thought I finally had a grasp on how ohm's law works but when I set up an actual experiment, the numbers don't match up. To keep a long story short, I converted a computer power supply into a lab ...
101 views

### Voltage Drop doubt

In an ideal circuit with just a battery short circuited, where there is no resistance in the wire, why is it necessary to have resistance along the way? I understand Ohm's Law, and that the current ...
215 views

### Do rail gun systems require high voltage?

I couldn't justify, as to why most rail-gun examples I found would have high current and high voltage. When the rails & projectile can be designed to have low resistance by varying the volume of ...
2k views

### Why are voltage and current directly proportional? [closed]

So, I thought I had an understanding of Ohm's law, but after thinking deeply about it, the direct proportionality between voltage and current doesn't make a lot of intuitive sense to me. So, when I ...
277 views

### Beginner Question about the logic behind Ohm's Law?

In all the physics i learned and practiced many subjects But understanding Ohm's law used to be a big problem for me... Here is what i mean : We have a 5 volt battery. And a 3 volt/ 20 mA led. So ...
58 views

### Wattage of DC device is not equal to AC [closed]

I have a led driver (LPC-20-700) that is 30V and 700mA, which makes it 21W. I run 10 leds (≈ 2.8V drop) in series with it and it is an constant current device. If I plug my multimeter between, I do ...
299 views

### Reason for inconsistency with Ohm's Law, of current measurements in resistor connected to battery circuit?

simulate this circuit – Schematic created using CircuitLab Connecting various resistors to the same two batteries rated 1.5 V and 9 V as shown in schematic above returns unexpected results: ...
1k views

### How does an MCU pin detect a voltage?

If I have an MCU pin connected in between 2 resistors (a voltage divider) how does the MCU ...
255 views

### Voltage Divider producing unexpected voltage

I am attempting to create a voltage divider that create 4.5 volts from a 9v battery. Using the voltage divider equation 4.7k / (4.7k + 4.7k) ) * 9v = 4.5v, I ...
5k views

### What is the difference between Ohmic and Non-ohmic devices? [closed]

Besides the fact that one does not following Ohm's law, are there any other differences between these conductors?
668 views

### Calculated resistance of an LED [duplicate]

I am working on some circuit lab homework this week and am a bit stumped by something I just can't seem to find anywhere in my Circuit Analysis I book. The lab has me construct the following circuit ...