# Questions tagged [resistance]

A measure of how much a conductor opposes current flowing through it. It can also refer to a part of a circuit, which has a non-negligible resistance value.

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### Why am I not dead after repeatedly touching a high-voltage source?

While playing with mosquito racket in my home, I unscrewed the racket and touched the 2 wires with my hands. I felt that my bones were dislocated, I got shocked, but I am not dead. My calculations ...
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### Correct formula for LED current-limiting resistor?

I'm trying to work out what value resistor to use in a LED circuit. The equation I'd use to do this is: $$R = \frac{V_{cc} - V_f}{I_f}$$ Seems logical, and makes complete sense. The answers to the ...
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### Why does Ohm's law not work for vacuum cleaners?

I have been learning about Ohm's law and testing the resistance across the plug of my household appliances and calculating the current. For example, my kettle was 22 ohms (10.45 amperes) and is ...
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### Using a bottle of water as a resistor

Today, while drinking some water from a $500mL$ bottle, I started reading the info about the water and found out that the conductivity ($\sigma$) at $25°$C is $147.9\mu S/cm$. So it came to my ...
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### Why do we "need" resistors (I understand what they do, just not why...)? [closed]

I have always had a basic understanding of electronics. I am now starting to learn a bit more, using an Arduino as a test platform, and I have a question about resistors that I can not seem to solve ...
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### Two black boxes display the same impedance at all frequencies. Which has the single resistor?

Two black boxes display the same impedance at all frequencies. The first contains a single 1 Ohm resistor. Each end is connected to a wire, so that two wires protrude from the box. The second box ...
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### Why are lightbulbs considered resistive load?

A lightbulb (a plain old incandescent lamp) is typically brought up as an example of resistive load. Yet the filament is actually made of several feet of very thin wire cleverly coiled to form a ...
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### Resistance Vs Impedance?

What is the difference between the Resistance and Impedance? When we will say it is an Impedance and when we will say it as an Resistance? Can you explain it with diagram (If possible) and real time ...
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### Anti-static bags - safe to support powered circuit boards?

Is it generally safe to rest a low voltage powered up circuit board on an anti-static bag when debugging? A simple meter reading suggests it is a good insulator, but it must conduct to some extent? ...
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### Why does my 120V 40W lightbulb only have 26 ohms across it?

I thought that if the voltage source is 120V and the lightbulb is 40W then the current would be 1/3 of an ampere meaning that the resistance of the lightbulb would be 360 ohms. But when I checked it ...
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### Why would you short two pins of a potentiometer?

I've found a potentiometer which controls the heat of a cook top. One of the outer pins and the centre pin were shorted with a soldered jumper. Why would someone do that? What does this achieve?
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### How to make a dummy load of 1000W - 2000W?

I need to check a 48V battery with dummy loads of 1000-2000 watts for 5-10 minutes. Perhaps using Nichrome wire, i can wrap some around a fire brick submerged in plenty of water and adjust the load ...
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### Is there ever any reason to prefer 5% tolerance resistors over 1%?

I work at an electronics store and the other day a customer came in who was rebuilding a circuit board. I sold him some resistors, but later he came back in wanting to return some of them because they ...
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### Why does water short out PCBs? (i.e. Why doesn't electricity follow the path of least resistance?)

If copper is more conductive than water (at any reasonable PH), submerging copper electronic circuits in should have no effect, as the electricity should continue to follow the path of least ...
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### Does adding thermal relief on PCB increase electrical resistance?

I am just getting started on PCB design (for fun) and came across this term called thermal relief. It increases thermal resistance so the components can be soldered easily. But according to what I ...
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### 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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### Side effects of using large resistances

Are there any problems that can be caused by using resistors of large resistances (in the order of megaohms)? I'm designing a feedback network that is just a voltage divider, and I want the feedback ...
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### Is there a way to average resistors together to get a tighter overall resistance tolerance?

I have a very sensitive, kHz frequency level, application where I need two matched resistors of the same resistance better than 0.05%. Like maybe by one magnitude (0.005%). I am basically needing to ...
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### What does "the resistance looking into" mean?

See this circuit, from Sedra & Smith microelectronics, 6th edition, page 287: It says that the resistance between the gate and source looking into the source is 1/gm, but that the resistance ...
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### Finding the resistance of a resistor without multimeter

So this is a trivial but frustrating issue I’ve come across. I have two 5 band resistors and I am having a hard time differentiating the tolerance band. I don’t have a multimeter on me and I don’t ...
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### Why are kiloohm resistors more used in op-amp circuits?

In operational amplifiers working in the negative feedback configuration, the voltage gain depends on the "ratio" of resistances. In most textbooks I have seen that both feedback and input ...
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### How did I survive this shock? [duplicate]

When I was about 10 years old playing with lamps, I accidentally picked up a wire completing a circuit for a lamp plugged into the wall (120 V AC U.S. standard). I'm not sure if it was the hot or ...
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### Why does the resistance of a space heater decrease as it gets hot?

I came across something I find odd while hacking a space heater into a high power dummy load: my cheap space heater is rated 1.5 kW 110 VAC, so I expected to see less than 8 ohms which, when heating, ...
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### Resistors with ends of the same colour

I know the values of resistors if they are gold-colored at the end. When both ends are the same, such as brown-o-p-brown and red-x-y-z-red, I am in problems. How to know which side has the last colour ...
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### Why can sticking fingers in an electric outlet kill you?

I just wanted to learn some differences between volts, amps, ohms and so forth and came up with this question. If your skin has 100k ohms resistance and the outlet is 220v wouldn't the current flowing ...
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### Why do fuses burn at a specific current?

We usually specify the maximum current that a conductor (such as a fuse) can handle without burning. But doesn't the conductor really fail when a certain amount of energy/heat has been dissipated in ...
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### How do you determine the effective resistance of a finite grid of resistors?

Disclaimer: I am a geophysicist with limited electrical engineering background. I am not sure if this problem is incredibly easy, incredibly complex, or completely nonsensical. My goal: Determine the ...
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### What is negative Resistance

I have heard some people talk a bit about negative resistance, but I have never really understood it. What is negative resistance?
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### Will doubling thinner wire in lieu of thicker wire work?

I've been needing a jumper cable for a while and have plenty of 10 AWG copper wire lying around. From what I've read, 6 AWG wire is recommended for jumper cables. I want to make a 7 foot cable. So, ...
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### How to measure gigaohm resistors?

I've got an issue that appears to be caused by damaged resistors that are either open circuited or of too low a value due to contamination. The problem is that they're gigaohm resistors, so to a ...
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### Resistance depending on voltage - the chicken and the egg?

If I understand correctly: The resistance of some circuit components (like diodes) depends on the voltage across them. When two components are connected in series: $\frac{V1}{V2} = \frac{R1}{R2}$ ...
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### What is the purpose of wiring pins 2 and 3 of a potentiometer together?

I build a lot of audio devices - amplifiers, effects processors etc. I've been doing it for about a year and learned a lot of principles. However one thing I still don't understand is wiring pots 1 ...
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### Why would you put multiple resistors in line vs one resistor of the same value?

Why would you put multiple resistors in line instead of one resistor that is the same value? In this case, I have a PCB with three 1k ohm resistors. What could be the reason for this instead of just ...
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### Is there a fluctuation effect between heat, resistance and current?

We are told that heat increases a resistor's resistance (or decreases its conductance) and current decreases when resistance is increased. So with less current, less heat would be dissipated, which ...
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### Why do cables specify 75 Ω impedance when impedance is a two-dimensional quantity?

As I understand it, impedance has an imaginary and a real part. To be able to perform any useful calculations, one needs to know both. Cables and speakers usually only say like "8 Ω" or &...
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### 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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### Why is 10W resistor getting hot with only 6.5W running through it?

I am running about 6.5W though a 10W resistor. The ohm rating is 220 ohms, which is correct for the circuit ohms which is calculated to about 225 ohms. Here is what is running through my 220 ohm 10 ...
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### How do I calculate the current in parallel branches?

I've been playing around with an Arduino for a while now, and while I know just enough about simple circuits to get little projects up and running, I still don't know enough to figure out what's going ...
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### Why do diodes have a voltage drop?

Is resistance what makes a diode have a voltage drop? Or is it impedence? If it is electrical energy being converted to light, please tell me what this effect is called.
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### How do you calculate the noise of an op-amp circuit?

I think I know how to do this, but you can find a lot of different instructions and calculators online that contradict each other. I have yet to find a clear, concise procedure for calculating the ...
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### How can I ensure low-resistance ground contacts to an aluminum enclosure?

I am building an electronics project with an aluminum enclosure. I need to ground it—and very well—for human safety. (The system handles 300 A of current, and I want to be sure that if there is an ...
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### How does static dissipative foam work?

I am planning to build a grip-release sensor using the foam. I'm pretty interested in the way it works. Does the resistance in circuit increase or decrease when force is applied onto it? And this will ...
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### Why does a multimeter put more voltage to measure a smaller resistance?

I noticed this behaviour on two different multimeters (different models and brands as well). At first I did not put a multimeter in the way to measure how much voltage changed for the different scales ...
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### What is the physical meaning of negative resistance?

I am a bit confused about the physical meaning of negative resistance. Mathematically, a component which has negative resistance shows a decreasing voltage across its terminal when the current inside ...
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### 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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### If voltage is measured between two points on a wire, with no resistance in between is the voltage zero?

I have a series circuit. Let's say the battery has a potential difference of 10 Volts. On two points of the wire before the current hits any resistors, I attach ...
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### How do I calculate the resistor value for a simple LED circuit?

I have a simple circuit: The max rating (current) of the LED is 30mA. How can I work out what the resistance of the resistor needs to be? By using ohm's law I found it to be $3V/0.03A = 100 \Omega$...
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### Formula for Logarithmic (audio taper) pot

What I would like I am trying to determine the formula for an audio taper (logarithmic) pot. I would like a formula that takes R and P as inputs. R being the total resistance and P being the "...
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### Why does reflection only applies to transmission lines?

Why does the concept of wave reflection seem to only apply to transmission lines? For example, for a simple circuit with two resistances R1 = 50$\Omega$ and R2 = 75$\Omega$, is the voltage wave ...
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