Questions tagged [capacitor]

A fundamental electronic component that stores energy in an electric field, commonly used in filtering applications.

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214 votes
17 answers
182k views

What is a decoupling capacitor and how do I know if I need one?

What is a decoupling capacitor (or smoothing capacitor as referred to in the link below)? How do I know if I need one and if so, what size and where it needs to go? This question mentions many ...
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89 votes
11 answers
71k views

Can you make a non-polar electrolytic capacitor out of two regular electrolytic capacitors?

There was some discussion on this question What are some reasons to connect capacitors in series? What are some reasons to connect capacitors in series? which I don't see as being conclusively ...
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80 votes
9 answers
270k views

How does a capacitor block DC?

I am confused with this! How does a capacitor block DC? I have seen many circuits using capacitors powered by a DC supply. So, if capacitor blocks DC, why should it be used in such circuits? Also, ...
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65 votes
5 answers
12k views

What's the use of a decoupling capacitor near a reservoir capacitor?

I've seen some circuits where a decoupling capacitor is used as well as a reservoir capacitor, like this (C4 and C5): I've read about decoupling capacitors and for me it looks as if they are meant to ...
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2 votes
4 answers
1k views

AC Circuit Having Only Capacitor

In my book, it is written that at any instant, PD across the Capacitor Plates = Applied EMF ---------(1) Assuming that it's a pure Capacitor of Capacitance C, I think if Voltage across Capacitor ...
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79 votes
8 answers
73k views

Decoupling capacitors: what size and how many?

Lots of chips nowadays require smoothing capacitors between VCC and GND for proper function. Given that my projects run at all sorts of different voltage and current levels, I was wondering if anyone ...
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8 votes
4 answers
4k views

Different (15 and 10 pF) load capacitors on 32.768 Hz quartz crystal

I've got some random chinese electronic desktop clock/alarm. It turned out that it's completely missing any capacitors on crystal. That was the reason why the clock were going faster ~5 min each month....
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110 votes
6 answers
41k views

Are tantalum capacitors safe for use in new designs?

I've heard it suggested that "solid tantalum" capacitors are dangerous and may cause fire, may fail short circuit and are fatally sensitive to even very short over voltage spikes. Are tantalum ...
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66 votes
4 answers
102k views

What's the purpose of two capacitors in parallel?

What's the purpose of the two capacitors in parallel on each side of the regulator in this power supply circuit I've seen similar setups in other similar circuits and can guess that it's related to ...
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11 votes
3 answers
63k views

Whats this white stuff on my board?

I'm not an engineer, but my monitor stopped working yesterday (it just wouldn't display anything but still gets power) so I figured I'd like to take it apart and see what went wrong. At the very ...
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26 votes
4 answers
105k views

How do I calculate how fast a capacitor will discharge?

Say I have a 1F capacitor that is charged up to 5V. Then say I connect the cap to a circuit that draws 10 mA of current when operating between 3 and 5 V. What equation would I use to calculate the ...
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26 votes
6 answers
15k views

Frequency dependence of electrolytic capacitors

It's said that electrolytic capacitors behave as inductors at high frequencies, which is why we put small ceramic caps in parallel with them: Electrolytic, paper, or plastic film capacitors are a ...
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19 votes
4 answers
6k views

Will a 0402 0.01 µF ceramic capacitor next to a 0402 0.1 µF ceramic capacitor have any power decoupling benefits?

I always understood that the point of using smaller capacitors in parallel was to provide low impedance at higher frequencies than the bigger capacitors, because the higher capacitance capacitors '...
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22 votes
4 answers
34k views

How diodes protect H-bridge DC motor driver?

I don't really understand how these diodes in this circuit and similar circuits (like driving a relay circuit) protects the controller circuit from the energy stored by the inductance of the coil. I ...
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0 votes
1 answer
960 views

Is this an astable multivibrator?

In this circuit the LED flashes on and off, however could it be classified as an astable multivibrator?
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0 votes
3 answers
856 views

Help understanging ac signal connected to a charged capacitor (coupling capacitor)

(Assuming ideal elements) My friend is trying to convince me that the voltage across \$R_L\$ varies between 4 and 6, says the average value doesn't decrease and stays fixed at \$5V\$ forever. I'm not ...
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22 votes
4 answers
2k views

Energy in capacitors - loss?

The energy stored in a capacitor is $$ U= \dfrac{1}{2} CV^2 $$ So when I have a 1F supercap charged to 1V the energy is 0.5 J. When I connect a second supercap, also 1F in parallel the charge will ...
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46 votes
8 answers
19k views

Can we build capacitors on a PCB board?

For the magnitude of nF or µF capacitors, I hope I can build them on a PCB board. The capacitor is like a two metal layer and something between them. Is this possible? Not buying the capacitor, ...
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6 votes
2 answers
5k views

Self resonance frequency for MLCC capacitor

Is there a general self resonance frequency characteristics for MLCC capacitors? If I make a RC low pass filter, say using 1kohm and 15nF 0603 with fc = 10.6kHz, I guess it won't work as LPF beyond ...
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1 vote
3 answers
2k views

Understanding this AC coupled circuit

I'm having a little trouble understanding this circuit and could use some help. So far I understand that this is an AC coupled circuit, and the capacitor is a blocking capacitor. I also know that ...
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0 votes
2 answers
4k views

Why is the voltage of a capacitor equal to the voltage of a battery connected it?

I am confused by this question. If we connect a resistor in series to capacitor then the voltage will drop across the resistor and now voltage across the capacitor is less than the source, then why ...
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19 votes
4 answers
10k views

Pulse-powering heavy loads with a coin cell

Lithium coin cells are rated for fairly low standard current draws, on the order of 1 to 5 mA. Also, while they allow greater pulsed current draws (i.e., periodic bursts), this appears to be ...
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17 votes
5 answers
84k views

Do capacitors automatically release their energy over time?

Will a capacitor automatically release its energy over time, or will it stay in there until manually discharged? So let's say I've had an old computer sitting around for a year and decide to take ...
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2 votes
4 answers
17k views

Varying run capacitor for speed control of single phase motor

If I want to vary speed of single phase electric motor within say, 10 to 20 % max of its rated speed (or torque), is it a good idea to change its run capacitor setting? Either via multiple capacitor ...
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  • 507
3 votes
2 answers
6k views

What is the function of this circuit?

Just wondering if someone can give me an explanation for this capacitor layout - what is it's function? The chip to the right is an ADC. Circled in blue. I'm guessing it is well known but i don't ...
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99 votes
6 answers
73k views

Ceramic caps vs electrolytic. What are the tangible differences in use?

A quick google around and all I seem to be able to find are people talking about the physics & the chemistry of the capacitors but not how this affects choosing which to use. Avoiding talking ...
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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
64 votes
4 answers
69k views

Can I replace all electrolytic capacitors with ceramic?

I am designing the power supply circuit for a system that needs several supplies, my questions are: Is it possible to replace all electrolytic caps (Mostly 100uF) by ceramic ones? What are the ...
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  • 2,567
29 votes
4 answers
16k views

"Starter Kit for electronics" - What to buy?

I'm going to buy some things for back to the electronic world. Starting with an Resistor Assorted Kit. Need to buy some kits containing the most common capacitors and transistors... what to buy??? [...
35 votes
6 answers
38k views

Do electrolytic capacitors have a limited shelf life?

Do electrolytic capacitors have a limited shelf life? I would like to know for both aluminium and tantalum.
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22 votes
3 answers
44k views

How long would a 150 Farad Capacitor light up an LED

I have a bright white LED from a flashlight. Aproximatley How long will it light up with a 150 farad 2.5 volt capacitor. Do I need a resistor? And if so how many Ω? The capacitor is a maxwell 150 ...
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35 votes
4 answers
33k views

Where did the value of 0.1uF for bypass capacitors come from?

Nearly everyone recommends 0.1uF for bypass capacitors. Why this value? I assume there is no harm to using larger values so is it merely a "sensible minimum"? And if so why do people go for the ...
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  • 1,023
12 votes
3 answers
9k views

Why too many capacitors in parallel for Vdd supply net? Can't we just add all to replace with one big capacitor?

Here is a schematic of a Basys-2 board's power regulator IC and filters. It is just an example but this is fairly similar to many designs I've seen. Why are there so many capacitors added in parallel ...
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31 votes
10 answers
42k views

What are some reasons to connect capacitors in series?

Usually you either combine capacitors in parallel because you want to increase the total capacitance while fitting the components in a certain shape/position, or you just combine capacitors by buying ...
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9 votes
1 answer
27k views

How Exactly Current Flow Through a Capacitor? [duplicate]

Capacitors behaving as an open circuits, How can we understand the concept of the Electrons flowing through a capacitors.
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0 votes
2 answers
203 views

Replacing these capacitors?

This is a controller board for a home ventilation system. It runs fine most of the time but sometimes the ventilation stops for one minute, then starts again and runs normally for anywhere between one ...
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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 ...
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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29 votes
6 answers
25k views

How to convert AC to DC

I am designing a circuit that needs to output 5VDC @ 1A. I'm trying to use a wall transformer to step the voltage down to 12VAC. The next step is the diode bridge and ripple capacitor. The ripple ...
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25 votes
9 answers
131k views

High voltage capacitor, in a low voltage system?

Quick question: is using a capacitor rated for high voltage (let's say 35V) in a system that let's say supplies 5V (like for LEDs or what have you) dangerous? Since it can store up to 35V, will it ...
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13 votes
2 answers
45k views

Ceramic capacitors: how to read 3-digit plus 1-letter markings?

It seems that reading a ceramic capacitor value out of its written values is harder than decoding enigma machine. I wonder if experienced users here does have a trick to quickly figure out these ...
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7 votes
4 answers
64k views

Calculating the capacitor values to control ceiling fan speed

I have an old ceiling fan motor that runs with a 1.5µF run capacitor, at what I believe is, its full intended speed. With the help of some folks here at StackExchange I've wired everything up as in ...
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7 votes
1 answer
598 views

Amplifier capacitors

Why is that when I use electrolytic capacitors (the cylindrical ones) and measure the end with respect to ground, there is some DC voltage appearing (where in fact it should be 0 volts since capacitor ...
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2 votes
2 answers
3k views

Generating high voltage (100V+) from 5V USB supply

I am using an USB port as a power supply. Is there a way I can take the weak 5V of a regular laptop USB and transform if to 100V+ while begin in the smallest package possible. I've heard of boost ...
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  • 125
9 votes
1 answer
12k views

How to derate a ceramic capacitor for DC bias

I understand that the capacitance of a ceramic cap is very dependent on the DC bias applied to the terminals. At rated voltage, I've measured a decrease of up to 70%, and I've heard it can go higher. ...
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5 votes
2 answers
1k views

Bootstrap capacitor - Explanation

i'm taking a course in Switched-mode power supplies and I am a bit confused about the theoretical explanation of what is happening in increasing voltage using a bootstrapped capacitor. You connect a ...
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  • 361
3 votes
3 answers
2k views

Car bluetooth power delay- USB capacitor? [closed]

I just bought a bluetooth adapter for my car. It's input is 5V/0.3A and is powered via a 5V/1A USB power adaptor that is plugged into the 12V/10A/120W max. socket in the center console of my 2004 ...
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1 vote
3 answers
3k views

RC circuit as clock source

I am not understanding something very fundamental here , I have read that an RC circuit can be used as a clock source. From this I assume a DC input. Playing around with my oscilloscope the only time ...
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0 votes
2 answers
632 views

Capacitors, ground between connection, Colpitts oscillator

For the Colpitts oscillator circuit shown (from a YouTube video), the video author said that the capacitors C1 and C2 are in series, which makes sense since there is a common connection point for C1 ...
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  • 387
9 votes
4 answers
36k views

De-coupling capacitor and Bulk capacitor

How does de-coupling and bulk capacitors work? what difference do they make adding them to the circuit.. Can anyone help me using a simple circuit that shows the effect of decoupling and bulk ...
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