Two Transistor DC Motor Driver Circuit

Posted by Micro System

on Monday, September 27, 2010

This is a circuit for controlling an ordinary DC motor using a pair of transistors (1 NPN and 1 PNP). Note the dual supply of this circuit (+6V and -6V). A DC motor runs in one direction if the required voltage is applied across its winding and runs in the opposite direction if the polarity of the applied voltage is reversed. This is the figure of the circuit;

 

In this circuit, a positive voltage at the control input turns on Q1 (an NPN transistor) but turns off Q2 (a PNP transistor).  This causes current from the +6V supply to flow through the motor from node A to B (ground), making it turn in the forward direction. On the other hand, a negative voltage turns off Q1 and turns on Q2, causing current to flow from node B to node A of the motor, then to the negative supply through Q2, making it turn in the opposite direction.  An input of 0 V stops the motor.

Low DC to High DC Voltage Converter

Posted by Micro System

on

This is a design circuit for converting a low DC voltage into a very high DC voltage. The 555 timer IC in this circuit is configured as an astable multi vibrator, i.e., it generates a continuous signal of given frequency. This periodic signal is fed into the low-side winding of a transformer. This is the figure of the circuit;

The resulting pulsating current in the low-side winding results in a high AC voltage across the high-side winding.  The level of this voltage depends on the voltage ratio of the transformer used.  The example above uses a transformer with a voltage ratio of 6.3V:220V, which can easily generate voltages in excess of 200 volts from a 9-volt battery. The current in the output winding is rectified using a diode, and a capacitor is used to 'smoothen' the rectified voltage waveform, producing a DC voltage. R3 is required to 'bleed' the capacitor charge to ground when the circuit is not in operation. Warning: C2 can store a high voltage, so extreme care is needed when working with this circuit (e.g., the output must not be touched).