Elecrtostatic recording - capacitor related question

[mr_monster]

I want to be able to record a signal on a plate that is made up of a metal disc coated with an isolated oxide. The metal is grounded. A transistor with high voltage and current delivery capability is connected to a brush with discharges a small charge on the disc while it is moving. Another device with very high input impedance reads the signal from the disc after some distance (with some time delay). I have managed to make a prototype that works, however pretty badly. I have noticed that as I increase the output voltage the signal I get is starting to be louder and less distorted. The disc seems like a small 2 - 5pF capacitor at any given moment according to my LCR meter. Is the high voltage needed because the transistor has a hard time driving enough current to charge up the disc to a usable point? Can I do anything to lower the required voltage for this?

Thanks!

 

[betwixt]

This sounds very much like the technology used in copy machines. A charge is put on a conductive drum coated in selenium or similar oxide and removed by contact with paper. Perhaps investigating the methods used by these machines would give some clues on how to improve it. The underlying capacitive charge to store information (toner in the case of a copier) methods are very similar, a copier spreads the charge equally across the drum rather than at a point and discharges the unwanted areas but I can't see any reason why applying it selectively shouldn't work just as well.

I would guess the high voltage is needed to store sufficient charge on the small capacitance to make it readable afterwards. The only reason you might need high current it to maintain a high slew rate on the signal.

Brian.

 

[mr_monster]

What type of copy machines you refer to? As far as I know they had some special coating on the drum which caused a charge to build up when the surface was lit or something along that lines.

I also think the high voltage needed is because of the lack of current driving capability of my circuit. I will need to tweak it so it would be able to push current into a small cap. I have found some info on that, however seems tricky not to mess up the recorded data

 

[betwixt]

I was referring to standard office paper copy machines although very similar technology is used in laser printers too. They use high voltage to electrostatically transfer the toner to the drum, the light is used to discharge it where necessary by making the drum coating conductive. It occurs to me that if you are trying to store analog information in the same way, rather than modulating the high voltage to apply the charge, it would be easier to charge the whole surface and selectively discharge it.

I'm not sure what you are trying to achieve with this project, can you explain more please.

Brian.

 

[mr_monster]

OK, I see...

The idea of the project is to record an audio signal onto a disc media, just like you said. The thing works OK with high voltage (50 - 100V) but I would like to lower that voltage, between 50 - 75%. The direction I was looking at is a current source which is modulated by the voltage magnitude of the audio signal. That will allow linear charging and might dodge all kind of problems related to circuits that have a time constant.

Regarding the idea of charging the surface. How would you discharge in correlation with the audio signal? And how would you read the data?

 

[betwixt]

I'm not sure a current source is the right thing to produce an electrostatic charge, a higher voltage would probably work better. I presume the charge is stored between the disc surface and an underlying conductive layer so the type and consistency of the oxide would be critical and would decide the best way to add/remove charge. You probably also need a very fine needle point to concentrate the charge in a small area and minimize dispersal into the surrounding oxide.

Back in the days of reel-to-reel tape recordings there was a product that used electrostatic charges to store audio on conventional tape. It wasn't well received in the market, probably because of compatibility issues with other machines. Instead of using a magnetic field concentrated in a narrow gap (the normal approach) it had two closely spaced electrodes which just touched the tape surface. From the era it was used, the prevalent technology used vacuum tubes so it was probably easier to interface to high voltage than the high current needed by magnetic heads. The 'recording bias' was a fixed DC voltage making it easy to connect to tube amplifiers.

Brian.