Low Cost Parallel Plate Capacitor

[roms05]

Hello! I'm hoping someone can help me with my problem. I'm currently working on a low cost parallel plate capacitor. I used a galvanized iron as the plate material since this is a low cost material. The problem is when I measure the voltage across the capacitor, the voltage read peaks at a certain reading and then continuously decreases. I know the voltage is dependent on the capacitance but i still don't know why this decrease in volts happens. please help. :)

 

[NascentOxygen]

http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

What insulation material are you using? What source are you charging this from? What are you using to monitor the voltage? What area of parallel plates do you have?

The voltmeter continuously takes current from the plates, so the capacitor voltage will inevitably be seen to slowly fall.

 

[roms05]

Oh. I forgot about the details. Sorry about that.
I used an insulating varnish on the plates. I'm using a constant 5V DC source. I used a digital multimeter to measure the voltage. And the area is 1m x 1inch so it's like 1x0.0254 m squared.

 

[NascentOxygen]

So, it charges to 5V, then after you disconnect the source the voltmeter reading steadily falls back towards 0V?

That is not much capacitance. You should dismantle a cylindrical polyester capacitor to see what area and close spacing commercial products use.

 

[roms05]

Actually, that's what puzzles me. The 5V DC source is constant and i don't unplug it. But still, the voltage still drops.

 

[roms05]

And hey, by the way, thanks for replying. :)

 

[NascentOxygen]

Have you connected the DVM in series with the capacitor, or in parallel??

 

[roms05]

In parallel.

 

[NascentOxygen]

If the supply stays fixed at 5V, and you say the voltmeter reading drops, then it sounds like you have the meter connecting the capacitor to the supply, and not in parallel.

 

[roms05]

I swear I'm reading the voltage in parallel. :(

 

[NascentOxygen]

Take a photo of the layout. Reduce it to about 50kB, attach it here.

 

[roms05]

I don't have a pic of the setup with me. But I promise I know if it's connected in series or parallel. And i measure the voltage across the capacitor in parallel. And just to add, I'm using water as the dielectric material between the plates. So there's a portion of the capacitor that has water as dielectric material and the remaining with air as its dielectric material.

 

[NascentOxygen]

Why don't you try it without the water?

In any case, its capacitance is too small to function as a capacitor here. There is something else going on. What does the DVM reading do after you disconnect the 5V supply?

 

[roms05]

When I tried it without water, the voltage across the capacitor is 5V and constant.
With water, if I disconnect it from the source, the decrease in voltage just goes faster. With constant 5V source, the decrease in voltage happens but slower.
Do you think the small capacitance is a factor why this happens?

 

[davenn]

When I tried it without water, the voltage across the capacitor is 5V and constant.
With water, if I disconnect it from the source, the decrease in voltage just goes faster. With constant 5V source, the decrease in voltage happens but slower.
Do you think the small capacitance is a factor why this happens?

you do understand water is a conductor don't you ?
its likely its just shorting out the capacitor

Dave

 

[roms05]

you do understand water is a conductor don't you ?
its likely its just shorting out the capacitor

Dave

I realized that. That's why I used an insulating varnish on the plates.

 

[davenn]

OK but you shouldn't be using water anywhere near them
there may be gaps in your varnish layers

as NascentOxygen says in a post a way back ... this is a really small capacitance you are dealing with
only a few pF ( picoFarad) lots of things in you setup will have a significant effect on measurements etc

we all REALLY need pics to see what you are doing ...
else its just a guessing game, which no one likes to play

Dave

 

[roms05]

OK but you should be using water anywhere near them
there may be gaps in your varnish layers

as NascentOxygen says in a post a way back ... this is a really small capacitance you are dealing with
only a few pF ( picoFarad) lots of things in you setup will have a significant effect on measurements etc

we all REALLY need pics to see what you are doing ...
else its just a guessing game, which no one likes to play

Dave

I understand. And I'm really sorry for not providing any images of the setup. Also, I am very grateful for your responses. :)
When I get the chance to take a pic of the setup, i will post it here. Probably next week when classes are back from the lent holidays.