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Old 23-06-2013, 20:47   #1
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5 Farad caps

I see a lot of these in Asia and I am considering putting one by my distribution board . For suppression and to take out surge ripple. Anybody tried this yet ? They work great in cars for this purpose...
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Old 23-06-2013, 20:53   #2
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Originally Posted by Emmalina View Post
I see a lot of these in Asia and I am considering putting one by my distribution board . For suppression and to take out surge ripple. Anybody tried this yet ? They work great in cars for this purpose...
Why, is surge ripple causing you problems , have you looked at the amount of such ripple. What are the causes and amounts involved.

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Old 23-06-2013, 21:51   #3
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Re: 5 Farad caps

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Originally Posted by Emmalina View Post
I see a lot of these in Asia and I am considering putting one by my distribution board . For suppression and to take out surge ripple. Anybody tried this yet ? They work great in cars for this purpose...
Five Farads! What kind of ripple do they have in Asia?!!
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Old 23-06-2013, 22:14   #4
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Re: 5 Farad caps

They are usefull near high loads such as elect winch, fridge compressors etc where there maybe a high transit pulse. Ensure the voltage rating is above your expected line voltage. (16v for 12v systems) They have their place.

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Old 24-06-2013, 06:44   #5
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Re: 5 Farad caps

For a fraction of the price, a small motorbike battery has over 5,000 farads and puts out a much more constant voltage than a capacitor.
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Old 24-06-2013, 06:48   #6
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Re: 5 Farad caps

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For a fraction of the price, a small motorbike battery has over 5,000 farads and puts out a much more constant voltage than a capacitor.
Interesting idea. So a small battery will absorb voltage spikes like a cap? Why don't your your house batteries do the same thing or would you have to install the battery close to the load to work?
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Old 24-06-2013, 06:50   #7
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Re: 5 Farad caps

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Interesting idea. So a small battery will absorb voltage spikes like a cap? Why don't your your house batteries do the same thing or would you have to install the battery close to the load to work?
They do indeed, which is why I questioned what the OP was trying to do.
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Old 24-06-2013, 06:52   #8
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Re: 5 Farad caps

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They do indeed, which is why I questioned what the OP was trying to do.
Along these lines, reminds me of what an old friend used to say. The best regulated 12V DC power supply is a battery.
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Old 24-06-2013, 08:13   #9
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Re: 5 Farad caps

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For a fraction of the price, a small motorbike battery has over 5,000 farads and puts out a much more constant voltage than a capacitor.
Strange school of Electronics you attended !!!
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Old 24-06-2013, 08:51   #10
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Re: 5 Farad caps

Huh, do the mathematics Steve.

Definition of a farad is a drop in voltage of 1 volt while putting out 1 amp for 1 second.
So assuming the motor bike battery has a capacity of 20 amp hours and drops in voltage under load from 12.8 to 11.8 volts while putting out 1 amp for 20 hours, the faraday capacity is 20hours*3600 = about 75,000 farads. I was off by 70,000 but I did say "OVER 5,000 farads" as a wild guess before being forced to do the math.
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Old 24-06-2013, 09:13   #11
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Was it not a farad that's ate a baby on Fraser Island some years back. I sure don't want one of those aboard.
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Old 24-06-2013, 10:21   #12
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Re: 5 Farad caps

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Huh, do the mathematics Steve.

Definition of a farad is a drop in voltage of 1 volt while putting out 1 amp for 1 second.
So assuming the motor bike battery has a capacity of 20 amp hours and drops in voltage under load from 12.8 to 11.8 volts while putting out 1 amp for 20 hours, the faraday capacity is 20hours*3600 = about 75,000 farads. I was off by 70,000 but I did say "OVER 5,000 farads" as a wild guess before being forced to do the math.
Well your maths got better but are you looking at the discharge characteristics. Why do we use caps on a battery driven circuit at all.
Why does my inverter have a 2 farad cap in it ! As do most of them in the 2kw spectrum.
Why do they bother making them at all ?
They have enormous fast discharge characteristics that’s why.
Cost wise 5 Farad cap about 30USD Small bike battery about the same....
They are excellent means of supplying surge current.
They store energy electrostatically as apposed to batteries which store it chemically.
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Old 24-06-2013, 10:55   #13
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Re: 5 Farad caps

Capacitors have several advantages over batteries. They don't require complex recharge sequences. They are not damaged by over discharge (even all the way to zero). They require no maintenance. They do not have a finite lifetime. Recharge time is very fast. Transportation even by air is not an issue.

One down side is they have higher leakage rates than batteries. But this can be overcome with a very small solar panel.

Using so-called super-caps as a buffer to keep electronics alive is a good use for them. I am sure someone is working on a commercial device that uses a set of these capacitors to feed DC power to on-board electronics so that starting load transients do not cause the electronics to reboot. UPS devices have been doing that for some time.

An earlier post mentioned voltage "spikes" or "surges" in the DC system. This is not usually a problem. What causes modern electronics fits is "sags" where the voltage momentarily drops below 10.5V or some other arbitrary threshold and the electronics then shut down their power supplies no matter how brief the sag. This "feature" was mandated by electronics that got "lost" when the power supplies sagged too low and then remained "lost" after power was restored. The solution seems to be to completely shut down internal power the instant DC voltage sags too low and then remain off until the DC power is restored for a long enough time to be sure it is going to remain on. The result is your chart plotter/radar reboots every time you start the engine or a big surge comes on the inverter. Supercaps could eliminate that problem at low cost in terms of money and time.

Actually, buffering the starter current is another potential brilliant use of these devices. Starters don't "reboot" when the voltage gets to 10.5V like electronics. A relatively small bank of super-caps can start a small diesel with no help from the battery at all. And they recharge very quickly (as in minutes).
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Old 24-06-2013, 12:45   #14
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Re: 5 Farad caps

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Capacitors have several advantages over batteries. They don't require complex recharge sequences. They are not damaged by over discharge (even all the way to zero). They require no maintenance. They do not have a finite lifetime. Recharge time is very fast. Transportation even by air is not an issue.
My understanding is that capacitors do have a finite lifetime, at least some types. For example electrolytic capacitors I believe are rated for about 50,000 hours (a bit over 5 years) and age as the electrolyte dries out.

Ceramic capacitors also experience a loss in capacitance over time due to changes in the crystal structure of the dielectric

Less of an issue with film capacitors but depending on the quality of the encapsulation, they can degrade over time by absorption of moisture in a high humidity environment (maybe like a boat?)
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Old 24-06-2013, 16:56   #15
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Re: 5 Farad caps

Emmalina-
" For suppression and to take out surge ripple.... They work great in cars for this purpose... "

Reminds me of the old joke about a man walking down Park Avenue in NYC always snapping his fingers. One day someone asks him why he's always doing that, and he says "It keeps away the lions" and the man says "But there are no lions in NYC" and he says "You see? IT WORKS!"

Uh, no, massive electrolytics are not used that way in cars, not by the folks who make the cars. Some car audio freaks use them to supply transient power surges for bass response and a few other real issues, but in general? There's no ripple coming out of your alternator unless it is defective. There's no surge unless it is defective. The cap can filter out spikes but there are smaller cheaper spike protection devices that you can get and use.

Yes, capacitors can filter out some things. I used two single-farad caps as filters in a big old analog power supply, and they work beautifully, Which leads to what Skip said: How long they last is a quality control issue. Cheap ones may leak and die in five years, good ones show no problems at 40 years.

Horses for courses and huge electrolytic caps are just the wrong ones for boat electrical panels. Fix your alternator and regulator, use spike protection devices where and as needed, save the big cans for something else. Like a car with too much sound system.
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