Trouble with homemade capacitor

Thread Starter

ElecN00b

Joined Aug 3, 2019
5
Hey guys, new member here. I've been trying to put together a homemade capacitor and I'm just having no luck whatsoever, and I have no idea why. I've followed the same instructions I found online at various websites, but when I check the voltage across the capacitor after disconnecting the 9v battery, it shows no voltage.

I've used heavy duty aluminum foil for thr plates. I've tried using paper, a freezer bag and wax paper for the dielectric with no avail. Can someone give me an idea of what I'm doing wrong?
 

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crutschow

Joined Mar 14, 2008
25,264
when I check the voltage across the capacitor after disconnecting the 9v battery, it shows no voltage.
How did you measure the voltage?
The normal input resistance of a voltmeter will likely so rapidly discharge the very small capacitance of your capacitor, that it would appear it stored no charge.
Do you have an oscilloscope you can use to monitor the voltage?
 

jpanhalt

Joined Jan 18, 2008
10,090
1) Are your expert sources YouTube or Instructables? How about giving us a clue as to the genius with which we are competing? That is, give links.

2) Too many years age to remember, I used aluminum foil in a glass jar for my Tesla coil capacitor. Why do you need a HV capacitor?

3) How does it fail?
 

paulktreg

Joined Jun 2, 2008
786
If you are using a DVM then the input resistance is likely 1Mohm or so. (It's specification will tell you the input resistance) . Try a 10Mohm resistor in series with your DVM and see what voltage it reads and calculate the actual voltage.
 

AlbertHall

Joined Jun 4, 2014
10,236
If you have a means of testing capacitance measure your construction.
Let's assume your voltmeter is 10MΩ and that your capacitor is 1000pF (it's unlikely to be this high). The time constant would be 10^7 * 10^-9 seconds. That is one hundredth of a second and too fast for your meter to read.
 

Thread Starter

ElecN00b

Joined Aug 3, 2019
5
1) Are your expert sources YouTube or Instructables? How about giving us a clue as to the genius with which we are competing? That is, give links.
I never claimed them to be "expert" sources.
https://www.instructables.com/id/Aluminum-Foil-Plate-Capacitor/
http://sciencewithkids.com/Experiments/Energy-Electricity-Experiments/how-to-make-a-capacitor.html

2) Too many years age to remember, I used aluminum foil in a glass jar for my Tesla coil capacitor. Why do you need a HV capacitor?
Is 9V considered an HV capacitor? I don't "need" one, I'm just playing around getting familiar with electronics, which is why i'm making this.

3) How does it fail?
It fails in the sense that I couldn't measure a voltage across the capacitor after disconnecting the power source.
 

oz93666

Joined Sep 7, 2010
737
From your picture the two foils do not appear pressed together , there is air space resulting in a mm gap which will give a very low capacitance . It's all about getting the two foils close together , thickness of foil doesn't really matter ...

I would think the best way is to get two rolls of kitchen foil and two rolls of cling film ...wind them together to make a sandwich roll ...vacuum packing the roll will help get the air out and gap tight ...
 

Thread Starter

ElecN00b

Joined Aug 3, 2019
5
From your picture the two foils do not appear pressed together , there is air space resulting in a mm gap which will give a very low capacitance . It's all about getting the two foils close together , thickness of foil doesn't really matter ...

I would think the best way is to get two rolls of kitchen foil and two rolls of cling film ...wind them together to make a sandwich roll ...vacuum packing the roll will help get the air out and gap tight ...
My standard method of charging the capacitors was to weigh it down with books and my weight to remove those gaps. I also ran it around a pen into a circular configuration.
 

MisterBill2

Joined Jan 23, 2018
6,727
Producing a capacitor using aluminum foil and cling wrap would be interesting if it could be done without adding wrinkles,or air spaces. Producing a short circuit would defeat the whole thing. So it is a bit more complicated than that simple description. And having an air gap in the construction is functionally putting two capaitors in series, reducing the cpacitance a lot!
One more thing, 9 volts is a LOW voltage as capacitors go. "High " voltages on a capacitor start around 400 volts and head up from there. Those sorts of voltages are dangerous, by the way, and can easily be lethal. Thus there is a whole additional set of precautions involved with anything above low voltages. ( That caution is not to scare or discourage you, but rather to alert you)
 

Thread Starter

ElecN00b

Joined Aug 3, 2019
5
Thanks for the warning. Right now I'm trying to build a dissectable capacitor for the fun of it, so I'm not planning on going to HV anytime soon.

Producing a capacitor using aluminum foil and cling wrap would be interesting if it could be done without adding wrinkles,or air spaces. Producing a short circuit would defeat the whole thing. So it is a bit more complicated than that simple description. And having an air gap in the construction is functionally putting two capaitors in series, reducing the cpacitance a lot!
One more thing, 9 volts is a LOW voltage as capacitors go. "High " voltages on a capacitor start around 400 volts and head up from there. Those sorts of voltages are dangerous, by the way, and can easily be lethal. Thus there is a whole additional set of precautions involved with anything above low voltages. ( That caution is not to scare or discourage you, but rather to alert you)
 
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