Author Topic: When to use a capacitor and when to use an inductor.  (Read 29140 times)

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Offline adept

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When to use a capacitor and when to use an inductor.
« on: May 17, 2011, 05:16:40 am »
Hey guys! Hate to be the fool that asks this question, but I was wondering when I should choose to use an inductor and when it is best to use a capacitor. They both hold electrical charge and can hold lots of it. Is it ever okay to replace an inductor with a similar charge cap? Dave, maybe you could do a blog on inductors to go with your capacitor show. I know I would watch it!
 

Offline Simon

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Re: When to use a capacitor and when to use an inductor.
« Reply #1 on: May 17, 2011, 05:33:16 am »
No they are not interchangeable. While both store a charge they have opposite AC characteristics
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Offline Trigger

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Re: When to use a capacitor and when to use an inductor.
« Reply #2 on: May 17, 2011, 06:12:50 am »
Hey guys! Hate to be the fool that asks this question, but I was wondering when I should choose to use an inductor and when it is best to use a capacitor. They both hold electrical charge and can hold lots of it. Is it ever okay to replace an inductor with a similar charge cap? Dave, maybe you could do a blog on inductors to go with your capacitor show. I know I would watch it!

In basic terms you use a capacitor to impede a change in voltage.  You use an inductor to impede a change in current
 

Offline Mechatrommer

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Re: When to use a capacitor and when to use an inductor.
« Reply #3 on: May 17, 2011, 06:30:07 am »
another simpler word. you put a capacitor across a battery, nothing will happen (open circuit) but if you put an inductor, it will short and probably burst. put another way, a capacitor is high frequency pass filter, where inductor is low pass.
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Offline vk6zgo

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Re: When to use a capacitor and when to use an inductor.
« Reply #4 on: May 17, 2011, 07:47:35 am »
Hey guys! Hate to be the fool that asks this question, but I was wondering when I should choose to use an inductor and when it is best to use a capacitor. They both hold electrical charge and can hold lots of it. Is it ever okay to replace an inductor with a similar charge cap? Dave, maybe you could do a blog on inductors to go with your capacitor show. I know I would watch it!

No, the circuits used are totally different.

When a capacitor is charged from a DC supply,the current is initially large,diminishing to virtually zero at "full charge".

The cap can be removed from circuit,left for some time with the terminals open circuit.If a short circuit is then placed across the terminals,

the capacitor will then give up its electrical charge in the form of current,(accompanied by a nice big spark if the cap is high enough in value,or

the charging voltage was high enough.)  :D

An inductor's charging current is initially low,increasing to a maximum as the inductor reaches full charge.

A magnetic field is set up around the inductor,which is where the charge is stored.

If the charging circuit is now interrupted,the magnetic field collapses,releasing the stored energy in the form of an induced voltage,which will 

often cause a nice big spark! :D

So:- Capacitor---- fully charged---no current!

        Inductor-----fully charged---lots of current !


Capacitors are used more often than Inductors in much general circuitry,because they don't need a standing current to stay charged,they are

normally lower in loss,& usually smaller.

Inductors are widely used,however,where their special characteristics are very useful.

Both Capacitors  & Inductors are used extensively in AC circuits.

I suggest you read up on Basic DC theory,Inductors & Capacitors.----You may find information on the Internet,or try your local library.

Now guys,before you pull me to pieces for what I've left out,this was aimed at the OPs level of knowledge.


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Offline jahonen

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Re: When to use a capacitor and when to use an inductor.
« Reply #5 on: May 17, 2011, 08:35:41 am »
Basic equations of inductor and capacitor are U=L*di/dt and I=C*du/dt. So examining those, inductor and capacitor can be exchanged if voltage and current is also exchanged :) Of course, this has nearly theoretical meaning. In practice there are numerous difficulties doing those substitutions, but some can be done if the circuit topology is also modified.

Regards,
Janne
« Last Edit: May 17, 2011, 08:37:34 am by jahonen »
 

Offline saturation

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Re: When to use a capacitor and when to use an inductor.
« Reply #6 on: May 17, 2011, 12:46:40 pm »
Since fundamentally you are looking for reactance per frequency, either device can be designed in for that purpose.  However, in terms of accuracy in real world components and stability in environmental conditions, its usually R then C, L in ~ order.  So its easier to design in a specific value closure to ideal using C over L, if both can be substituted for each other, with suitable redesign of course, assuming you get the best of each type for those parameters.

In terms of long term reliability for your design, it depends.  RLC used in power applications are more likely to fail than those used in low power use.  For example in a lowly linear power supply, rates of failure could be C > R > L, for electrolytic, and power thick film resistor, and transformer or air core inductors respectively.

  


« Last Edit: May 17, 2011, 07:52:42 pm by saturation »
Best Wishes,

 Saturation
 

Online Zero999

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Re: When to use a capacitor and when to use an inductor.
« Reply #7 on: May 17, 2011, 04:53:35 pm »
They both hold electrical charge and can hold lots of it.
No, only a capacitor holds electrical charge, an inductor stores magnetic flux.
 
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Offline adept

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Re: When to use a capacitor and when to use an inductor.
« Reply #8 on: May 18, 2011, 02:14:17 am »
So basically what I'm getting is that these two have different applications. Such as an inductor could be used to smooth a signal in a wire and a capacitor could be used to filter the ripple of a rectified AC supply because the inductor wouldn't have much loss in a signal where they are supposed to be connected while a capacitor has no electrical connection between the leads so it has less loss when they aren't supposed to be connected together. Right?
 

Offline vk6zgo

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Re: When to use a capacitor and when to use an inductor.
« Reply #9 on: May 18, 2011, 02:30:20 am »
So basically what I'm getting is that these two have different applications. Such as an inductor could be used to smooth a signal in a wire and a capacitor could be used to filter the ripple of a rectified AC supply because the inductor wouldn't have much loss in a signal where they are supposed to be connected while a capacitor has no electrical connection between the leads so it has less loss when they aren't supposed to be connected together. Right?

No!

The difference in loss is just due to the construction of real world components,real world inductors are made from lengths of wire,wound into a coil.

wire has resistance,hence the inductance looks like it is in series with a resistor.

The internal resistance of capacitors is very much less, due to their construction.

Capacitors do have an electrical connection for AC,but not for DC.

In your example of a filter,classic power supply filters used inductors in series with the rectified supply,combined with capacitors in parallel.

The AC hum present on the rectifier output  was conducted to earth by the caps,whilst the inductors  had a high reactance at the hum frequency,

reducing the amount of hum present at the other end of the inductor.

Again ,I  say, read it up! Books & web pages have much better explanations than we can do in a couple of sentences.

They also have diagrams which make the concepts easier to follow.

VK6ZGO   
 

Online Zero999

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Re: When to use a capacitor and when to use an inductor.
« Reply #10 on: May 18, 2011, 04:34:23 pm »
An inductor resists changes in the electrical current flowing through it.

A capacitor resists changes in the voltage applied across it.

An inductor is good for smoothing tipple on a constant current power supply.

A capacitor is good for smoothing ripple on a constant voltage power supply.
 

Offline vk6zgo

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Re: When to use a capacitor and when to use an inductor.
« Reply #11 on: May 19, 2011, 04:41:39 am »
An inductor resists changes in the electrical current flowing through it.

A capacitor resists changes in the voltage applied across it.

An inductor is good for smoothing tipple on a constant current power supply.

A capacitor is good for smoothing ripple on a constant voltage power supply.

In practice both L &C are used in all types of power supplies,as the  L in series,C in parallel setup is a low pass filter,In the case of a DC supply,

the  high frequency cutoff is <<2f, where f is the frequency of the AC supply before rectification.

All this stuff is outside the apparent knowledge level of the OP,& I reiterate :-

Find a good book on,or Google for,"Basic Electrical Theory"!!

Inevitably,answers you get on this forum will run the gamut between purely practical information,to abstruse  mathematical definitions,

depending in the erudition of the people answering,but most of it is wasted,if you don't have the basic tools to understand what they are

talking about.

For,instance,if you know basic electrical theory,you can usually get the drift of an EE's argument,even if your maths are a bit rusty,but if

you don't have this basic knowledge,the maths just look like maths!


VK6ZGO
 

Offline Mechatrommer

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Re: When to use a capacitor and when to use an inductor.
« Reply #12 on: May 19, 2011, 05:07:27 am »
Find a good book on,or Google for,"Basic Electrical Theory"!!
i wud like to add... get your hand dirty as well, theory is just a part of it.
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Offline vk6zgo

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Re: When to use a capacitor and when to use an inductor.
« Reply #13 on: May 19, 2011, 02:30:01 pm »

I'd like to add this to my earlier comments:-

Of the options I gave,Books are the best,most of the websites I found while checking are not that marvellous.


This one isn't bad:-       http://www.electricianeducation.com/theory/electrical_theory.htm
                                       
               Or this:-        http://www.allaboutcircuits.com/

Overall,I believe books will be the better choice.

I would suggest the ARRL Handbook,The RSGB Handbook,or one of the US Naval Navbooks.

Read the information & try to get a feel for it,then ask questions if you don't understand what they are saying in the book.


VK6ZGO











 

Offline Trigger

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Re: When to use a capacitor and when to use an inductor.
« Reply #14 on: May 20, 2011, 03:15:38 am »
http://www.amazon.com/Electrical-Engineering-101-Second-School-but/dp/1856175065/ref=sr_1_1?ie=UTF8&qid=1305861195&sr=8-1

Great book with practical useful information.  Dave you should link it in your store.

In fact I payed a lot more for mine, I just bought another since I keep lending mine out at the hackerspace.
« Last Edit: May 20, 2011, 03:17:37 am by Trigger »
 

Online Zero999

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Re: When to use a capacitor and when to use an inductor.
« Reply #15 on: May 20, 2011, 04:24:12 pm »
In practice both L &C are used in all types of power supplies,as the  L in series,C in parallel setup is a low pass filter,In the case of a DC supply,
Yes, inductors do more when large currents are drawn and capacitors are better when less current is drawn.
 

Offline Leo Bodnar

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Re: When to use a capacitor and when to use an inductor.
« Reply #16 on: May 20, 2011, 07:53:31 pm »
No!
The difference in loss is just due to the construction of real world components,real world inductors are made from lengths of wire,wound into a coil.
wire has resistance,hence the inductance looks like it is in series with a resistor.
The internal resistance of capacitors is very much less, due to their construction.

What do you mean by "very much less?"  ESR of capacitors and inductors does not come from DC resistances but from skin effect, intercoil interferences, inductance of leads, etc.
Modern inductors and capacitors both can have Q (defined as X/ESR) as high as 1000s.

Online Zero999

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Re: When to use a capacitor and when to use an inductor.
« Reply #17 on: May 20, 2011, 08:44:19 pm »
ESR of capacitors and inductors does not come from DC resistances but from skin effect, intercoil interferences,
At low frequencies most of the ESR is mostly due to the DC resistance of the coil for or the plates and electrolyte of an electrolytic capacitor and is very significant for high value components. The ESR of a large 10,000uF capacitor can be measured at 1kHz which is too low for the skin effect to be prominent and the ESR 100mH choke can be measured at DC.

Quote
inductance of leads, etc.
The inductance is a separate measurement to the ESR, it's the ESL (Effective Series Inductance).
 

Offline vk6zgo

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Re: When to use a capacitor and when to use an inductor.
« Reply #18 on: May 21, 2011, 04:43:53 am »
ESR of capacitors and inductors does not come from DC resistances but from skin effect, intercoil interferences,
At low frequencies most of the ESR is mostly due to the DC resistance of the coil for or the plates and electrolyte of an electrolytic capacitor and is very significant for high value components. The ESR of a large 10,000uF capacitor can be measured at 1kHz which is too low for the skin effect to be prominent and the ESR 100mH choke can be measured at DC.

Quote
inductance of leads, etc.
The inductance is a separate measurement to the ESR, it's the ESL (Effective Series Inductance).

Exactly! Obviously ,as I pointed out, inductors are made of relatively long lengths of wire,& inherently have real
series resistance.This is always part of the "Equivalent Series Resistance",which is a imaginary resistance in
series with an ideal component,when used in circuit analysis.

Even when "Surface effect" is taken into account,it is the resistance at the frequency being tested,which is never less
than the DC resistance.
The other resistive component at HF is "radiation resistance",where the inductor is trying to be an Antenna.

Because capacitors are made up of plates of fairly large area,the "DC",or low frequency resistance is less than in an inductor.

ESR usually is not a major component of capacitor characteristics for lower capacitance values,& is largely ignored,
with the capacitor treated as an ideal component.

Where ESR is important,is in relatively large value components,such as, in the real world,Electrolytic & Tantalum capactors of 1uF & up.An alternative specification sometimes used for Electros & Tants is "Ripple Current".

ESR is not normally the Specification of choice for Inductors, "Q" being preferred.

None of this is rocket science,it is included in every legitimate  book on Electronics Theory.

VK6ZGO
 

Offline SL4P

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Re: When to use a capacitor and when to use an inductor.
« Reply #19 on: September 27, 2016, 11:35:02 pm »
Just stumbled across this -
Also consider that capacitors may appear in series -or- parallel, depending on their role.  Inductors are always in series with the load.
Don't ask a question if you aren't willing to listen to the answer.
 

Offline T3sl4co1l

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Re: When to use a capacitor and when to use an inductor.
« Reply #20 on: September 28, 2016, 01:11:42 am »
Just stumbled across this -
Also consider that capacitors may appear in series -or- parallel, depending on their role.  Inductors are always in series with the load.

*Cough*

So what about transformers? ;)

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Offline Brumby

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Re: When to use a capacitor and when to use an inductor.
« Reply #21 on: September 28, 2016, 02:15:20 am »
Just stumbled across this -
Also consider that capacitors may appear in series -or- parallel, depending on their role.  Inductors are always in series with the load.

*Cough*

So what about transformers? ;)

Tim

Transformers are in series with the load.

Just follow the current path.
 

Offline Brumby

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Re: When to use a capacitor and when to use an inductor.
« Reply #22 on: September 28, 2016, 02:40:45 am »
So basically what I'm getting is that these two have different applications.
More correct to say that operate using quite different physics.
- Inductors don't like changes of current through them
- Capacitors don't like changes of voltage across them
These differing characteristics means their general principles of application are quite different, but they can be used together with a common goal, such as.....
Quote
Such as an inductor could be used to smooth a signal in a wire and a capacitor could be used to filter the ripple of a rectified AC supply

Supply filtering can be performed using both.  Inductors are in series with the load and capacitors are in parallel.  Here is a classic example - the Pi filter:



Quote
because the inductor wouldn't have much loss in a signal where they are supposed to be connected while a capacitor has no electrical connection between the leads so it has less loss when they aren't supposed to be connected together. Right?
First a clarification - "Loss" is more often associated with energy lost from the system ... I think you are talking more about "Attentuation"
Second ... I'm not clear on what you are trying to say here.
« Last Edit: September 28, 2016, 02:44:19 am by Brumby »
 

Offline AG6QR

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Re: When to use a capacitor and when to use an inductor.
« Reply #23 on: September 28, 2016, 09:57:33 pm »
Just stumbled across this -
Also consider that capacitors may appear in series -or- parallel, depending on their role.  Inductors are always in series with the load.

"Always" is a pretty strong word.

It's true that, if you want to deliver DC to the load, you won't put an inductor in parallel.  And that's a very common case for many types of loads.  But it's not the only possible case.

Parallel inductors see occasional use in antenna impedance-matching networks, when you want to efficiently transfer RF from a source (transmitter) to a load (antenna) with a particular impedance mismatch.  See, for example:

http://www.giangrandi.ch/electronics/shortanttuner/shortanttuner.shtml
 

Offline Brumby

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Re: When to use a capacitor and when to use an inductor.
« Reply #24 on: September 29, 2016, 03:30:02 am »
Parallel inductors see occasional use in antenna impedance-matching networks, when you want to efficiently transfer RF from a source (transmitter) to a load (antenna) with a particular impedance mismatch.  See, for example:

http://www.giangrandi.ch/electronics/shortanttuner/shortanttuner.shtml

RF voodoo.  It's magical.
 


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