Questions tagged [resistors]

A resistor obeys Ohm's law (V=IR); the current through it is equal to the voltage across it divided by the resistance (equivalently \$I=\frac{V}{R}\$)

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153 votes
6 answers
106k views

Why exactly can't a single resistor be used for many parallel LEDs?

Why can't you use a single resistor for a number of LEDs in parallel instead of one each?
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110 votes
9 answers
110k views

Why does a resistor need to be on the anode of an LED?

Please be kind, I am an electronics nub. This is in reference to getting an LED to emit photons. From what I read (Getting Started in Electronics - Forrest Mims III and Make: Electronics) electrons ...
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106 votes
3 answers
99k views

How do I calculate the required value for a pull-up resistor?

There are many tutorials that use a pull-up or pull-down resistor in conjunction with a switch to avoid a floating ground, e.g. http://www.arduino.cc/en/Tutorial/button Many of these projects use ...
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97 votes
29 answers
102k views

What's the best way to store and categorise resistors/capacitors/ICs/etc?

I did a big cleanup of my collection of parts today and I now have a big pile of parts on my desk (the majority of which is resistors). My previous method of finding the resistor value I wanted was to ...
92 votes
11 answers
163k views

Is there a correct resistance value for I2C pull-up resistors?

The datasheet of the 24LC256 EEPROM states that: The SDA bus requires a pull-up resistor to VCC (typical 10 kΩ for 100 kHz, 2 kΩ for 400 kHz and 1 MHz). I thought that any resistor with a kΩ value ...
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89 votes
19 answers
25k views

How do I prove to my physics teacher that adding a battery in parallel doesn't double the current?

simulate this circuit – Schematic created using CircuitLab My physics teacher said that the current through the resistor is 4A because each battery has a current of 2A if hooked up to the ...
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  • 809
74 votes
14 answers
30k views

My understanding of RC circuits is broken

I asked a relatively simple question. Unfortunately, the answers provoke far more questions! :-( It seems that I don't actually understand RC circuits at all. In particular, why there's an R in there....
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61 votes
5 answers
97k views

How to choose value of resistor in voltage divider?

I understand that the output voltage is determined by the ratio between the two resistor values, and that if the both resistors are same then output voltage will be exactly same for all; but what's ...
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55 votes
6 answers
131k views

Do I really need resistors when controlling LEDs with Arduino?

I'm just trying out Arduino Uno for the first time with 2 blinking LEDs on a breadboard. All the tutorials on the Internet seem to use a resistor. I do know the function of resistors, but does it ...
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  • 871
55 votes
7 answers
63k views

What is the usage of Zero Ohm & MiliOhm Resistor?

I am new to PCB design and I noticed that some schematics use 0Ω or 100mΩ resistors. What is their purpose and why do we need to use them in our PCB design? Normally if we wish to probe how much ...
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  • 1,381
54 votes
5 answers
67k views

When to use pull-down vs. pull-up resistors

After learning and experimenting with microcontrollers, I've understood the concept of pull-up- and pull-down resistors. I now understand when and how to use them, and how they work. I've mainly used ...
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  • 695
53 votes
2 answers
68k views

Calculating the pulldown resistance for a given MOSFET's gate

I searched and read many similar questions, but did not find a specific answer for how to calculate the correct value for a pulldown resistor for a MOSFET's floating gate. It seems like everyone ...
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51 votes
5 answers
11k views

Why do you need 2 resistors when hooking up a transistor as a switch

What is the point of R2 in the following diagram: I get that R1 controls the current to the Base, but what does R2 do?
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50 votes
9 answers
40k views

Is a current-limiting resistor required for LEDs if the forward voltage and supply voltage are equal?

For blue LEDs with a forward voltage of 3.3 V and supply voltage of 3.3 V, is a series resistor still needed to limit current? Ohm's Law in this case says 0 Ω, but is this correct in practice? ...
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48 votes
6 answers
116k views

Easy way to figure out a LED's Vf in order to pick an appropriate resistor

I was wondering what the easiest way to figure out a LED's forward voltage is, using measurement tools. I know we can assume red LEDs to be around 1.8V - 2.2V, and that we have similar information ...
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42 votes
8 answers
66k views

What is a pull up and pull down?

Can some one explain this terminology please I think I understand it but not completely sure. I think pull down is where you place a resistor between +V and the other component and pull up is where ...
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41 votes
3 answers
104k views

Question about MOSFET gate resistor

I have N-MOSFET gate connected to a 4043 logic. Id is about 100mA. Both the 4043 and MOSFET have +5V. I plan to use a 2N7000. How large a gate resistor do I need between the 4043 and MOSFET. The ...
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  • 413
40 votes
8 answers
72k views

Why put a resistor in series with signal line?

A lot of times in circuits I see a resistor placed in series in a signal line and sometimes even in series with an MCU's VDD line. Is the intention of this to smooth out noise in the line? How is this ...
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  • 3,675
38 votes
6 answers
13k views

Why do resistors still use color coding?

I always wondered why color coding is still used on resistors in 2014. Here is Wikipedia's word on the original reason why : Colorbands were commonly used (especially on resistors) because they ...
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  • 543
38 votes
2 answers
84k views

Purpose of the diode and capacitor in this motor circuit

I'm hooking up a small DC motor to an arduino using an NPN transistor using the following diagrams I found online: The circuit works, and I'm successfully able to make the motor run. Now, I'm ...
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  • 757
36 votes
2 answers
11k views

Weird resistor with dots around it on the schematic

I was looking through the LTC4041 datasheet and saw this: The 10mOhm resistor with the two nodes really close to it - is that a "special" resistor or something? Why have they drawn it like that?
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  • 2,575
36 votes
8 answers
9k views

Are two (or N) resistors in series more precise than one big resistor?

Let's say I have one 2 kΩ resistor with 5% tolerance. If I replace it with two 1 kΩ resistors with 5% tolerance, will resulting tolerance go up, down, or remain unchanged? I'm bad with ...
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  • 675
36 votes
5 answers
9k views

How can a one million megohm resistor be useful?

I used to do periodic maintenance on a detector system for low-power-level particles. Its circuitry included a one million megohm resistor. It was in a sealed solid brick made maybe of Bakelite, about ...
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  • 1,281
36 votes
9 answers
25k views

Organizing electronic parts? [closed]

I've got one of these cabinets, which I intend to use for storing electronic components: It has 24 drawers (each 175 x 69 x 37mm). Every drawer can be subdivided into up to six compartments by ...
36 votes
5 answers
38k views

What makes a pull-up/down resistor strong or weak?

A "strong" pull (up/down) resistor would be one of a relatively low value, while a "weak" one would be of a relatively high value. For example, a pull-down resistor would be used to keep an I/O pin ...
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35 votes
7 answers
10k views

Why don't we use low voltage Power Sources for high wattage applications?

Super nooby question involving Ohm's law, but this has been on mind this morning. Say I have a 60W device, and I want to power it. Usually this calls for a 120V source or something. However, why not ...
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  • 767
35 votes
11 answers
23k views

Doesn't using resistors serial to LEDs all the time waste a lot of energy?

I was wondering for a long time why in schemes which use LEDs for lighting it's very common to put a resistor to go with the LED, and finally it seems like the answer in this question explains why. (...
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  • 528
35 votes
2 answers
2k views

Heuristic explanation of the usage of a pull-up (or pull-down) resistor needed

First, I acknowledge that there are several questions regarding this topic in the forum, however, the answers assume too much background knowledge of electronics to be of use to a true beginner (like ...
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  • 635
34 votes
8 answers
10k views

Power rating on Zero Ohm resistor

I was ordering some resistors online, and I saw that 0 Ω resistors have a power rating. Why is that? Power through a resistor is calculated with the equation \$P = UI\$ or \$P = RI^2\$. Since \$R ...
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  • 3,838
34 votes
5 answers
444k views

how to reduce DC voltage using resistors?

How would one go about using a 12 V DC power source to power something which needs 4.5 V DC using resistors? Is there a way to determine how much adding a resistor would drop the voltage?
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  • 487
34 votes
3 answers
11k views

Whats inside a resistor?

How does a resistor "resist" current/potential? I know it's an elementary question, but I'm sure others are wondering too.
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  • 5,804
34 votes
8 answers
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 ...
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33 votes
5 answers
10k views

What reasons are there to avoid vertical through-hole resistors?

I'm working on a layout for a PCB and I need to include a handful of pull-up resistors. The board I'm working on will be a proof of concept, and it is likely I will only need one (and order two). That ...
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31 votes
6 answers
261k views

Reducing voltage with resistors

I have been looking around for an easy way to convert 12V to 5V. I have seen some people saying that a simple resistor is all that is needed. $$ Volts = Ohms \cdot Amps $$ $$ Amps = \frac{Volts}{...
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31 votes
10 answers
7k views

Resistor suggestions for colorblind person

I'm a developer who has always had a love of the low-level electronis, but I have always been intimidated by it since resistors all seem to be color based and as a color-blind person, this is ...
30 votes
4 answers
5k views

Why aren't resistors being used in this flip-flop used in static MOS RAM?

In the book, Digital Computer Electronics, there is a diagram on page 140 (figure 9-4 a) of a flip-flop used in static MOS RAM to store a bit. The book says that Q3 and Q4 act like resistors, which ...
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30 votes
7 answers
31k views

How do I calculate the power rating for zero-ohm resistors?

Based on my earlier question, since there supposed to be zero voltage (V=IR) drop across a 0 Ω resistor, how do we select the power rating of such a component? For example, let's say I were to ...
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  • 1,381
29 votes
6 answers
9k views

Compute the minimum number of 120Ω resistors to get 80Ω of resistance?

I recently had to take test in basic electronics. I didn't get one question right, but I don't quite understand why. ...
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29 votes
7 answers
7k views

What's the purpose of these two diodes in this circuit?

What's to point of using these two diodes? Can't you just replace them with a resistor?
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29 votes
12 answers
31k views

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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29 votes
5 answers
41k views

What is a "100R" resistor?

Sometimes I see references to "R" resistors. For example: Obviously the 100 refers to 100 Ohms. What does 100R mean?
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  • 2,128
29 votes
7 answers
14k views

Basic diode question about voltage-drop

We have been begun studying diodes in electronics 1 and I have really basic question that may be really dumb but I cannot find an answer online. If a diode is in series with a resistor and a voltage ...
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29 votes
3 answers
3k views

Why are resistors tolerances relative instead of absolute?

Every resistor has a tolerance, this provides the user with an idea of the accuracy of the product. This tolerance is represented by a percentage. This means: a big value resistor will be less ...
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  • 3,838
29 votes
2 answers
16k views

What are the advantages of a shunt resistor vs. a hall effect sensor?

I'm constructing a boost converter, and I need to measure both the input current and the output current. Currents range anywhere from 25A to 200A, depending on the model. My controller is referenced ...
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27 votes
10 answers
11k views

Can a 10% resistor be used as a 1% resistor?

If my schematic calls for a 1% resistor, can I use a 10% resistor that measures to the correct resistance within 1% or is there some quality to tolerance beyond what it measures Ohm-wise? For example,...
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  • 2,128
27 votes
6 answers
23k views

How does an LM7805 voltage regulator work?

How do most voltage regulator ICs work? Are they the same as hooking up a variable resistor and a voltmeter and turning the knob until you get the desired voltage?
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  • 5,526
26 votes
1 answer
1k views

Why are the E-series numbers different from the powers of 10?

The E-series numbers are the common values used in resistors. For example, the E6 values are: 1.0 1.5 2.2 3.3 4.7 6.8 As you can see, each is about \$10^\frac16\$ apart. But I wonder why they aren'...
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  • 361
25 votes
2 answers
17k views

6 Band Resistors, which way should the bands be read

I've just bought a bag of random resistors from Maplin, and can't work out which way round the six band ones should be read. For instance I have one which is either: Violet Orange Red Brown Brown ...
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  • 2,893
24 votes
9 answers
8k views

Why don't LEDs have built-in resistors?

A night of struggling to fit 16 current limiting resistors into a small breadboard for a uni project made me wonder why LEDs don't have any kind of internal resistance? I can't think of any downsides. ...
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  • 687
24 votes
5 answers
2k views

Resistor Mounted On Top of IC?

Recently, I've been working to repair a broken 1980 "Arp Solus" synthesizer that I acquired from a friend. I have, however, found myself confronted by an unfamiliar arrangement: some kind of resistor ...
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