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I require a bit of assistance please.

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The_Happy_Duck

Thu Jul 26 2012, 09:09PM

Registered Member #4089 Joined: Fri Sept 09 2011, 04:40PM
Location:
Posts: 19

Hello,
For a while now I have wanted to get into HV and I need some help with capacitors, I know this probably belongs in the â€˜High Voltageâ€™ section but I do not want to spam it with my newbie knowledge.
I have been playing with capacitors for a little bit, disposable cameras and what not, and the only equation I know is to calculate the energy of a cap. I have a few questions I would like answered to clear a few things up.

1. Are there any equations to calculate the current and wattage of a capacitor? Using the basic P=IV equation would require you to know one or the other I assume unless you can use V=IR.

2. I am looking around at different models of capacitors including Ultra Capacitors and Electrolytic Capacitors and calculating the energy stored. Ultra Capacitors have a much (much) higher energy rating but I assume they discharge with lessâ€¦ power? Do capacitors with a much higher voltage than capacitance discharge faster and with more power?

3. How would I calculate how much energy is being outputted in a period of time? I would assume the faster it discharges the more â€˜damageâ€™ it can do. I assume this would be related to power, E/T and all that.

4. If you were to use a capacitor in an electromagnet or something what sort of capacitor would give the biggest pulse and therefore biggest pull instantaneously? Would one with a higher capacitance which lasted a bit longer than one with a high voltage attract metal at a further distance?

Thank you to anyone who can help clear things up for me, I really appreciate it.

Conundrum

Fri Jul 27 2012, 06:39AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Internal resistance of the capacitor can be described as ESR (Equivalent Series Resistance)
This can be measured by putting the capacitor in series (discharged first!!) with a known resistor, and applying a frequency of known amplitude to the system.

By using the well known voltage divider equation (Vout=Vin*(Rb/(Rt+RB) you can calculate the ESR.
For example, say your input is 10V p-p, and the output is 5V p-p then your capacitor has the same resistance at frequency N as the test resistance.

The general equation for power transfer states that power transferred to a load is maximised when the input and output resistances are equal (MPTT)

My own experience with capacitor/electromagnet systems suggests that up to a point increasing capacitance and/or voltage will yield more "pull" but eventually the core material saturates if you are using bundles of ferrite rods to focus the magnetic flux.

With air cored coils this is a non issue, the failure point is normally the interconnects or the coil windings arcing over internally.
Some people have managed to push peak currents of several kA for a very short time on hand wound coils.
The useful tip here is that the thicker the wire the better, and the more turns in a given space the better.

To get a larger peak current you can add a series non polar capacitor which critically damps the pulse so that all the energy is dumped into the coil in the first half cycle.
This will have the effect of delaying the start of the pulse however.
-A

The_Happy_Duck

Fri Jul 27 2012, 12:10PM

Registered Member #4089 Joined: Fri Sept 09 2011, 04:40PM
Location:
Posts: 19

Thank you for the help!
But can we apply some of this to an example so I can grasp it better?

Say you had a capacitor with 2500V and 1500uf (an example someone sent me)
The energy equation states that this contains 4687.5J of energy, correct?
What can I work out from this? Is it possible to work out the speed of discharge? I do not own a capacitor so I cannot check, but is there a standard resistance/current?

Another thing I was wondering; say if you had 2 of the capacitors above (2500v, 1500uf), if you put them in series would you get 5000v & 750uf (9375J)? If so what would be the major difference between this and one capacitor of 2500v & 3000uf (considering they have the same energy), would the one with the higher voltage discharge faster but with the same energy?
Thank you again!

Steve Conner

Fri Jul 27 2012, 12:33PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Try Barry's Coilgun Simulator.

Let's consider a bunch of capacitors that all store 1kJ. As you figured out, this can be achieved with a high capacitance and low voltage, or a low capacitance and high voltage. Now broadly speaking, the low capacitance, high voltage unit will discharge quicker and blow things up better.

When using capacitors with coils, the situation is less clear. You can design the coil to suit the capacitor and get whatever peak current and pulse length you want.

Within limits though: high capacitance, low voltage electrolytics are limited by their ESR, ultracapacitors even more so. Neither can discharge as quickly as you may want.

The_Happy_Duck

Sat Jul 28 2012, 04:59PM

Registered Member #4089 Joined: Fri Sept 09 2011, 04:40PM
Location:
Posts: 19

Steve Conner wrote ...

Try Barry's Coilgun Simulator.

Let's consider a bunch of capacitors that all store 1kJ. As you figured out, this can be achieved with a high capacitance and low voltage, or a low capacitance and high voltage. Now broadly speaking, the low capacitance, high voltage unit will discharge quicker and blow things up better.

When using capacitors with coils, the situation is less clear. You can design the coil to suit the capacitor and get whatever peak current and pulse length you want.

Within limits though: high capacitance, low voltage electrolytics are limited by their ESR, ultracapacitors even more so. Neither can discharge as quickly as you may want.

Hi, thank you for replying.
I took a look at the coil simulator; it was very helpful in showing the relationships between Capacitance and Voltage.
One question I had was what does the frequency represent? Is that for an AC capacitor which I read about, because the ones I have seen are DC?

Anyway, I was wondering what sorts of projects and applications capacitors could be used in?
I have been reading about HURF's and Lasers and was wondering if they could be used to produce a big pulse thus creating a very powerful instantaneous beam? Also in regards to a electromagnet would a capacitor create a considerable range of attraction in comparison to one running off a battery? If so could you work out the radius using the inverse square law (I am guessing you cannot direct it)?

Thanks again!

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