# How does a Primary Charge Roller work?

I have been recently trying to get more deeply into the workings of a common laser printer.
While the basic concept seems to be fairly simple to understand I have been having trouble finding more information on the physics behind it.

In this case for example the Primary Charge Roller.
I'm aware of what it does but I haven't found any information on the how.

How exactly does it create the negative charge?
Is the simple contact of the roller and the other medium enough to transfer it evenly?
What are the restrictions regarding the target material, could you transfer a charge with the same method to glass, wood, metal?

I hope someone here can help me out with some information or point me into the right direction.

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Why don't You give a link to where You read about this? Hopefully including some picture. There were so many variants/improvements over time to xerography! – Georg Oct 3 '11 at 10:21

I guess this is mainly an engineering problem - the basic physics of charging a roller is to just apply a voltage to it, and these charges will jump over to another (uncharged) object that comes in contact with it (the drum, in the laserprinter).

The engineering problem is to get a perfectly even charge, to make that charge transfer to the photoconductive drum (the image forming element where the laser "writes" the image), and then to erase or even out any residual charge before the next turn of the roller.

I think your question about if it's possible to do this to other materials (wood etc) is not specific enough - of course you can apply a charge to other materials, but the laser printer is a delicate piece of engineering designed to solve a very specific problem - how to get a toner onto a paper in a certain pattern - and the specific engineering solutions there don't readily transfer to for example charging a highly irregular surface like a piece of wood, at least not given the same requirements (thousands of dots per inch of perfectly homogenous intensity).

The following image is from the howstuffworks.com article about laser printing, which outlines the basics of the printer:

The original laser printer used a high-current wire (corona wire) above the surface of the image drum, this caused a very even charge on the drum at the expense of a high-power voltage supply and heavy ozone production. Then a charged roller started to be used, which is basically a roller with a rubbery surface and enough conductive properties to be charged easily and evenly by the applied voltage.

At this point, all the tricks come in - I'm not an expert in this by far, but one of the obstacles to be overcome seemed to be that the surface would not charge evenly with an applied DC voltage due to the electrical properties of the required rubber surface. Apparently one solution is to add an AC voltage on top of the DC voltage, I guess this "shakes" the charges into a more even distribution.

I found this patent from 1996 which describes a pure DC alternative by using a special rubber coating, but it does provide some useful background information of the problems and technology of the charge roller itself if that might lead you in the right way.

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Bjorn, thank you very much for your reply! I will look further into the patent and hopefully find more on the specifics of PCRs. Thanks again! – Tom11 Oct 4 '11 at 14:28
Good that it helped you :) Please consider marking it as answer if you feel it answered your question as well! – BjornW Oct 5 '11 at 8:59