# simple q about capacitors **URL:** \ **Category:** Factual Questions\ **Created:** [March 8, 2012, 12:00am UTC](https://boards.straightdope.com/t/simple-q-about-capacitors/615084 "2012-03-08T00:00:37Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 8, 2012, 9:35pm UTC](https://boards.straightdope.com/t/simple-q-about-capacitors/615084/21 "2012-03-08T21:35:28Z")
> [@Michael63129](#): > > In some cases, especially power supplies, the ESR (equivalent series resistance) may also be important in stabilizing the regulation loop (if unstable, large oscillations in output voltage can occur); some spec sheets advise not to use low-ESR capacitors for this reason, so replacing a capacitor with a a better one may cause problems (higher voltage capacitors often have lower ESR due to a larger plate area). Also, most capacitors have a pretty loose tolerance (+/-20% is typical, have seen up to +100% tolerance on some old filter capacitors), so it might work if you use a different value, as long as it isn’t lower and too far off (except for tuned circuits and the like where an exact value is critical, but motherboards aren’t any concern here). Good points. Especially how too low of an ESR can cause oscillations. (While having a high ESR can be bad in terms of decreased efficiency and greater self heating, it certain situations it can be advantageous due of its higher damping factor. In other words, it is less likely to oscillate with the inductance of the power rails.) But I disagree that “higher voltage capacitors often have lower ESR due to a larger plate area.” A higher voltage capacitor has a thicker dielectric, all else being equal. Perhaps you meant that a higher capacitance capacitor has a lower ESR (all else being equal), which would be correct. Lowering the dielectric’s dissipation factor will also lower the ESR. ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [March 8, 2012, 11:05pm UTC](https://boards.straightdope.com/t/simple-q-about-capacitors/615084/22 "2012-03-08T23:05:46Z")
> [@Crafter\_Man](#): > > But I disagree that “higher voltage capacitors often have lower ESR due to a larger plate area.” A higher voltage capacitor has a thicker dielectric, all else being equal… ESR isn’t due to the dielectric (which ideally has infinite resistance), it depends on the thickness and size of the plates, and in electrolytics, the conductivity of the electrolyte (which acts as one of the plates). As an example, [here](http://www.nichicon.co.jp/english/products/pdfs/e-hz.pdf) is a datasheet (PDF) typical of motherboard capacitors; ESR generally decreases with voltage and larger case size for the same values, for example, 1200 uF at 6.3 v has twice the ESR as the same value at 16 v, and 1500 uf has three case sizes at 6.3 v, all with different ESR. One exception you will find is capacitors (of the same series) which span a very wide voltage range; high voltage capacitors (as found on primary side filters, 200-450 v) usually have higher ESR because they need a different electrolyte. Also, I didn’t mention ripple current in my previous post, which is largely a function of ESR and temperature rise. ---
**Author:** ![mchomard](https://avatars.discourse-cdn.com/v4/letter/m/22d042/32.png) [@mchomard](https://boards.straightdope.com/u/mchomard)\ **Post date:** [March 9, 2012, 9:11pm UTC](https://boards.straightdope.com/t/simple-q-about-capacitors/615084/23 "2012-03-09T21:11:07Z")
ok, thanks everyone. I will go check it. Hopefully it will be obvious upon inspection. I’m not entirely confident of that tho, lol. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 9, 2012, 9:50pm UTC](https://boards.straightdope.com/t/simple-q-about-capacitors/615084/24 "2012-03-09T21:50:13Z")
> [@Michael63129](#): > > ESR isn’t due to the dielectric (which ideally has infinite resistance), it depends on the thickness and size of the plates, and in electrolytics, the conductivity of the electrolyte (which acts as one of the plates). No, this is not correct. ESR is an AC phenomenon, not a DC phenomenon, and thus depends on the dissipation factor (D.F.) of the dielectric in addition to plate geometry. ([Here](http://www.illinoiscapacitor.com/pdf/Papers/low_ESR_fact_or_fiction.pdf) is a paper by Illinois Capacitor which talks about it.) ESR is also affected by the conductivity of the electrolyte. > [@Michael63129](#): > > As an example, [here](http://www.nichicon.co.jp/english/products/pdfs/e-hz.pdf) is a datasheet (PDF) typical of motherboard capacitors; ESR generally decreases with voltage and larger case size for the same values Yes, on electrolytic capacitors with working voltages below 100 V, the maximum D.F. specification for the dielectric becomes lower as the working voltage spec is increased. So all else being equal, the maximum D.F. specification for a capacitor with a working voltage of 25 V will be lower than the maximum D.F. specification for a capacitor with a working voltage of 10 V. Because ESR is a function of D.F., the ESR of the former will be less than the ESR of the latter. > [@Michael63129](#): > > for example, 1200 uF at 6.3 v has twice the ESR as the same value at 16 v, and 1500 uf has three case sizes at 6.3 v, all with different ESR. No, those are not ESR values. They are overall impedance values at 100 kHz. The impedance is comprised of X[sub]C[/sub], ESR, ESL, and R[sub]P[/sub]. For a graphical representation of this, check out Fig. 1-11 in [Nichicon’s technical document on aluminum electrolytic capacitors](http://www.nichicon.co.jp/english/products/pdf/aluminum.pdf). Note that ESR is the dashed line and impedance is the solid line. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [March 9, 2012, 10:17pm UTC](https://boards.straightdope.com/t/simple-q-about-capacitors/615084/25 "2012-03-09T22:17:56Z")
> [@Crafter\_Man](#): > > Wow. Didn’t know that. ☹ Add in the fact that he was on his way home from Jim Williams’ memorial service when he crashed, and it was a really bad month for this sparky’s heros. At least Pease went out while driving his beloved beetle, and got extra irony points for having written a book about how to avoid car crashes. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 10, 2012, 12:05am UTC](https://boards.straightdope.com/t/simple-q-about-capacitors/615084/26 "2012-03-10T00:05:39Z")
Yea, Kevbo, we lost a couple good guys there. ☹ I own books by both of them. > [@Crafter\_Man](#): > > ESR is an AC phenomenon, not a DC phenomenon Just wanted to clarify that, technically speaking, ESR is still “there” (due to plate geometry and electrolyte conductivity, but not D.F.) if a steady-state DC voltage is applied across a capacitor. But it doesn’t matter, because the leakage current is usually so small that the ESR is dissipating almost zero energy. ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [March 10, 2012, 2:16am UTC](https://boards.straightdope.com/t/simple-q-about-capacitors/615084/27 "2012-03-10T02:16:18Z")
> [@Crafter\_Man](#): > > No, those are not ESR values. They are overall impedance values at 100 kHz. The impedance is comprised of X[sub]C[/sub], ESR, ESL, and R[sub]P[/sub]. For a graphical representation of this, check out Fig. 1-11 in [Nichicon’s technical document on aluminum electrolytic capacitors](http://www.nichicon.co.jp/english/products/pdf/aluminum.pdf). Note that ESR is the dashed line and impedance is the solid line. I know that ESR isn’t the same as AC impedance, but I often see the terms used interchangeably, especially since only one or the other is usually specified (e.g. a datasheet for a regulator may state that ESR should be less than 1 ohm, with no mention of impedance\*). \*Which can be modeled, but in the SPICE program I use (LTSPICE IV) the built-in capacitors (a random selection from different manufacturers) have only ESR specified, although ESL and parallel resistance can be defined (I have defined my own by putting impedance where ESR goes and haven’t had discrepancies from the actual circuits). That’s at typical usage frequencies, of course. ---
**Author:** ![Mk\_VII](https://avatars.discourse-cdn.com/v4/letter/m/5f9b8f/32.png) [@Mk\_VII](https://boards.straightdope.com/u/Mk_VII)\ **Post date:** [March 11, 2012, 10:15pm UTC](https://boards.straightdope.com/t/simple-q-about-capacitors/615084/28 "2012-03-11T22:15:17Z")
Back around 1980 I had a job testing PCBs with electrolytic capacitors in them and if one had been wired in wrong way round you soon knew about it. Oil and paper contents blasted all over the place, and your nerves shredded too. It got so I would turn one on and take a turn round the office. Those tantalums burnt up quickly and you had to act fast if you didn’t want the board to burn (which meant they couldn’t sell it and it got retained for internal use). ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 12, 2012, 1:54am UTC](https://boards.straightdope.com/t/simple-q-about-capacitors/615084/29 "2012-03-12T01:54:57Z")
> [@Mk\_VII](#): > > Those tantalums burnt up quickly and you had to act fast if you didn’t want the board to burn (which meant they couldn’t sell it and it got retained for internal use). I’ve seen tantalum caps blast holes in PCBs when the fail due to excessive self heating. The (newer) polymer tantalum caps are supposedly “better behaved” when they fail, since they don’t contain MnO[sub]2[/sub]. [Previous page](https://boards.straightdope.com/t/simple-q-about-capacitors/615084.md?page=1)