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capacitors

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capacitors

Unread postAuthor: trigun » Mon Jun 01, 2009 5:54 pm

I am looking to create a large ignition system.
Looking at capacitors i see that many i find are low in voltage but many have varying farad ratings. I looked up farad and cant seem to quite grasp what it means.

So as far as i know a large amount of volts are needed to jump a gap in air not farads.
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Unread postAuthor: dewey-1 » Mon Jun 01, 2009 6:09 pm

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Unread postAuthor: rp181 » Mon Jun 01, 2009 8:57 pm

farad is a mesure of capacitance. in general:
MOAR FARAD = MOAR POWAH!
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Unread postAuthor: c11man » Mon Jun 01, 2009 8:59 pm

yes omore fareds is more power but if you want to have larger spark gap then you need more voltage
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Unread postAuthor: jimmy101 » Mon Jun 01, 2009 9:52 pm

Cap's are pretty simple devices. They have two important (and about a dozen less important) characteristics.

The capacitance, measured in Farads, describes how many excess electrons can be stored on one of the cap's two plates. (The other plate stores "holes", missing electrons.) A Farad is a pretty big unit, most cap's will be rated in microfarad (10^-6) or picofarad (10^-12).

The voltage rating of the cap tells you what the maximum voltage the cap will take before it's internal insulation breaks down. Exceed the voltage rating and you'll ruin the cap.

The energy stored in a cap is given by;
E = (1/2)CV<sup>2</sup>
where E is in Joules, C is the capacatance in Farads and V is the voltage tha cap is charged to.

If you want to store energy in the cap then the voltage rating is more important than is the capacatance.

A typical photoflash cap from a disosable camera is rated at 120 MFD (microfarads) and is charged to about 300V. The fully charged cap contains (0.5)(0.000120F)(300V)<sup>2</sup> = 5.4 Joules

Edit:
Generally, you can't get a spark across an air gap with just a charged cap. The voltage isn't high enough. You need to dump the energy stored in the cap through a step-up transformer, like an automotive ignition coil, to boost the voltage high enough to jump a gap.
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Unread postAuthor: rp181 » Mon Jun 01, 2009 9:58 pm

To add to the above,
capacitor's are used as high energy sources for spark, due to the large peak currents. As you can see from the formula above, doubling the voltage is more energy than double the capacitance. Think of it as a air system, voltage equals pressure, capacitance equals the tank size.
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Unread postAuthor: Technician1002 » Tue Jun 02, 2009 12:56 am

rp181 wrote:farad is a mesure of capacitance. in general:
MOAR FARAD = MOAR POWAH!


Almost.. Stored energy is capacity times voltage SQUARED. Doubling the capacity doubles the energy. Doubling the voltage on the same capacity increases the stored energy 4X.

In general higher voltage = more power. Unfortunately when working with iron core transformers, core saturation limits the peak power, so more power in does not always = more power out. Saturation is a power limitation of ignition transformers. Insulation is a limitation of the peak voltage that can be used. Stay within the voltage and current limits of your ignition transformer to prevent destruction.
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Unread postAuthor: rp181 » Tue Jun 02, 2009 10:43 am

I was assuming a constant voltage rating.

Higher voltage will be more power, but its not practical power. for a HV source through a pulse transformer, stick with 300-800v. With pulse transformer's, saturation is less of a issue. I have a LARGE iron powder core, and I can pulse my entire capacitor bank into it.
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Unread postAuthor: spot » Tue Jun 02, 2009 10:59 am

You said that you couldn't find any high voltage capacitors. If you put capacitors in series, their voltage rating adds up. So 2 100V capacitors in series, have a voltage rating of 200V. 3 have a voltage rating of 300V etc. (BUT the total capacitance is lower than the capacitance of one capacitor. The formula is: 1/Ctotal = (1/C1) + (1/C2) ...) When you put them parallel, their capacitance adds up. (and voltage rating stays the same) For example: 2 100V 1 farad capacitors parallel have a voltage rating of 100V and 2 farad capacitance.
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Unread postAuthor: rp181 » Tue Jun 02, 2009 1:10 pm

series capacitor's are usually discouraged from use, and are almost never used in commercial products. Differences in the capacitor can still cause one capacitor to charge more, possibly still exceeding the voltage rating. Higher voltage capacitor's are usually film or oil.
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Unread postAuthor: trigun » Tue Jun 02, 2009 8:20 pm

thanks guys, jimmy ,rp, c11, and technician. So voltage over farads, farads are measure of how much stores got it.

Thanks for giving me RELIABLE information that makes sense unlike deweys wiki page.
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Unread postAuthor: dewey-1 » Tue Jun 02, 2009 8:53 pm

trigun wrote:thanks guys, jimmy ,rp, c11, and technician. So voltage over farads, farads are measure of how much stores got it.

Thanks for giving me RELIABLE information that makes sense unlike deweys wiki page.


I just gave you a starting reference point. Think of it as preliminary information.

I have no clue as to what if any research was performed by you.

There have been better questions asked that were given the "do some of your own research, we don't spoon feed" standard remarks given by some.

Sorry I tried to give you the "polite" version.
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Unread postAuthor: trigun » Tue Jun 02, 2009 9:16 pm

i dont need to be spoon fed. I just wanted to double check everything before i wasted money, killed myself, blew my house up.
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Unread postAuthor: dewey-1 » Tue Jun 02, 2009 9:46 pm

rp181 wrote:series capacitor's are usually discouraged from use, and are almost never used in commercial products. Differences in the capacitor can still cause one capacitor to charge more, possibly still exceeding the voltage rating. Higher voltage capacitor's are usually film or oil.


I beg to differ on that statement. They are used quite often on commercial products.
Home appliances: Air Conditioners, Refrigerators, even speakers that have cross over networks

From Wiki;
Special capacitors designed for AC operation are available, usually referred to as "non-polarized" or "NP" types. In these, full-thickness oxide layers are formed on both the aluminum foil strips prior to assembly. On the alternate halves of the AC cycles, one or the other of the foil strips acts as a blocking diode, preventing reverse current from damaging the electrolyte of the other one. Essentially, a 10 microfarad AC capacitor behaves like two 20 microfarad DC capacitors in inverse series.
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Unread postAuthor: rp181 » Tue Jun 02, 2009 10:32 pm

I don't have a link, but that all came from the head engineer at EPCOS, when talking to him about a capacitor bank.

Its different using it for those applications and pulse applications. They are discouraged in constant change load, like filter capacitors.

Your thoughts jimmy?
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