In order to control direction of the rotation of the motor you need to separate the armature from the field circuitry, so serial DC motors are out of the question and your chopper needs to have the ability to reverse polarity only to the armature without changing polarity on the field. This can be accomplished by four switching transistors in "H" configuration, only two will be active at any time providing negative and positive to the armature, when the other two are active the polarity will reverse, reversing the rotation of the motor. If are looking for an inexpensive fully tested and running Drive go here and scroll down to "Motor Control" I will recommend that you star with K166 or CK1400, but you will find a hole range of choices. Good Luck Nestor Martins Electronic engineering technician Don't forget to download the full specifications.The application of the electric motor is an essential part of many hobbyists' projects, as well as industrial and home devices, on machines that need to be portable and independent from the AC power line; the use of DC motors is the only alternative. Introduction In any project where the control of the speed is at the essence, like wheelchairs, robots traction and general movement, it became important to have a way to control speed in an efficient way. As in any project it is important to start simple, therefore the implementation of a simple driver that will do the job became very necessary, then when the project is already running, the project should be perfected by adding indicators and special safety devices that will prolong the life of the hole, also intelligence through microcontroler devices can be added to get into acceleration and deceleration, and keeping this within the limits that the project requires. Driving a DC Motor The way to control the speed of a DC motor is controlling the current through its armature, in industrial application this is done very effectively using SCRs or controlled rectifiers, these devices will control the current going to the armature by means of firing the SCR at different times on the cycle; however when the source of your power is already DC the efficient way to do it is through PWM (pulse with modulation). Other methods like a simple variable voltage power supply are inefficient because lots of energy is dissipated on internal circuitry resistors, and if you are operating your project from batteries they will run out of power very quickly. So, efficiency is a very important issue, on Pulse With Modulation (PWM) the dc voltage is chopped producing a square wave where you can control the on time, even though the pick voltage of the wave will remain the same, the effective current will average, if you can control the on to off ratio, this will control the energy to the armature, even though what you really get is a pulsating DC there is no need to filter this because the armature inductance will take care of that.