Find Best Metal for DIY Capacitor Experiments

[alxm]

When you answer that I'm sure you'll figure out why two dielectrics in series won't help.

I didn't say it would. Putting a film of plastic there will most definitely hurt the dielectricity and capacitance.

You're totally ignoring my point, which is that water conducts electricity. A capacitor with a lower dielectric is better than a capacitor with a high dielectric constant which is leaking current - because the latter isn't going to act like a capacitor to begin with!

 

[uart]

anyone?

Sorry you didn't collect enough useful data to make any sort of experimental estimate (other than zero) for the capacitance.

Even if you repeat the experiment it is unlikely that you'll manage to get a discharge time constant much more than 100ms (your ohm meter will probably have a 10M input resistance and you're unlikely to get much more than 10nF with the present set up). Now 100ms is going to be too quick to get any meaningful reading from a DVM, the meter time constant may even be longer. So you need to either build a much larger capacitor or give up on that approach.

Alternative approaches would be :

1. Get an ocsilloscope and do it properly.

2. Get an AC plug-pack (some people call them "wall warts") and measure the AC current through the capacitor. I'd try to get a higher voltage unit, like 21VAC or 24VAC (not so common but some devices use them) or at least the more common 15 VAC pack.

Procede as follows

- Set up capacitor with 1k series resistance (to prevent accidental short of pack)

- Connect a DC supply (12 to 24 volts, whatever you have at hand) and attempt to measure the DC current. It should be approximately zero. This part is the control.

- Replace the DC supply with the AC plugpack and re-measure the current (making sure to switch your ammeter range over from DC mA to AC mA).

You'll need a meter that can measure down to 10's of uA AC (a 20mA AC range with a 3 digit display might just do it do) or better yet you could use the 1k resistor as a shunt (and measure it's voltage) if your meter has a 200mV-AC volt range.

The value of the capacitance can be estimated from the measured current as follows :

$$C = \frac{I}{120 \pi V}$$ *

* Assuming freq=60Hz - use 100 pi if your mains is 50Hz.

 

[jalebi]

Why would you use plastic? Plastic is a lousy dielectric. (the cited number of around 6 sounds right. Water is 80). The problem with water is its conductivity - you don't want the capacitor discharging across your dielectric medium.

Perhaps a combination of water, and a thin sheet of plastic to stop any currents?

I used plastic because i was investigating different dielectrics. I was not designing an optimized capacitor - just carrying out an experiment to learn something.

thanks for all the help uart.

i don't mean to sound rude or unappreciative, but neither of you has answered my question on the materials of the dielectrics. do you guys know what the specific names of those materials are (see about two posts of mine ago)?

 

[alxm]

i don't mean to sound rude or unappreciative, but neither of you has answered my question on the materials of the dielectrics. do you guys know what the specific names of those materials are (see about two posts of mine ago)?

Wikipedia has a pretty comprehensive list:
http://en.wikipedia.org/wiki/Types_of_capacitor#Types_of_dielectric

 

[Dadface]

Hello jalebi.I'm not 100 percent sure but I think the projector transparency is cellulose acetate and cling film is low density polythene.

 

[gary350]

I have built several high voltage capacitors. You did not say what voltage or UF rating you want? I use aluminum flashing available at any lumber yard, hardware store in the roofing section. I buy aluminum 10" wide rolls 20 ft long. I also buy polyethylene clear plastic the thickess I can get is usually 6 mil thick. For high voltage I want the polyethylene to be over lap the edges of the aluminum by 1" all the way around. If I make flat plate caps of rolled caps I still want 1" of over lap to keep sparks from jumping around the plastic from 1 alumimum plate to the other. After cutting the aluminum I file and sand the edges very smooth and cut off the shape corners. Round off the corners real nice and smooth. As for the physical size the rolled caps are always 1/2 the size of the flat plate caps. The flat plate caps have twice the current capablity of the rolled caps. The finished cap is filled with High Voltage oil, or caster oil, or mineral oil, or BBQ grill charcoal lighter fluid, or liquid lamp oil.

 

[jalebi]

Hello jalebi.I'm not 100 percent sure but I think the projector transparency is cellulose acetate and cling film is low density polythene.

thanks again Dadface. you've been too much of help. thanks to the rest of you guys as well - what a helpful forum this is