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Best type(s) of capacitor

Discussion in 'General Electronics Discussion' started by CiaranM, Aug 7, 2012.

  1. CiaranM

    CiaranM

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    May 19, 2012
    Hi! I like making audio circuits, but I'm not sure what the best types of capacitor to use would be. E.g. electrolytics have high capacitance but also high tolerance and are supposed to be used with DC, so I want to avoid using them. Anyone got any recommendations? Thanks.

    NOTE: anyone know about thermistors (apart from the fact that they are temperature-dependent resistors, since i at least know that..)? I need a '2K +3300PPM/'C' one for a VCO. What does +3300PPM mean? I can't find such a thermistor on eBay. Are they also known as 'tempco's'?

    ANOTHER DAMN NOTE:
    I found 1K +3000PPM thermistors on eBay, would two of those be a suitable substitute?
     
    Last edited: Aug 7, 2012
  2. shrtrnd

    shrtrnd

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    Audio circuits are ok with electrolytics, which are generally cheaper.
    Electrolytics are for lower frequencies, most other makes are made for higher frequencies.
    Not familiar with the details about thermistors, I'd have to look them up like you are.
    'tempco' probably refers to the temperature coefficient of the device.
    Somebody will probably come on later with more info to help you.
     
  3. CiaranM

    CiaranM

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    May 19, 2012
    thanks shrtrnd!
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Re capacitor types. Some self-described audiophiles claim that particular types "sound" better, but AFAIK no one has ever done controlled tests to eliminate psychological bias, i.e. the people listening have always known what type of capacitor they are listening to, so the results are not meaningful (consistent results from a double blind test would be needed to establish that there is any audible difference).

    Different parts of an amplifier need different ranges of capacitance, and each capacitor type is manufactured in a specific range of values, so it's not always possible to pick and choose which type you want to use. For example if you need a 2200 uF 50V capacitor to smooth a power supply rail, you won't be able to use a silvered mica capacitor because a silvered mica capacitor with that value would be as big as a house, and probably as expensive too! Generally though, capacitors in the signal path have lower values and there are many types to choose from. Apart from avoiding multi-layer ceramic capacitors, I can't advise you here. Choose your favourite "expert" and follow his advice.

    Thermistors are defined by several parameters. First, there's the direction of the temperature characteristic. Positive temperature coefficient of resistance (PTC) means that the resistance increases with increasing temperature; negative temperature coefficient is the opposite. Your one is PTC.

    Second, there's the nominal resistance, in ohms, at a specific temperature. Usually this is specified at 25 degrees Celsius.

    Finally, there are a number of parameters that specify the shape of the relationship between temperature and resistance. This relationship is never just a straight line.

    Your thermistor is described as "2K +3300PPM/'C", so it will have a resistance of 2 kilohms at some nominal temperature, and its resistance vs. temperature characteristic is described as 3300 ppm (parts per million) per degree Celsius.

    One part per million of 2000 ohms is 0.002 ohms, so 3300 of them is 6.6 ohms, so the resistance needs to vary by 6.6 ohms for every one degree Celsius temperature change. This is not a complete specification, because the relationship between temperature and resistance is not linear; it is probably an approximation that gives the slope of the line around some specific temperature; probably the temperature at which the resistance is 2000 ohms.

    The way it is specified as just "2K +3300PPM/'C" implies that the nominal resistance of 2k applies when the temperature is 0 degrees C but thermistors are normally specified at 25 degrees Celsius so I can't be sure what temperature that resistance relates to.

    Do some research if you need more information. Wikipedia, Digikey and Google are your friends :)
     
  5. CiaranM

    CiaranM

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    May 19, 2012
    Your post helped me a LOT. thanks! that PPM thing was confusing me. Hmm, a capacitor as big as a house certainly would be problematic. Could I live inside the dielectric?

    Audiophiles do annoy me.. I'll quote part of a description from a YouTube video of someone's creation "Built with all top quality NOS parts; vintage Allan Bradley carbon composite resistors, Mullard and other top quality electrolytic capacitors, switchcraft pots and of course hand picked NOS vintage Germanium transistors to create this secret magic pudding. The overtones are sweet although cutting, with depth.
    (From the research by Dr Asai; Germanium is an important trace element in the life cycle of fauna and flora. Organic Germanium is a powerful healer of the human body, I believe the use of Germanium in analogue - basic electrical sound conversion is why the harmonics are so favorable to the human ear)"

    It's a bloody distortion unit..
     
  6. BobK

    BobK

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    I am curious, what is the problem with multi-layer ceramics? Do they have a resonance or something? As I undertand they are quite good for low ESR applications.

    Bob
     
    Last edited: Aug 8, 2012
  7. CiaranM

    CiaranM

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    May 19, 2012
    it would be unfortunate too.. I recently bought 1000 for a mere £3 from China. I hope they're not useless!!
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    MLCCs are fine for the applications where they're normally used - decoupling, mostly, where the important factors are high capacitance per volume, low ESR and ESL, and low cost, and in non-critical timing circuits perhaps. But most MLCCs are Class II and Class III and have poor capacitance tolerance and temperature characteristics, as well as a high dissipation factor (see link), all of which make them unsuitable for use in the signal path in audio equipment.

    http://en.wikipedia.org/wiki/Ceramic_capacitor
    http://en.wikipedia.org/wiki/Dissipation_factor

    CiaranM here's a story for you. Around 2000 at my old job, we bought a lot of no-name 0.1 uF Y5V (or similar) radial MLCCs from a local supplier, for decoupling applications mostly. After a few years, we started getting field failures caused by these capacitors going very leaky (under 1k) and screwing up the reference voltage on the board. Presumably (statistically) VCC decouplers on these boards were failing too, but these failures wouldn't be detectable, apart from increased power consumption. I couldn't say whether your low-cost Chinese parts will be any good, but as a result of these problems I vowed to always buy name-brand MLCCs. In fact it's probably a good policy for ALL passives. Once bitten, twice shy :)

    Re audiophiles. I would accept any proclamations that they could back up with evidence that they could reliably distinguish between the components they like and the ones they dislike. My policy is "if you can prove you can reliably hear the difference between two alternatives (under double blind test conditions), then I will accept your opinion on which is better."

    In some cases, the differences are measurable - for example, early transistor amplifiers DID have issues with crossover distortion, which can be clearly audible if it's bad enough, and the effects of damping factor are at least theoretically detectable by ear. But when they use vague terms like "brilliance", "fullness" and other even more nebulous descriptions, without backing their opinions up with tests that are designed to negate the MANY psychological effects that can make people honestly THINK they're hearing something that they AREN'T hearing, I start to suspect that they're just showing off and imagining themselves to be more perceptive than they really are (whether they know it or not).

    My father wrote an amusing article on this subject for a local magazine (NZ Listener, I think). It's a bit dated now, but still funny. I will try to dig it up and post it on this thread.
     
  9. CiaranM

    CiaranM

    74
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    May 19, 2012
    that sounds like a hassle. looks like I've taken a risk with these Chinese capacitors. Never trust the Chinese. Haha.
    I did a bit of research (well, wiki-search); I think polystyrene or polycarbonate would be good.

    Hey, I have a question about decoupling. How is possible to remove DC from AC? Say I've got a 5V peak-to-peak sine with a maximum positive value of 2.5V and a minimum value of -2.5V. If I added 1V DC to the wave, then the lower value would be at -1.5V and the higher value would be at 3.5V, right? The wave just moves up the scale.. so I don't see how DC could be removed. Sorry if I'm wrong (which I probably am).

    I completely agree with you on the audiophile issue. I would use an oscilloscope as a lie detector in such instances; see if what they hear can be demonstrated visually. "You're right, this sine looks so warm and fuzzy on this oscilloscope screen... oh wait that's just my shaky calibration hand" etc

    OK, sounds good, I look forward to reading the article! Was your father an engineer as you are?
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yeah, buying unbranded Chinese components is like a bawx of chawcolates - you never know what you're going to get :)

    Yes, if you have a sinewave of 5V p-p with a centre voltage of zero, it goes between -2.5V and +2.5V. If you add 1V DC to it, you get a 5V p-p sinewave that varies from -1.5V to +3.5V. But your original sinewave already has "no DC" because the average, mean, or centre voltage is zero.

    An oscilloscope will show some kinds of distortion very clearly - clipping, for example, and crossover distortion if it's bad enough, but there are other types of distortion that won't show up, or at least are more obvious by listening. Transient Intermodulation Distortion (TIM or TID) is an example. I'm prepared to accept that people can hear it, if they can demonstrate that fact. I don't rely on unsubstantiated claims, that's all.

    No, my father was a statistician! He was an audiophile though. His penchant was valve amplifiers.

    I'm away from home at the moment; it'll be a week or so before I can find the article. I'm sure I have a copy of it somewhere!
     
  11. CiaranM

    CiaranM

    74
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    May 19, 2012
    well I would quite like some chocolates now.. damnit!
    I'm still a bit confused with the subject of decoupling.. if a constant voltage is applied to a capacitor then it will charge up and lastly stop any current from passing. So DC is being blocked.. but if DC is superimposed on an AC wave, that wave still has the same frequency and no 'flat' sections (DC being a flat wave), so a capacitor will always be charging up or discharging depending on the motion of the wave (or rather, voltage). So how is DC removed when the capacitor isn't able to charge completely to block it?


    Hmm, I've never heard of TID; I'll look it up though. Thanks!

    Ah, valve amplifiers.. actually, I'm too young to remember them :O
    All I know is that they provide a more 'pleasant' form of distortion than transistors do, at the expense of being rather unreliable. do they need high voltages to work??
     
  12. davenn

    davenn Moderator

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    Sep 5, 2009
    cuz the DC wont pass through the capacitor, what would happen is that the AC voltage would be offset from zero by the amount of the DC voltage...
    that is normally the AC voltage would be cycling plus and minus from the 0V level. But having a DC voltage there, lets say 5V then the AC signal would be oscillating plus and minus from that 5V setting..... normally thats not desireable, tho occassionally there may be a reason to offset a voltage

    have a look at this circuit, a simple radio transmitter, there are 3 DC blocking caps

    [​IMG]

    the 22nF cap between the electric microphone and the base of the first transistor is also DC blocking. Its stopping the DC supply for the microphone from getting to the base of the BC547 and only allowing the audio (AC signal) from the mic to get through

    the 1uF between the first and second transistors collector of the first to the base of the second ( the BC547's)
    then there is a 10pF cap between the collector of the second BC547 transistor and the base of the last transistor, the 2N3563

    these 3 caps are all DC blocking, the first 2, the 22nF and 1uF are operating at audio freq's, the 3rd one is at RF freq's ~ 100MHz, hence is much smaller value ... only 10pF


    cheers
    Dave
     

    Attached Files:

    Last edited: Aug 9, 2012
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Decoupling and coupling are different things.

    Have a read of my long posting in this thread:
    https://www.electronicspoint.com/newbie-questions-t248766.html

    Ignore the part from the LED onwards. I think some people didn't like it because of the strange analogy I used :)

    In it, I model a capacitor as one of those hydraulic suppressor contraptions they attach to the top of doors, to stop them from banging closed. They will move, but only at a certain speed. The more pressure (current) you apply, the faster the plunger will move in the suppressor (the faster the voltage across the capacitor will change). If you hold the door open and push and pull it quickly, the rapid changes in the force on the door are transferred to changes in force on the wall where the suppressor mounts, but the suppressor itself doesn't compress or expand much in response to quick changes in force. It's the same with alternating current and a capacitor. The capacitor can slowly charge up to the DC voltage you apply (i.e. the suppressor extends when you open the door slowly), but brief changes don't cause its length (the voltage across it) to change much.

    This analogy applies best to larger-value capacitors, e.g. say 100 uF and higher, where the charge and discharge times can be in seconds like a door suppressor. With small value capacitors the effect is there, but less force is required and/or the suppressor moves a lot quicker for a given amount of force. If you include ALL types of them, capacitors as a group cover a capacitance range from under 1 pF (picofarad, 1x10^-12 farads) to over a farad, a range of more than twelve orders of magnitude.

    You can use a capacitor in two basic ways. Either IN SERIES with the signal (coupling) or IN PARALLEL with the signal (decoupling, or "coupling to ground").

    Decoupling is the simplest to understand, and the door analogy applies here. A capacitor to ground (ground is an unmovable reference voltage - most of the time) is equivalent to a hydraulic suppressor attached to a wall (an unmovable reference point). Anything that tries to open or close the door quickly meets "resistance" (not resistance in the electrical sense - conductance is the electrical equivalent, which is the reciprocal of resistance), so rapid changes in the door position are reduced. This is how decoupling works. A voltage rail that is decoupled with a capacitor to ground is held at a relatively steady voltage, regardless of short surges of current, either positive or negative. A sustained current is needed to cause the voltage to vary significantly; this is equivalent to putting a steady force on the door, which will cause it to open slowly (charge up or discharge).

    This is why decoupling capacitors are used around ICs, which can have very short periods of high current drain, but lower average current drain, and are fussy about their supply voltage staying steady. The capacitor holds the rail voltage steady during short bursts of high current needed by the ICs. This is the function of many of the big electrolytic capacitors on computer motherboards.

    A coupling capacitor is like one of those hydraulic suppressors held between two moving points. The AC signal at one point is coupled to the other point, even though there may be DC voltage difference between the points. Very low frequencies won't "pass through" a coupling capacitor; they will cause the voltage across the capacitor to change with the signal, and the capacitor "absorbs" the signal, leaving gradual changes (low frequencies) reduced at the other end of the suppressor, so coupling capacitors have to be chosen so that the capacitance is high enough to ensure that the lowest frequencies that have to pass through the capacitor will not be attenuated (significantly) by the capacitor. DC coupled circuits (many audio amplifiers are DC coupled) have no coupling capacitors in the signal path (except usually one at the input) and as such can reproduce very low frequencies accurately.

    HTH. Please re-read that referenced post and this description again if it doesn't make sense first time. :)
     
    Last edited: Aug 10, 2012
  14. bishop01

    bishop01

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    Aug 9, 2012
    nice post. :)
     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Thanks bishop01 :)
     
  16. davenn

    davenn Moderator

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    those are interesting comments you made Kris :)
    but it to be noted that what may be a coupling capacitor for one signal can at the same time be a decoupling cap for other signals as in the transmitter.

    The caps are all interstage coupling ... of either audio or RF but they also provide decoupling buffering between stages for any DC voltage present, as it is in this case

    really its a pretty fine line to what you call them one or the other ;)

    Dave
     
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Dave, in the circuits I have mostly dealt with (audio-frequency analogue, interfacing, digital) each individual capacitor is usually used for one purpose or the other, but a capacitor can do both for different frequencies, and that's a clever use of resources. :)
     
  18. davenn

    davenn Moderator

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    Im the opposite .... mainly an RF guy 30+ yrs getting too old for this stuff hahaha
    seriously I still enjoy it. The huge boost was after moving to Australia ... there's just so so much ex commercial microwave radio gear to comvert to the amateur bands. Also with the advent of eBay that really opened up the world to all sorts of other goodies :)

    if you wanna see some of the stuff I get into......

    http://www.sydneystormcity.com/microwave.htm

    cheers
    Dave
     
  19. CiaranM

    CiaranM

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    May 19, 2012
    HA; that thread is amusing.
    "I'm not using recreational drugs; this will make sense soon"
    "You lot are awesome. Except you Kris. You're awesome and mental"
    "Don't get the wrong idea because my analogy uses little men to roughly model semiconductors"

    Good analogies though, you put a lot of time into that post. I wondered how to 'model' a capacitor.. you did it :D

    Thanks for explaining decoupling! I always wondered why I had to put capacitors across the 'rails' when using op amps. I have a question regarding op amps; if you're using an AC power source; you'll have to smooth it, right? Should diodes be placed across the positive and negative 'rails' prior to smoothing?

    Also, thanks for explaining coupling.. If DC is added to AC, it is like adding a low frequency (well, no frequency) wave, and by passing it through a capacitor, this frequency is attenuated, bringing the wave 'down' to center along the X axis (so its equally positive and negative), right?
    Do you know what a good value to use for coupling in audio circuits is? E.g. if I used a 100u capacitor then the reactance would be approximately 80R at 20Hz. I'm thinking maybe 2200u, but I'm not sure how high the resistance should be at, say, 10Hz.

    Thanks for your help Dave. I see you've got your own website; it looks good. eBay certainly is useful for old electronics. I got a great tape deck for £10 not that long ago... still, I dream of a shop which has unwanted electronic goods and an owner who doesn't realise item value :D
     
    Last edited: Aug 9, 2012
  20. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    LOL :)
    Thanks :)
    No, unless you want to protect the components from accidentally reverse-connected batteries or supplies. That's the reason for connecting diodes ACROSS a power supply rail. It protects against reverse polarity, and if they're zener diodes, against excess voltage.

    Removing ripple from a supply that's derived from an AC source such as the mains is done by adding capacitance. You can also use "pi" filters which are called "pi" filters because they look like the "pi" symbol - there's a capacitor to ground, a resistive and/or inductive element in series, and a second capacitor to ground.

    Also, using a regulator also removes ripple. A linear regulator is ideal; a switching regulator adds its own ripple and noise.
    That's exactly right, and is a good way of explaining it.

    For a sinewave, when the DC is removed the waveform is symmetrical around 0V. For waveforms of other shapes, such as a rectangular wave with a duty cycle different from 50%, the 0V line will be where the AREAs enclosed by the waveform above and below the 0V reference are equal.

    It depends on the impedance of the stage that is being fed. The reactance of the coupling capacitor and the impedance of the following stage form a voltage divider; you have to ensure that the coupling capacitor's reactance is low compared to the following stage's input impedance at the frequencies you're concerned about, so that relatively little signal is lost across the coupling capacitor so most is seen at the input of the next stage.
     
    Last edited: Aug 9, 2012
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