Collectors types are either spring or gravity based. Spring based systems usually have the spring element within a basic brass strip collector. This is probably the easiest method for scratch conversion. Due to upward forces created by the spring the chassis needs to have sufficient weight to ensure the wheels maintain contact with the track.
Spring forces can be avoided by using a gravity collector. Here the collectors falls onto the power rail. A centre piston system as shown on one below or a mechanical lever have been used on commercial systems. Not so easy for the scratch converter, but has less problems in operation.
1 Conversion of Hornby power bogie. The weight of the bogie means a simple brass strip acting as collector works well. Since I took this picture I had to use this bogie in a model I was selling on eBay. Currently the bogie I have the same is not yet working.