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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38
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2answers
20k views

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 ...
12
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4answers
59k views

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\$...
18
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12answers
12k 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 ...
17
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7answers
7k views

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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1answer
6k views

I/O Resistance of common source MOSFET with source degeneration

Common Source MOSFET with source degenerations looks like this I am a bit confused about different input and output resistance statements (provided by different sources). Some of them say that ...
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5answers
24k views

Does an ATX PSU's PS_ON Signal Require Resistance

As the title suggests; when connecting the PS_On signal directly to ground in order to switch on an ATX PSU, is there a required amount of resistance? - To try to clarify my particular problem; I ...
13
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2answers
2k views

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 ...
19
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6answers
13k views

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 ...
3
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2answers
721 views

Analysing a circuit to determine what is in series or in parallel

This is probably a very basic question but I have the following circuit, I am not sure what is in series and what is in parallel or if they are all in parallel, initially I thought R2 and R3 were in ...
4
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3answers
1k views

Total resistance with resistors bridging two branches

I know how to calculate resistance in parallel and series connected resistors, and how to reduce most circuits to these, but I missed my lecture on cases where you can't just reduce the circuit to ...
2
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1answer
1k views

In a non auto-ranging digital multimeter, what do the increasing numbers in the Ohm section represent?

For my science fair project I am measuring the resistance of nichrome wires under different conditions. I am using an Innova 3300 digital multimeter to measure the resistance. Other websites have told ...
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1answer
118 views

Voltage Calculate [closed]

I want to calculate Vo by Thevenin's theorem. First I removed the RL(1k) Now I have to calculate Vth. Can I say that Vth is 6V? or Vth=6V + voltage through 2k? If both these options are wrong, then ...
22
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4answers
3k views

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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4answers
2k views

How does 3-wire RTD or resistance measurement works in bridge of wheatstone?

I cannot understand how 3-wire measurement will improve the resistance measurement. They say that IF lead resistance \$ R_L \$ is the same, it will compensate. How can this happen? If bridge is ...
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3answers
2k views

Find total resistance of this circuit

I want to find the total resistance between A and B. This is a HOTS mcq, which has the following options: A: 45\$\Omega\$ B: 15\$\Omega\$ C: 5\$\Omega\$ D: 22.5\$\Omega\$ A book says that the answer ...
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3answers
367 views

How much resistance does the capacitor itself contribute to an RC circuit?

Are the effects of the internal resistance of a capacitor in a non-repetitive RC circuit with an RC time constant of less than a nanosecond significant? I have only been able to find equivalent series ...
0
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1answer
4k views

Diode equation with a series resistance

i have this problem with a diode and a series resistance next to it. They ask me to calculate an expression for the diode current in function of the diode voltage. This resistance is a parasite ...
0
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1answer
2k views

How to calculate the resistance of a complicated circuit?

I learned in high school the basic rules of how to compute resistance between two points, given a network of resistors. You try to rearrange the circuit so into parallel parts and series parts, then ...
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2answers
1k views

Common drain JFET output resistance “problem”

The output resistance of small signal model for common drain JFET amplifier equals $$ \frac{1}{g_\text{m}} || R_{\text{ds(on)}} \parallel R_\text{S} $$ and since $$ R_{\text{ds(on)}} =\frac{1}{g_\text{...
24
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2answers
32k views

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 ...
56
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6answers
17k views

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 ...
17
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4answers
12k views

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 ...
14
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2answers
32k views

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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7answers
5k views

Resistance of a semiconductor diode

When we calculate the dynamic resistance \$r=(\frac{dv}{dI})\$, for any n-p junction, how is it different from the normal resistance \$R=\frac VI\$? Does the equation for the voltage drop (The fermi ...
16
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4answers
44k views

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, ...
12
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7answers
66k views

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 ...
8
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8answers
3k views

How does the electricity “know” the ratio of the resistance in a voltage divider?

I'm having a hard time conceptualizing what/how things are working in a voltage divider setup. I've read a couple other questions/explanations where the answerer has said to "not think about it in ...
7
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6answers
4k views

Is it possible to double the resistance of a potentiometer?

I'm starting a circuit-bending project using the rhythm PCB from an old Viscount electronic organ. The idea is to make a portable rhythm box by adding some extra controls and processing. What this ...
6
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4answers
9k views

Precise Resistance Measurement Techniques

This question is inspired by some basic questions on resistors I've read here lately. What's the most accurate/precise technique for measuring the value of a resistor you can think of? What level of ...
2
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8answers
3k views

Understanding Electrical Engineering with Analogies

Understanding concepts with analogies help can be a useful technique and hopefully this will help me better understand electricity and Electrical Engineering better. Lets start with a simple water ...
4
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1answer
13k views

Why can a shorted resistor be ignored?

I am having a little trouble understanding what results from short circuiting. The bottom circuit is the one we are given in the question, and we are asked to obtain the Norton equivalent. The above ...
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3answers
1k views

Electronically switched decade resistance box design

I'd like to design a decade resistance box that uses a microcontroller to control the resistance. Would it be possible to switch between different valued resistors using transistors? (or another ...
0
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6answers
990 views

Is equivalent resistance always lower if we add a resistor to a passive electronic circuit?

How to prove that equivalent resistance of any passive network is always lower (or equal) if we add a resistor between arbitrary two nodes? Note that this is not necessarily a parallel circuit, 2 ...
2
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5answers
318 views

I have a problem with power equations

As we know the power equations are P=VI, P=V2/R or P=I2*R. My question is: how I can increase the power by increasing and decreasing the resistance? In the formula P=V2/R, if I want to increase the ...
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6answers
6k views

How is it possible that if I combine 100 Ohm resistors in different ways, I get _LESS_ resistance than original? [duplicate]

This is blowing my mind. Let's say I've a bunch of 100 Ohm resistors, all identical. How is it possible that if I combine them in parallel, I can get something that has less resistance? Check this ...
0
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3answers
113 views

Fluctuation in resistance value

I'm trying to measure the resistance of a surgical screw that seems to have a low resistance, around maybe one ohm. I use a 1908P multimeter and wires having for resistance around 0.005 ohm. Since ...
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2answers
250 views

PC fan speed control

I am making a stir plate out of an old PC fan, variable resistor, and DC power supply from this tutorial. I have two power supply's to choose from, their outputs are: PS1 = 12Vdc, 2A PS2 = 12Vdc, ...
0
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2answers
1k views

Designing voltage summer without op-amp

The above circuit is from Operational Amplifier Adder. Now remove feedback resistor and op-amp, and get the output. From the answer of the question, I know how the end result for Vout would be, ...
0
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2answers
1k views

Equivalent resistance of resistors

Can somebody help me solve this. I need to find the equivalent resistance of these resistors. I really don't know where the trick here is. If somebody could help me I would really appreciate it
0
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2answers
589 views

Calculating voltage in a simple circuit

If I had a 12V battery and wanted to use an LED that used 2V and 20mA, then I need a resistor that will drop 10V and provide enough resistance to produce a current of 20mA. Therefore I would use R=V/I ...
0
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1answer
961 views

Calculating the value of the resistance for controlling the speed of AC fan

i have an ac fan that i would like to control its speed i am a bit new in this field so please bear with me i had the idea of fixing 3 speeds with 3 switches one for each speed: (25% 50% 90%), ...
0
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3answers
1k views

Resistance calculation [duplicate]

Are these resistors in parallel? A question is asked to find the equivalent resistance between X1 and X2; to my knowledge, these resistors are like parallel connections. But the choices are given as: ...
0
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1answer
178 views

Find the Resistance between Two Terminals [closed]

Let's say R = 49 ohms, and we want to find Req between a and d, how do we do that? Do we have to find Req for the whole circuit?
0
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1answer
432 views

Why the input resistance of a common emitter amplifier is like this?

I am trying to find the input resistance of this BJ common-emitter amplifier: I replace the transistor with the hybrid-pi model It appears clear to me that the input impedance will be \$ R_{in} =...
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1answer
92 views

what is the equivalent resistance of this configuration? [closed]

consider the blocks as resistance and the numbers as their respective resistance.count it's equivalent resistance across A and B. I just want some hints.this is not my homework problem.I am just ...
33
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8answers
3k views

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 ...
30
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2answers
11k views

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 ...
36
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7answers
7k views

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 ...
6
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5answers
5k views

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 "...
17
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3answers
11k views

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 ...