And Smaller We Go...

A couple of posts back I talked about converting the KATO 11-107 to a tiny 4 wheel chassis in the hope it would be small enough to fit a model I'm working on. Turns out that although it's small it's not small enough, not by quite a long way, so I've had to have another look at power options. The result is that I'm trying another well made KATO power unit but this time taken from one of their Centram models.

Not only is the tram model tiny (it's N gauge after all so a scale of 2mm to the foot) but it's been modelled so as to leave the inside empty so you can add your own passengers. This means that all the drive components are hidden under the floor. In fact the model contains two completely independent power bogies which when removed look like this.

Each wheel is just 4mm in diameter which gives you some idea of just how small and compact these are. The circuit boards drive the lights as well as reducing the track voltage to protect the tiny motor, and can be replaced by a simple resistor if you want to make things even more compact.

The problem though, is that when removed from the model there is no connection between the pickups and the circuit board and motor meaning they don't work. The tops of the pickups are the rounded brass pieces sticking up on either side between the wheels. When assembled into the Centram these rub on phosphor bronze strips which connect the pickups to the circuit board which in turn powers the motor.

Now most people solve this problem by soldering wires to the pickups, but you have to be fast. Not only are the pickups touching the plastic casing, but inside the chassis a set of plastic gears run alongside each pickup and they are easily melted if you get the pickups too hot. While you might get two chassis from one model, the model still costs between £80 and £90 (you have to shop around but this is a common price; I got mine cheaper by buying a second hand "as new" model) and so you don't want to destroy a £45 chassis by lingering too long with the soldering iron.

My solution was to design a small clip (which you saw in the earlier post balanced on a 5p coin) which could retain two pieces of phosphor bronze rod which would in turn rest upon the pickups.

It's not the most innovative piece of design work ever but it does the job. The slots for the rod to fit in are slightly undersized, to ensure a tight hold, and directly inline with the pickups so that the rod has to flex slightly ensuring a good push against the pickups. It means that all the soldering can be done away from the chassis and then the rods just clipped in place, and can be easily dismantled again if need be.

You could cut the wires on the chassis and solder them directly to the rods (via a resistor) but I'm going to retain the circuit board for now to use the lighting circuit so I've just soldered on loose wires to prove the system works, which were then wrapped around the terminals on the circuit board. The nice thing is that it opens up using the chassis for anyone who owns a soldering iron no matter how bad their soldering is, without any chance of damaging the chassis, so I'd call that a success even if, as I said before, the design isn't going to win any awards.