I am a mechanical engineer and going to design a electrical fluorescent dimmer. The problems what I need to know are
1) how many terminals needed? I know 3 terminals for dimmer power input (L, N, E) but don't know how many terminals for ballast (the ballast is still required for this dimmer) and fluorescent light.
2) What is "Dry contact"? The output of this dimmer required to be Dry contact. I don't understand this.

Can someone help on this?
Thanks very much.

1) This depends entirely on your fixture. Is it 115V? 208V? 347V? (Yes, for my American friends, 347V is used here in Canada). The voltage, and # of phases in particular, will determine how many 'terminals' are needed.

2) A dry contact is a device that is controlled by either a relay, or electronically, to open or close a circuit based on logical algorithms. A typical application is to control high voltage circuits via low voltage switches/circuits. Another is in PLC control panels.

I'm interested in how you intend on making a dimmer for a fluorescent fixture. I'm not a lighting expert, but I'm under the impression that fluorescents (much like MH or HPS) require a certain minimum voltage to maintain the ionization process which produces light. Ballasts are devices which are there for the sole purpose of maintaining the proper voltage to the fixture. So when you say you're going to have a device (that still requires a ballast) to will act as a dimmer (which typically involves altering the voltage waveform getting through to the fixture) I'm slightly intrigued.

I am a mechanical engineer and going to design a electrical fluorescent dimmer. The problems what I need to know are
1) how many terminals needed? I know 3 terminals for dimmer power input (L, N, E) but don't know how many terminals for ballast (the ballast is still required for this dimmer) and fluorescent light.
2) What is "Dry contact"? The output of this dimmer required to be Dry contact. I don't understand this.

Can someone help on this?
Thanks very much. regular ballast is not dimable, check this out at Lutron

They make dimmable CFL's that work with regular 2 contact dimmers. Might reverse engineer and scale up one of them.

We have used 3 wire dimming ballast for years, the dimmer also has 3 wires, the 3rd wire was brown, Later a company produced "Mark" or "Mark 5" that had 2 wire dimming ballast, these were both for 8' flourescen fixtures, and others.
Currently dimming the CFL's is done.
You won't be solving a problem, but you may come up with a different way to dim.
I would give good thought how you would do it, then research methods and techniques used, then design. You may come up with more efficient way. Good Luck