When to use a capacitor and when to use an inductor. - Page 1
Forum section: electronics / beginners

author: adept
author_rank: Regular Contributor
timestamp: May 17, 2011, 05:16:40 am
content: 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!

author: Simon
author_rank: Unknown
timestamp: May 17, 2011, 05:33:16 am
content: No they are not interchangeable. While both store a charge they have opposite AC characteristics

author: Trigger
author_rank: Regular Contributor
timestamp: May 17, 2011, 06:12:50 am
content: Quote from: adept on May 17, 2011, 05:16:40 amIn basic terms you use a capacitor to impede a change involtage. You use an inductor to impede a change incurrent

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 17, 2011, 06:30:07 am
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 17, 2011, 07:47:35 am
content: Quote from: adept on May 17, 2011, 05:16:40 amNo, 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,orthe charging voltage was high enough.)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 willoften cause a nice big spark!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 arenormally 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.VK6ZGO

author: jahonen
author_rank: Super Contributor
timestamp: May 17, 2011, 08:35:41 am
content: 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 exchangedOf 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

author: saturation
author_rank: Super Contributor
timestamp: May 17, 2011, 12:46:40 pm
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2011, 04:53:35 pm
content: Quote from: adept on May 17, 2011, 05:16:40 amNo, only a capacitor holds electrical charge, an inductor stores magnetic flux.

author: adept
author_rank: Regular Contributor
timestamp: May 18, 2011, 02:14:17 am
content: 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?

author: vk6zgo
author_rank: Super Contributor
timestamp: May 18, 2011, 02:30:20 am
content: Quote from: adept on May 18, 2011, 02:14:17 amNo!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

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2011, 04:34:23 pm
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 19, 2011, 04:41:39 am
content: Quote from: Hero999 on May 18, 2011, 04:34:23 pmIn 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 aretalking 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 ifyou don't have this basic knowledge,the maths just look like maths!VK6ZGO

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 19, 2011, 05:07:27 am
content: Quote from: vk6zgo on May 19, 2011, 04:41:39 ami wud like to add... get your hand dirty as well, theory is just a part of it.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 19, 2011, 02:30:01 pm
content: 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.htmOr 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

author: Trigger
author_rank: Regular Contributor
timestamp: May 20, 2011, 03:15:38 am
content: http://www.amazon.com/Electrical-Engineering-101-Second-School-but/dp/1856175065/ref=sr_1_1?ie=UTF8&qid=1305861195&sr=8-1Great 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.

author: Zero999
author_rank: Super Contributor
timestamp: May 20, 2011, 04:24:12 pm
content: Quote from: vk6zgo on May 19, 2011, 04:41:39 amYes, inductors do more when large currents are drawn and capacitors are better when less current is drawn.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: May 20, 2011, 07:53:31 pm
content: Quote from: vk6zgo on May 18, 2011, 02:30:20 amWhat 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.

author: Zero999
author_rank: Super Contributor
timestamp: May 20, 2011, 08:44:19 pm
content: Quote from: Leo Bodnar on May 20, 2011, 07:53:31 pmAt 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.QuoteThe inductance is a separate measurement to the ESR, it's the ESL (Effective Series Inductance).

author: vk6zgo
author_rank: Super Contributor
timestamp: May 21, 2011, 04:43:53 am
content: Quote from: Hero999 on May 20, 2011, 08:44:19 pmExactly! Obviously ,as I pointed out, inductors are made of relatively long lengths of wire,& inherently haverealseries resistance.This is always part of the "Equivalent Series Resistance",which is a imaginary resistance inseries 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 lessthan 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

author: SL4P
author_rank: Super Contributor
timestamp: September 27, 2016, 11:35:02 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2016, 01:11:42 am
content: Quote from: SL4P on September 27, 2016, 11:35:02 pm*Cough*So what about transformers?Tim

author: Brumby
author_rank: Unknown
timestamp: September 28, 2016, 02:15:20 am
content: Quote from: T3sl4co1l on September 28, 2016, 01:11:42 amTransformersarein series with the load.Just follow the current path.

author: Brumby
author_rank: Unknown
timestamp: September 28, 2016, 02:40:45 am
content: Quote from: adept on May 18, 2011, 02:14:17 amMore correct to say that operate using quite different physics.- Inductors don't like changes ofcurrentthroughthem- Capacitors don't like changes ofvoltageacrossthemThese differing characteristics means their general principles of application are quite different, but they can be used together with a common goal, such as.....QuoteSupply 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:QuoteFirst 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.

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 28, 2016, 09:57:33 pm
content: Quote from: SL4P on September 27, 2016, 11:35:02 pm"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

author: Brumby
author_rank: Unknown
timestamp: September 29, 2016, 03:30:02 am
content: Quote from: AG6QR on September 28, 2016, 09:57:33 pmRF voodoo. It's magical.

author: Brumby
author_rank: Unknown
timestamp: September 29, 2016, 05:21:37 am
content: While I don't disagree with what you have said, the question of stability - conditional or not - would seem to be somewhat outside the OP's understanding - and well beyond the question asked.The fact is this: Inductors and capacitors canin some casesbe used to achieve similargoals, but operate on completely different principles and, therefore, are not interchangeable in any way.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 29, 2016, 06:29:57 pm
content: Quote from: Brumby on September 29, 2016, 05:21:37 amIndeed, the space within which exchange is possible, is rather abstract compared to simply swapping out one component for another.Namely, if you swap:Voltage source <--> Current sourceSeries branch <--> Parallel nodesCapacitor <--> Induuctor (value = 1/x)Resistance <--> Conductance (G = 1/R)You'll end up with the same RLC network, but with a series-parallel transformed equivalent.Note this is not generally possible with nonlinear circuits, because we don't have every possible combination of device: a diode has a constant-voltage characteristic in forward bias (transform: current regulating diode -- a boutique item, though something which does exist); a transistor has a voltage sense input to current source output (a transconductance amplifier), where the input and output exhibit parallel capacitance.Tim