# 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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### 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 ...
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 $3V/0.03A = 100 \Omega$...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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{...
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 ...
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 ...
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 ...
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?
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 ...
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, ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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 ...
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, ...
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, ...
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
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 ...
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%), ...
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: ...
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?
432 views