OT (as in Out of Touch) -- 1.5 Farad (!) Capacitor???



#7

I feel like Rip Van Winkle. I happened upon a Fry's ad insert from the local paper. It offers, among other things, a 1.5 farad (as in, not microfarad) capacitor with a digital voltage display. (A "digital capacitor" of all things.) I guess they're used to supplement the car's electrical supply when that monster stereo wants to pump a loud bass note through those enormous sub-woofers.

Man! I know it's been a while since my "hardware days" in school, but I've been seriously out of touch! (I'd never seen anything like that at the old Radio Shack.)

Now . . . I wonder: to what creative uses might such a component be put, hmmm?

Edited: 17 Aug 2007, 6:09 p.m.


#8

Phasers on the Enterprise.


#9

May be power one of our LED calc?

#10

I think it may be used in flux capacitor. ;)

#11

Switch polarity and place a 33s on it.

#12

Farad caps have been out for a few years now. One company called CAP-X makes an interesting assortment that can source 20A of current, initially at 4.5V. Pretty interesting stuff-- though they could suck a set of Pioneer batteries (3x LR44) dry in about 20 seconds! GRIN.


#13

In fact, you can now get "ultracaps" that are rated in the hundreds and even thousands of farads, though generally at not more than 2.7V. You can put several in series to raise the voltage, though that also lowers the capacitance. However, if you want to increase the voltage rating by n, you need more than n capacitors in series, because they won't share the voltage equally.

I'm not convinced that the ones being sold for car stereo use are anything other than a clever new way to part a fool from his money.

More than one vendor brags about using hybrid capacitors combining the characteristics of electrolytic capacitors and carbon capacitors. They claim specs like 5 farads and 0.002 ohm ESR. However, IMNSHO that's basically fraudulent, because it's just a low-capacitance low-ESR capacitor in parallel with a series chain of 6 or more high-capacitance high-ESR (e.g., 0.1 ohm) capacitors. When your amplifier tries to draw 80A from it, you're not going to actually get that 0.002 ohm ESR, because the carbon capacitor with the low ESR isn't actually storing much energy. Almost all of it will actually have to come from the electrolytic stack, with its 0.6 ohm ESR.

While 0.6 ohms might not sound like much, at 80A that will drop 4.8V out of your nominal 13V. That sort of voltage drop is exactly what installing the capacitor was supposed to fix.


#14

There are some hybrid switching power supplys that make use of several large capacitors combined with a low current switching PS. They're recommended for ham radio use in CW or SSB mode that need high transient currents but low average currents. The test results I've seen indicate that they are doing their job to deliver 20+ amps peak from a 5 amp continuous PS @ 13.8 volts with no significant voltage drop.

For example:

Here's one that runs from 120VAC.

So I think that they can be engineered to work well, but you're probably right about the stuff they sell to people who's goal is to go deaf while driving.

Edited: 18 Aug 2007, 2:10 a.m.


#15

Quote:
. . . people who's goal is to go deaf while driving.

Say whaaat?

;-)

#16

Yes, using capacitors to reduce voltage sag for high-current transients is a valid concept. What is wrong with the "hybrid capacitors" being sold for car audio is paralleling a high-capacitance high-ESR chain of ultracaps with a low-capacitance low-ESR capacitor, and then claiming that the result is high-capacitance low-ESR. That does NOT work as claimed.

If you really had a 2 farad 0.002 ohm ESR capacitor, it could supply a peak of a hundred amps with less than 1V sag for more than 10ms. The "hybrid capacitor" they're selling won't do that. If you're lucky, you might get a microsecond, which is clearly not long enough to have any beneficial effect on audio frequencies. It's snake oil -- just another way to part a fool from his money.

Edited: 18 Aug 2007, 1:43 p.m.


#17

I was working as a car audio installer back when these came out, and I agree that they are mostly snake oil.

However...

If you want to construct a theoretical model, you have to include the whole power supply chain to the amplifier(s).

Here's a typical chain:

Alternator -> Battery B+ -> Short length of #4-#00AWG -> Fuse or Circuit Breaker -> 10-20 ft. #4-#00AWG -> Distribution Block Input -> Fuse -> 2-4 ft. #10-#8AWG (Sometimes #4) -> Amplifier B+Terminal -> [amplifier power supply, amplifier circuit, speaker(s), etc.] -> Amplifier B- Terminal-> 1-2 ft. #10-#8AWG (or sometimes bigger) -> Vehicle's steel body -> 1 ft. #6-4AWG Ground Strap (unless upgraded) -> Battery B-

As you can see, there are a lot of potential voltage drops. Depending on where you put the capacitor it could make a difference, especially if the car's charging system is operating near its limit.

There are demonstrable gains to be had, but the only people who will really notice are those with very powerful systems (more than 1-2kW) or people competing in maximum SPL events where winning margins might be a dB or less.


#18

I'm not disputing that adding a capacitor will help. Done right, it certainly can.

I'm only asserting that the specific "hybrid capacitor" approach that many are using to claim low ESR is essentially fraudulent. The real ESR of the hybrid capacitor when delivering tens of amps will most likely be more than half an ohm, rather than the 0.002 ohms they claim in their specs.

The digital voltmeter they attach isn't going to tell the customer the real story, either, as it will (by design) respond far too slowly to show the actual voltage sag during high-current transients.

#19

1) Fine tune the HP-45 embedded timer/stopwatch (once upon a time, such devices were referred as "gimmick" capacitors) Alas, the counter will now change once a week instead of once a second...

2) "Zap" HP-25 NiCd batteries, when aging causes shortcircuits on them...

3) Power CMOS RAM chips to offer really-continuous memory, able to withstand a delay of a couple of days when reinserting a battery pack.

4) Offer a script line in which the value is entered as 1.5 E6 uF, to explain scientific and engineering notation in a 1970-style HP calculator manual. It may go as "Friendly engineer Daniel Ectron wants to calculate ..."


#20

You know, I remember seeing ad about such capacitor, but I remember seeing it about a digital Farad capacitor. I forget where, but that not important. Question I have is how is a capacitor digital, as I thought they all analog? I would think as digital, they could harm electrical equipment.


#21

<Serious voice>

Wow. One farad. Digital. I guess it means you can touch it safely with your fingers.

</Runs out of forum>


#22

LOL

Really out of touch :))

#23

They call it digital because of the digital voltmeter they've attached to it.

The voltmeter is intended to make the purchaser think that the capacitor is doing its job, but in actuality DVMs do not respond to fast transients, so it doesn't really prove that the capacitor is making a difference to the car audio system.

#24

Quote:
You know, I remember seeing ad about such capacitor, but I remember seeing it about a digital Farad capacitor. I forget where, but that not important. Question I have is how is a capacitor digital, as I thought they all analog? I would think as digital, they could harm electrical equipment.


Back in tech school (late '70's) an electronics instructor showed us that a linear voltage regulator (a voltage regulator that had a transistor in series with the load) was comparable to having greater than 1 Farad capacitors in a passive voltage regulator.

Hmmm, maybe that has nothing to do with a "Digital" capacitor...

I'm not sure if those kids who "pimp" their automobile audio systems use "Digital" capacitors or not, if they do, it may just be a marketing term.

But 1+ Farad capacitors used as SRAM backup have a slow discharge
rate, unlike conventional electrolytic capacitors, because rapid discharge is not needed for those circuits (even undesirable). Maybe that is what is meant by a digital capacitor- a capacitor designed to be used as a backup for
digital circuitry.

Ren

dona nobis pacem


#25

No, all they mean is that they've attached a Digital Voltmeter to the top of the capacitor. A marketing breakthrough!


#26

Quote:
A marketing breakthrough!

Next up: The microwave hair dryer.

;->


Edited: 23 Aug 2007, 3:24 p.m.

#27

Quote:
But 1+ Farad capacitors used as SRAM backup have a slow discharge rate,
The discharge rate is a product of the capacitance and the resistance of the circuit. Over a given period of time, this combination will discharge a given percentage of the available charge, and voltage does not figure into this calculation.

Quote:
Back in tech school (late '70's) an electronics instructor showed us that a linear voltage regulator (a voltage regulator that had a transistor in series with the load) was comparable to having greater than 1 Farad capacitors in a passive voltage regulator.
Yes. When an active device (transistor) is sensing and negating variations in available voltage, it appears to the circuit loading it as if it were a source with low impedance, such as a large capacitor.

However, a capacitor has an impedance which varies with frequency, so one needs to be specified.

#28

This is not so OT after all. My MLDL2000 for the HP41 uses a 1.0F/5.5V capacitor (also called supercap or goldcap) for keeping SRAM contents for many weeks. It is about the size of 5 stacked 10 Eurocent coins (smaller than a $ quarter??). I have also played with 0.1 F supercaps, which are smaller than an HP16 battery.

Meindert

#29

Well, when I went to school (late "70s) a 1 farad cap was about as big as a horses leg, and any circuit boards/componentry were strapped to it (rather than vice-versa). As for applications...
reducing the ripple voltage on a BIG TR/power supply comes to mind.

Hal :)


#30

Seriously, big caps were used in the early '70s in "homemade" lasers when some researchers were too cheap, uh... , I mean decided not to spend for commerical ones. You'd be crazy to construct your own today unless you weren't serious.

#31

I teach high school physics and recently obtained some 1.0 F capacitors. When I get them out I'm always blown away that they even exist. But my students are like, "Oh, OK, one farad, hmm, so is that the biggest one you have?" They have NO CLUE how radical a change that is from the recent past. I try to explain . . .


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