There is often a need to control power (RMS voltage) low-voltage load powered by conventional electrical supply is via step-down transformer. Examples of such loads are a collector DC motors, low voltage soldering irons, light bulbs and etc.
Brushed DC motors have been widely distribution in the home. Miniature motors used in Amateur radio practice, on their basis, for example, is made small-sized electric drill. Most often these devices are supplied directly from the AC through a step the transformer and rectifier (see diagram).
In practice, there is often the need to control the frequency of rotation of the shaft engine. Traditional methods of voltage variation with adjustable stabilizers DC, thyristor or triac regulators and other devices that are included between the rectifier and engine, are of limited use because the management node is much more complicated than the actual node power.
However, there is a very easy way of solving this problem. The gist of it is that a power transformer connected to the network via triac the power controller. It should be emphasized that the need is a triac (or vopolop EP iodine thyristor) controller, and in any case cannot use a half-wave thyristor - in this case, most likely, will burn the transformer itself thyristor regulator. Since the current in the primary coil transformer about to (transformation ratio) times less current in secondary power loss in the regulator is small and "normal" controller with the allowable current of 0,5... 1 A can be used to manage quite powerful engines.
As a loaded transformer is an almost purely resistive resistance, working triac controller for such a load of features not has To control the speed of the commutator motor DPR-12-1-27 author used a mains transformer TN and miniature triac voltage regulator ("Radio", 2000, № 6, p. 43). Reliable motor control with thyristor regulator was provided and when half-wave straightening (disable one of the diodes).
Similarly, you can control and other low voltage load - soldering irons, incandescent lamps, etc. the Node power this simplifies, since the rectifier becomes unnecessary.
Author: Dmitry Turchinsky, Moscow