Hi all,

I'm trying to store a negative charge on a 2 ft long, 1 inch wide metal plate for a decent amount of time. I have a step up transformer that transforms from 9 volts DC input to 20 volts DC output. My goal is to charge the plate to 20v and have it maintain its charge after I disconnect the battery.

Would I connect the positive wire (on the 20v side) to the metal plate and the negative wire (on the 20v side) to ground? If so, once I disconnect the 9v battery, will the metal plate retain its charge? Or will it immediately lose its charge via the ground connection?

I would like to ideally have the metal plate retain its 20v charge when I disconnect the 9v battery. Of course, there will be some leakage and over time the plate will lose its charge, but hopefully it can retain charge for at least like a few hours using this method. At least that's my goal. Any help is appreciated.

 

Well, it would be a capacitor and maintain a charge. But what do expect to do with it? If you try to measure its voltage with a voltmeter, it will be discharged before it can get a reading.

 

You said you have one small metal plate connected to -20V and the positive of the supply is ground. Then it forms a very small useless capacitance to ground.

 

If the plate is disconnected from the output of the voltage converter, rather than the 9V battery from the input, then the plate should retain it's charge, as dry air is a good dialectric.

The charge on the plate will be dissipated very rapidly by leaving the charge wire connected, or by humid atmosphere, or by dirt on the supports, or by supports that are a poor dialectric.

The charge on the plate will be very very small.

What do you propose to use it for?

 

It will need special care to provide extremely low leakage supports for the plate.
How do you propose to detect whether it is still charged? I can't think of any way of doing this.

 

what are the supports made of, and how tall are they?
The capacitance will be 8.85pF per square metre if the plate is 1m from the ground.
That‘s 0.13pF if your plate is 1m above ground, 1.3pF if it is 100mm from the ground.
The 10MΩ input impedance of a standard multimeter will discharge that in less than 50 microseconds, so how do you propose to measure it?

 

Everyone - Thank you for all the responses and questions!

To give some more context, I'm trying to build a device that negatively charges the plate so that it will attract positively-charged dust particles in the air, kind of like an air filter. My goal is to have it portable where I can charge it, disconnect it, and place it in a room.

It sounds like I may need to increase my voltage a bit to make this work. I can wrap the metal plate in rubber, or some other dielectric, if you think that the air would cause too much leakage too fast.

It sounds like I will need to add in a switch or something on the secondary side of the transformer, between the -20v line and the metal plate, to disconnect the metal plate once it's all charged up (so that it is floating and retains its charge).

It would be great if there was a way to measure the charge on the metal plate, or at least light up an LED to show when the plate is fully charged. I'm not sure how one would do that, though. Any suggestions would be appreciated.

 

Electrostatic air cleaners use 1000s of volts continuously. 20V charged 1 time, isn’t going to do anything.

 

You're digging down into the fundamentals of physics. With 20V or so your not going to be able to measure much, and as it has been noted, the charge will not last long due to leakage, and the act of measuring the charge will discharge it. Even using best practices. i.e. rounded corners, smooth surface, and no burrs. Several thousand volts will even dissipate relatively quickly. Which is probably closer to what you need to attract dust. That said, with 20V or so you can "listen' to the charging effect if you hold your tongue just right. If you have a nearby AM radio tuned to a non-station and manually move a conductor back and forth from the positive terminal to the negative terminal (not to ever touch both terminals at the same time) you can sometimes hear a "POP" on the radio. The conductor would be electrically tied to your plate.

 

https://www.google.com/search?q=electrostatic+charging+limits+leakage+material+shape

https://www.google.com/search?q=triboelectric+charging+limits+leakage+material+shape

 

Let’s count how many electrons there are on your capacitor.

q = CV

C = 0.13pF = 1.3e-13 F

q = 20 x 1.3e-13 = 2.6 e-12 C

charge of electron is

e = 1.6e-19 C

So you have 2.6e-12 / 1.6e-18 = 1.6e7 or

16,000,000 excess electrons.

Which sounds like a lot.

Let’s see how much dust that will take out. I looked it up, and the average house dust particle is 5 microns or:

5e-6 m in diameter.

Its volume is 4/3 pi r^3 or

1.33 x 3.14 x 2.5^3 x 1e-18 or 57e-18 m^3

or 57 e-9 cm^3

So, estimating the density at 1, you can take out

57 nanograms of dust.

That ought to clear the air!

 

Let’s count how many electrons there are on your capacitor.

q = CV

C = 0.13pF = 1.3e-13 F

q = 20 x 1.3e-13 = 2.6 e-12 C

charge of electron is

e = 1.6e-19 C

So you have 2.6e-12 / 1.6e-18 = 1.6e7 or

16,000,000 excess electrons.

Which sounds like a lot.

Let’s see how much dust that will take out. I looked it up, and the average house dust particle is 5 microns or:

5e-6 m in diameter.

Its volume is 4/3 pi r^3 or

1.33 x 3.14 x 2.5^3 x 1e-18 or 57e-18 m^3

or 57 e-9 cm^3

So, estimating the density at 1, you can take out

57 nanograms of dust.

That ought to clear the air!

What if I used 20kV instead?

If I need to I can increase the voltage as much as is required for the system to work. I don't really have any defined voltage limit in the project specifications.

 

Then you can get 57mg of dust, if you could keep it charged, which you could not.

Forget it. When you invent something new, a back of the envelope calculation should be used as a sanity check. Your idea failed miserably.

 

What if I used 20kV instead?

If I need to I can increase the voltage as much as is required for the system to work. I don't really have any defined voltage limit in the project specifications.

20 KV is a painful voltage if it doesn't kill you... If is does kill you, it will hurt like heck first. Better get some experience at much lower voltages and read up on high voltage construction and safety before getting into voltages like that.

 

Then you can get 57mg of dust, if you could keep it charged, which you could not.

Forget it. When you invent something new, a back of the envelope calculation should be used as a sanity check. Your idea failed miserably.

Thanks for your help. Turns out 57mg would be sufficient for another project. Is there a way to determine how long I could keep the plate charged for if I wrapped it in neoprene rubber or some other insulator? If I could get like 2 hours of life, that would work for this other project.

 

Look up electrostatic air cleaners and learn how they work. Your idea is not practical. All of us have said the same.

 

Look up electrostatic air cleaners and learn how they work. Your idea is not practical. All of us have said the same.

As mentioned above, it would be practical for a different project that's not an air filter. Let's not get too hung up on the practicality of it for an air filter. That was just one idea of how the tech could be applied. Any ideas on how to retain the charge for a little while, say an hour? Is that even possible?

 

Ah, so now we find that you are hiding the real reason you want this. Why? We can’t help you if you won’t tell us what you are trying to do.

 

Ah, so now we find that you are hiding the real reason you want this. Why? We can’t help you if you won’t tell us what you are trying to do.

No I just try to work on technologies that have multiple applications. A lot of people have a specific problem in mind and try to solve that problem. I find that gives people tunnel vision. I prefer to perfect a certain technology and then find a suitable use case for it. It's an approach that has worked well for me in the past. That's why the parameters are so adjustable. I'm not designing to a specification or particular goal. I'm trying to just get it to *work.* I'll figure out how to apply it later.

Here's an example: You're probably right that using this tech for an air purifier for a room in a home isn't going to work. But that doesn't mean it's useless. In certain chemistry settings, like very small chambers where certain chemical reactions take place, it could be really beneficial to purify the air of 50mg of dust. So I don't like to just give up if something won't work for my original application. I find a new application and keep going. There are probably dozens of other use cases we haven't thought of yet.

 

You are feee to waste your own time on this, but next time you request help, let us know that you have no use for the idea, so we don’t waste our time.