Polarized vs. Non-polarized caps
Polarized vs. Non-polarized caps
(OP)
Hi,
When do I know when to use a polarized capacitor vs. a non-polarized capacitor?? Is there a rule of thumb??
Thanks,
swb1
When do I know when to use a polarized capacitor vs. a non-polarized capacitor?? Is there a rule of thumb??
Thanks,
swb1





RE: Polarized vs. Non-polarized caps
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
RE: Polarized vs. Non-polarized caps
Thanks!,
swb1
RE: Polarized vs. Non-polarized caps
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.
RE: Polarized vs. Non-polarized caps
RE: Polarized vs. Non-polarized caps
High capacitance per unit volume is really electrolytics' only virtue.
Mike Halloran
Pembroke Pines, FL, USA
RE: Polarized vs. Non-polarized caps
swb1
RE: Polarized vs. Non-polarized caps
- store charge
OR
- pass an AC signal while blocking a DC signal
Mike Halloran
Pembroke Pines, FL, USA
RE: Polarized vs. Non-polarized caps
RE: Polarized vs. Non-polarized caps
RE: Polarized vs. Non-polarized caps
Thanks,
swb1
RE: Polarized vs. Non-polarized caps
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)
RE: Polarized vs. Non-polarized caps
swb1
RE: Polarized vs. Non-polarized caps
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
RE: Polarized vs. Non-polarized caps
The standard is pretty much 0.1uF
Larger caps have too much inductance to do effective bypassing.
RE: Polarized vs. Non-polarized caps
swb1, listen to itsmoked.
Mike Halloran
Pembroke Pines, FL, USA
RE: Polarized vs. Non-polarized caps
swb1
RE: Polarized vs. Non-polarized caps
RE: Polarized vs. Non-polarized caps
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
RE: Polarized vs. Non-polarized caps
RE: Polarized vs. Non-polarized caps
My bad?
RE: Polarized vs. Non-polarized caps
RE: Polarized vs. Non-polarized caps
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.
RE: Polarized vs. Non-polarized caps
kch
RE: Polarized vs. Non-polarized caps
OperaHouse; That is a classic "intrinsic safety" technique.
RE: Polarized vs. Non-polarized caps
RE: Polarized vs. Non-polarized caps
RE: Polarized vs. Non-polarized caps
RE: Polarized vs. Non-polarized caps
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.
RE: Polarized vs. Non-polarized caps
RE: Polarized vs. Non-polarized caps
Hence: put in fuse.
RE: Polarized vs. Non-polarized caps
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.
RE: Polarized vs. Non-polarized caps
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.
RE: Polarized vs. Non-polarized caps
It cost £450 in 1976... which is knocking on for £4000 today.
I must have been mad.
RE: Polarized vs. Non-polarized caps