Introduction: The EGR 450 project that I chose to design and build was a high current pulse width modulated DC motor controller. This device is used to determine the speed of a direct current electric motor. It does this by producing a voltage at a fixed pulsating DC frequency. The duty cycle is then modified from nearly zero to almost one hundred percent. The PWM that I built actually ran from 50 usec to 700 usec in duration. This compared to a 980 usec period, produced a low value for the duty cycle of 5 % to a high of 71 % . These values could be higher if the pot was a slightly higher value.
 
 

Parts list: The parts used for this PWM were all scavanged from the scrap parts bin, new parts bins, store room, Quality Farm and Fleet, T and W, and Jameco.
 

                        CD4047BE:        Low Power Monostable/Astable Multivibrator                        Jameco, Store room.

                        Resistors:            20 kOhm, 2 kOhm, and 1 k potentiometer                            Store room
                                                  2x 50 Ohm                                                                         Scrap Parts Bin

                        Capacitors:          0.33 uFarad, 0.01 uFarad, and 3x 0.39 uFarad                      New Part Bins

                        FET:                  312                                                                                    Store Room

                        BJT:                  TIP 120                                                                              Store Room
                                                 3x Powerex KD421K10  100 Amp                                        Scrap Parts Bin

                        Heat Sink:          12.5 inch x 9 inch                                                                Scrap Parts Bin

                        Wire:                 Multiple sizes                                                                      Store Room

                        Weldable Steel:  1/2 x 1/8 x 36 inches                                                            Quality Farm and Fleet

                    Screws:             #10 Machine                                                                       Store Room

                        Fuse:                60 Amp AC                                                                         Scrap Parts Bin

                        Circuit Board:   Small DIP Ready                                                                 T and W

Calculations: According to the National Semiconductor Corporation logic data book volume 1, the formula for calculating the frequency of the CD4047 astable multivibrator is 4.40(R)(C)=f for pins 10 and 11. The pin I used was pin 11. In the monostable mode of the multivirator, the formula that is used to find the period of the pulse was 2.48(R)(C)=f. The formula to calculate the frequency is located on page 5-141 under the CD4047BM/CD4047BC data. The CD4047 is a low power monostable/astable multivibrator.
 
 

                        Frequency of free-running astable multivibrator:                       4.40(20k)(.01uF)= 1.1364 kHz

                        Frequency of positve-edge triggered monostable multivibrator:   2.48(.33uF)(1k)= 818 usec
                                                    (calculated maximuim value)

                        Frequency of positve-edge triggered monostable multivibrator:   2.48(.33uF)(1)= 0.818 usec
                                                    (calculated minimuim value)

                        Minimium Duty Cycle (measured):                                          50 usec/ 980 usec= 5.102 %

                        Maximium Duty Cycle (measured):                                         700 usec/ 980 usec=71.428 %

                        Resistance at Drain of 312 FET:                                             (12-(.7 x 4))/4.6 mAmps=2 kOhms

                        Resistance at collector of TIP 120:                                          (12-(.7 x 3))/400 mAmps=25 Ohms (2 Parallel 50 Ohm)
 

Construction: The circuit was first bread boarded to verify that it was functional and then soldered in place on the printed circuit board. According to the logic databook certain pins must be tied to Vdd and Vss or in my case battery+ and battery-. These pins are 4,5,6,14 to battery+ and 7,8,9,12 to battery- for the astable multivibrator. The monostable multivirator ties pins 4 and 14 to battery+ and 5,6,7,9, and 12 to battery-. Output for both multivibrators is pin 11 and the input for the monostable multivibrator is pin 8. The capacitors for both multivibrators are connected to pins 1 and 3 and the resistor and potentiometer are connected to pins 2 and 3. The potentiometer is connected to the monostable multivibrator and speeds up the motor by twisting clockwise.

The output of the monostable multivibrator is connected to the gate of a 312 FET. The maximuim output of the 312 FET is 15 mAmps. The resistor that determines maximuim current draw stops the drain current short of that value. The FET is acting like a switch and therefore the resistor is determining the current flow and not the gate.

The source of the 312 FET feeds the base of a TIP 120 BJT. The TIP 120 has a large current gain, but it will act as a switch, therefore it needs to be forced into saturation. An input current of 4.6 mAmps will saturate the TIP 120 and cause the two parallel 50 ohm resistors connected to the positive battery terminal to limit the current to .4 Amperes. This is about three time the current needed to turn on one Powerex triple Darlington transistor module. The reason for the extra current is so that the other two transistor modules can be connected without any modification to the circuit board at a future time. The current from the TIP 120 feeds the B2 pin on the transistor modules.

Two buss bars were cut and drilled to mount across the top of the C2E1 and E2 terminals of the transistor modules.Motor plus is attached to the positive terminal of the battery and motor minus is bolted to the C2E2 buss closest to the circuit board. E2, in the middle, goes to a fuse and then to the battery minus.

The Circuit board is switched from the positive battery terminal because the minimium duty cycle is 5 % which is still in an on state. The motor will turn very slowly at all times unless power is removed from the mulitivibrators.

Cautions: While building the circuit on the cadet trainer the monostable multivibrator would burn out whenever the duty cycle was turned up passed a certain point. This was later found to be caused from 10 amp current spikes generated by the small DC test motor connected to the power supply of the cadet trainer. These spikes were clearly visible on the occiliscope when using a 1 ohm resistor between the power supply and the multivibrator. To solve this problem three 0.39 uFarad capatitors were placed in parallel across the power input of the multivibrators. The capacitors help to stop the problem, but the battery helped the most with it's large current supply capability for the motor.