Smart questions
Smart answers
Smart people
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

swb1 (Electrical) (OP)
15 Jan 06 0:06
Hi,

When do I know when to use a polarized capacitor vs. a non-polarized capacitor?? Is there a rule of thumb??

Thanks,

swb1
MikeHalloran (Mechanical)
15 Jan 06 0:20
Use non-polarized for AC or for fast-moving signals.

Explanation:
1.  Reverse polarity usually blows up a polarized capacitor.
2.  Polarized capacitors are usually electrolytic, i.e., part of the capacitance has to do with chemistry, which is slow, so they can introduce phase errors.

Mike Halloran
Pembroke Pines, FL, USA

swb1 (Electrical) (OP)
15 Jan 06 2:46
Thanks for your reply Mike! So are you saying that the value has nothing to do with it?? Are you saying that if I have a slow moving signal (less than 30 MHz) and there is a 10 uF cap on the signal, I should always use a non-polarized cap?? At what speed is a signal considered "fast"??

Thanks!,
swb1
Warpspeed (Automotive)
15 Jan 06 4:17
Polarised capacitors use a very thin oxide layer as the insulating medium between the plates, so that the capacitance per unit volume can be made very high. They usually require a dc polarising voltage to stabilize the oxide layer, and being a chemical process, either very high or very low temperatures can be a problem.  Another difficulty is that the actual value of capacitance may be neither very accurate or very stable.

The bottom line is, use polarised capacitors where you need a lot of capacitance in a small volume, but are not too fussy about the exact value. The main application is for coupling and decoupling, and bulk energy storage.

Non polarised capacitors are superior in every respect, except they start to become really huge and expensive if a really high value of capacitance is required.
itsmoked (Electrical)
15 Jan 06 4:25
Ceramic, mica, polypropylene capacitors are also considered non-polarized.  It's just a class of capacitors.  It's an important classification for people who lay out circuit boards.  This is because of the need to put down silk screen foot prints that correctly depict the polarity for subsequent assembly.
MikeHalloran (Mechanical)
15 Jan 06 15:05
To some people, 3kHz is a fast signal.  I have a friend who's into music in a big way.  He has replaced all the electrolytics in the signal path of his audio equipment with nonpolarized caps.  I think I can hear the difference.

High capacitance per unit volume is really electrolytics' only virtue.

Mike Halloran
Pembroke Pines, FL, USA

swb1 (Electrical) (OP)
15 Jan 06 23:32
Thanks for all the replies guys! My big newbie question is why is a capacitor needed anyway?? Sorry, for my inexperience :-(

swb1
MikeHalloran (Mechanical)
15 Jan 06 23:59
A capacitor is used to:

- store charge

OR

- pass an AC signal while blocking a DC signal

Mike Halloran
Pembroke Pines, FL, USA

itsmoked (Electrical)
16 Jan 06 4:23
And while storing charge delaying signals. Or shorting high frequencies to kill them.
Warpspeed (Automotive)
16 Jan 06 4:31
And making the magic of tuned resonance possible.
swb1 (Electrical) (OP)
16 Jan 06 14:30
"Storing Charge"?? For example, one side of a 4.7uF cap on a +5V line and the other side to ground. Why have the cap there and not let the +5V go straight into the IC??

Thanks,

swb1
itsmoked (Electrical)
16 Jan 06 14:42
A 4.7uF hooked up as you describe is for three reasons.

1) For logic hold up during shutdown (unlikely in your case)

2) To provide a low resistance,(impedance), supply for brief but large,(relative), current draws demanded by the logic that cannot be discovered and met by regulator quickly enough.  (very likely in your case)

3) To provide the required capacitance needed to prevent the regulator from oscillating fatally for the circuit. (very likely in your case)
swb1 (Electrical) (OP)
16 Jan 06 16:03
Okay..so how does one come up with the value 4.7 uF?? Will any value work??

swb1
MikeHalloran (Mechanical)
16 Jan 06 17:29
Put another way, the +5V 'line' has finite impedance, as does the ground return to the power supply.  The 4.7uF (other values would do) serves as a local supply.

A local supply is needed because logic chips draw large currents during transitions between states, and are sensitive to the supply voltage.  Without these 'bypass caps' on each logic chip, the transient current spikes from one chip would pull down the line voltage (and/or pull up the ground voltage) and interfere with the operation of other chips.

If you build a circuit board without the bypass caps, it will probably mostly work, but it will exhibit strange and logically inexplicable behaviors that you will need a very fast oscilloscope to even capture.

If you look at precision analog circuits or op-amp circuits, you may find 4.7uF caps at the downstream end of low value resistors in the supply, effecting RC low-pass filters.  Same idea, better filtering.

The 4.7uF is not entirely arbitrary.  It's a large value that can be had in a tiny package.  You can compute its cutoff frequency by estimating the resistance of the supply path and counting that as a resistor, again in an RC circuit.

For sensitive circuits where high frequencies are present, you may find several bypass caps of widely different values, parallelled right across the chip, to deal with transients of different speed.  At GHz speeds, 4.7uF caps don't really behave like caps, so you need a few pF to bypass the really fast transients around them.

To not quite answer your last question, "what will work" depends on what you are trying to do, and in what environment you are trying to do it.  But in general, if a  circuit doesn't seem to be working like you think it should, and you didn't bother to bypass it, you should start debugging by doing it.

Mike Halloran
Pembroke Pines, FL, USA

itsmoked (Electrical)
16 Jan 06 21:01
Lets also keep clear that huge values like 4.7uF are generally for "energy storage" whereas bypass caps need to be smaller on the order of LESS THAN 0.47uF

The standard is pretty much 0.1uF

Larger caps have too much inductance to do effective bypassing.
MikeHalloran (Mechanical)
16 Jan 06 23:43
We have sort of run off the end of what I've learned from a couple semesters of circuits, blowing up capacitors, and hanging around sparkys.

swb1, listen to itsmoked.

Mike Halloran
Pembroke Pines, FL, USA

swb1 (Electrical) (OP)
16 Jan 06 23:59
Thanks for your time and patience guys!

swb1
itsmoked (Electrical)
17 Jan 06 0:30
MikeHalloran do okay man!reading
Higgler (Electrical)
25 Jan 06 19:46
Tantalums work if you install them backwards too. At least for a while. Long enough to get thru a circuit card tester (cuz they use low voltage to test at room temp). Then comes temperature testing where they fail. That was at the systems test level, lots of unwiring, unsoldering, and resoldering and rewiring and retesting later, it worked. Ahh, I remember all the failures so vividly.

Two tantalums in parallel is supposed to be a no-no (unless you put a tiny resistance between them) according to Mil Std's. I had one blow up in a production run, that's when we found out that tidbit of knowledge. Never did change that, never heard of anymore blowing up in the field.

kch
felixc (Electrical)
25 Jan 06 22:28
Adding a series resistor to Tantalum caps, what a strange idea.  Aren't tantalums used for their low-esr characteristics? (also for their long-life, but...)  I use many tantalums in parallel on my PC boards.  Never saw any problem like that.  I wonder what could have been the reason behind this Mil-std.
itsmoked (Electrical)
26 Jan 06 2:12
I agree that is odd.  I have used them in P too lots of times.

My bad?
OperaHouse (Electrical)
26 Jan 06 5:13
We had a piece of equipment that had to go in a mine. It had to be modified with resistors placed in series with each of the caps as a current limiter so it was incapable of creating a discharge that would cause an explosion.  Probably that tant was in backwards or at least marked backwards.  I found a diode once in production test with the band printed on the wrong side.  Perfect diode, just marked backwards.  Stuff happens!
zeitghost (Computer)
26 Jan 06 5:22
Tantalum capacitors are notorious for going short circuit.

They really don't like reverse voltage either.

Some surface mount tantalum caps have a fuse built in to remedy this.

I had a tant bead on a pc motherboard short out once... a very unpleasant smell indeed.

I read somewhere that the usual arrangement of a 100nF cap per chip with a 4.7uF electrolytic every now and again is intended to damp the ringing that might occur on the power rails with the high Q of the ceramic caps.

Substituting a tant with a much lower esr won't have the same effect.
Higgler (Electrical)
26 Jan 06 12:20
Regarding tantalums in parallel, I recall some verbage such as they needed a small series resistance ? 1/2 ohm or so cuz at first turn on of voltage (maybe first dc power up) they can cause a problem. I'm certain I remember that, although maybe I remembered the wrong info. Our power supply tantalums definitely blew up during First Article test.

kch
itsmoked (Electrical)
26 Jan 06 14:01
Higgler you must lay off the juice while reading verbage...lol  Stick with verbiage.

OperaHouse; That is a classic "intrinsic safety" technique.
zeitghost (Computer)
26 Jan 06 17:35
Just found a "green" led that lights up orange... looks quite surreal.
OperaHouse (Electrical)
27 Jan 06 9:24
Twenty five years ago that was my first introduction to "intrinsic safety."  After designing IS devices and bringing them through agency approval, I can tell you there was nothing safe about what we did back then!
felixc (Electrical)
27 Jan 06 16:27
I remember reading about some families of tantalums that have restrictions on dIdT and that they do require some inrush current limiting in power supply applications.  In many cases the presence of a fuse or a PTC takes care of it. But I can't relate it to the addition of resistors when you parallel tantalums.
Warpspeed (Automotive)
27 Jan 06 17:16
I have never seen a series resistor with a tantalum, and cannot see the advantage of fitting one.  It all seems rather pointless if you are trying to bypass high frequency noise with a low shunt impedance.

Most power supplies start up reasonably softly, at least slow enough for there not to be an inrush severe enough to be a problem.  I cannot see a one time inrush being destructive either.  Very high frequency high amplitude ripple current may cook a tantalum, mainly because they are so small, but that should become very obvious at the initial prototype testing stage, and a resistor is not going to help.

All very strange.
zeitghost (Computer)
27 Jan 06 17:35
So why do the manufactures go to the trouble of putting fuses in surface mount tants?
itsmoked (Electrical)
27 Jan 06 17:40
Reverse bias => draws huge currents => causes rapid disassembly => technician eye damage => attracts lawyer vermin?

Hence: put in fuse.
zeitghost (Computer)
28 Jan 06 12:53
The other thing about tants is this:

In 1976 I bought an immensely expensive Yamaha CT7000 tuner.

In the audio path it had the then newish tantalum bead capacitors.

It's built like the proverbial tank with a 7 section variable capacitor in the rf section.

Sadly someone installed the tants the wrong way round: after about an hour or so I used to get a deep sort of rumbling under the audio.

Eventually took it apart & changed the tants for something a bit more audio friendly, if a lot larger: a couple of polycarbonate caps.
felixc (Electrical)
28 Jan 06 14:44
But the fused tantalums have a rather poor ESR. Duh! All that remains is that they last longer than electrolytics.

There even were 3-leaded tantalum drops.  To make sure that there was no way to insert them backwards.  I have seen holes bigger than a quarter in a multilayer PCB that had a reversed tantalum.  We've had to install smoke detectors and halon (that was long ago) systems to make sure that the burn-in chambers weren't scrapping whole lots because of a single tantalum.

Tantalums in high-end audio?  Even if connected correctly, their leakage currents may generate hiss depending on where they are in the path.  Nice decision zeitghost.

zeitghost (Computer)
29 Jan 06 10:29
Yes. I was somewhat surprised too.

It cost £450 in 1976... which is knocking on for £4000 today.

I must have been mad.
felixc (Electrical)
29 Jan 06 21:57
I have a vintage Hafler preamp which had tantalums too.  Heresy, but it sounds great!

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close