Definition

The electrical system within this project consists of main supplies: 12 volts to drive a number of windshield washer pumps, and the 5 volts necessary for all the control logic used that enables a computer to control these pumps. In addition to the control of the pumps it was also required to be able to control a solenoid, used within the system for the carbonator that supplies carbonated water to the system.

Design Solution

Power

The AC requirements consisted of supplying power at various voltages. The initial supply to the system was a standard 110V - 60Hz wall outlet. From this supply we needed to make 25 volts AC, and both 12 and 5 volts DC.

AC

The 110 volts AC power was used to supply power to various devices in the system: the refrigerator, the LCD, the PC the carbonator, and the two-25 volt AC Transformers. The transformers were also needed to supply the twenty-five volts AC to switch the solenoid on and off. The output of the transformer was also used to create the DC voltages needed for the pumps and the CMOS logic.

DC

There were two DC voltages required in the system: twelve volts for the pumps, and five volts for the CMOS logic. The twenty-five volts AC from the secondary side of the center-tapped transformers was rectified through a full wave bridge rectifier to obtain 18 volts DC.In addition to acquiring the 18 volts DC the voltage needed to be regulated down further to acquire both the 12 and 5 volts DC required.

Switching was also appended after the bridge rectifier to add the capability to disable power from the pumps.The switching was implemented, by using a relay that was turned on and off by a push button. The coil of the relay was connected to the 18 volts DC from the full wave rectifier output through the push-button switch. In the off state of the relay, 18 volts DC was connected to a yellow LED through a resistor, turning it on. The LED indicates that the power to all of the pumps is off. When the relay is turned on, The LED turns off and the 18 volts is connected to the pumps and control logic.

Volts DC

The twelve-volt DC supply was required to handle the necessary power required to operate up to two windshield washer pumps simultaneously. Each pump requires 3A of continuous current. In addition to the continuous current, the initial startup current of each pump requires a peak current of 4.5A. It was determined that when two pumps were run at the same time, a peak supply current of 9A was required at the startup, and a steady state requirement of 5.5A. With the large variation in the current supply, a change in the output voltage was noticed.

The voltage supplied to the pumps during single and double pump operation required as little variance in supply voltage as possible. The reason for this requirement was to ensure consistent flow rates between one-pump operation and two-pump operation. It was noted through testing that the marginal difference in flow rates of the pumps varied more significantly at 12 volts DC supply then at 13.5 volt supply. Because of this difference the 12volt supply to the pumps was increased to 13.5 volts DC. The addition of capacitors were to the 13.5 Volt output were included to help supply the surge current at the start up of the motors.

The 13.5 Volts DC was supplied to the pumps from the output of a NPN power BJT. The BJT (2N3055) was set up in an emitter follower configuration, with a 1K to ground on the emitter. The other connection to the BJT was 18 Volts DC to the collector and 14.1 volts to the base.The reason for the 14.1 Volts required at the base was to ensure the correct output voltage of 13.5 volts was available at the emitter of the BJT due to the voltage drop across the base emitter junction. A 1000F capacitor was also added to the emitter of the power BJT so that the voltage would not vary significantly during the start up of the pumps. The supply of the 14.1 Volts DC to the base was delivered via a three terminal 12 Volt regulator (MC7812).The regulator is capable of supplying twelve volts DC with a 1 amp supply current. To increase the output voltage of the regulator three diodes where attached to the ground reference.The voltage drops introduced as a result of adding the diodes increased the ground reference by 2.1 Volts resulting in an output of 14.1 volts.

This completed the power requirements for the pump operation.

Volts DC

The 5 volt DC supply was regulated using a MC7805. The input voltage to the five-volt regulator was 12 volts DC that came from a MC7812 that was connected to the 18 volt supply. The 5 volts DC was used to supply VCC power to all of the CMOS circuitry, including the de-multiplexer and the tri-state buffers.

Control

Using eight output pins (D0:D7) from the parallel port of a computer, all of the pumps and the solenoid were controlled.

De-multiplexer

Two of the three address lines of the de-multiplexer are driven from the first two data pins (D0:D1) of the parallel port via the PC. The third address pin was grounded. This configuration allowed us to set one of four output lines of the de-multiplexer to a logic low state. The three enable lines were wired in a configuration such that the de-multiplexer was continuously enabled, meaning that one of the tri-state buffers were always enabled.

Tri-state Buffers

The other 6 available data pins (D2:D7) of the parallel port were wired to each of the four tri-state buffers. The four outputs of the de-multiplexer were wired to the active low enables of each tri-state buffer. This configuration restricted the tri-state buffers to have one continuously enabled at a time. Using D2:D7 of the parallel port, up to six pumps could be controlled. This gave us the ability to control up to 24 different solenoids and pumps, having the capability to turn on up to all six, connected to one tri-state buffer, at a time.

The final design had 20 pumps and 1 solenoid connected to the system. This was done with 5 pumps attached to each of the tri-state buffer and the solenoid connected to the last bus.

MOSFETS

The twenty output lines for the pumps from the four tri-state buffers were connected to each gate pin of twenty power MOSFETS. These power MOSFETS were used to switch the power on and off to the pumps. An N-channel MOSFET was used since we wanted to use a logic high level to turn on the pumps.

A 10K pull-down resistor was required on each of the gates of the MOSFETS to ensure the pumps would turn off when the tri-state drivers where disabled. This was due to the relatively low turn on voltage of 0.6 volts of each MOSFET.

The 13.5 volts was connected from the BJT to the Pumps. The pumps were then connected to the drain of the MOSFETS. The source of the MOSFETS was then connected to ground so that when the MOSFET was turned on it completed the circuit turning on the pump.

Solenoid

The solenoid was used to control the output of the carbonator. The solenoid required 25 volts AC to operate. The 25 volts AC was switched using a relay. The coil in this relay was switched with 12 volts DC.The 12 volts to the coil was turned on and off in the same configuration as that of the pumps.

Discussion

+12 Volt Power Supply

The Voltage out of the BJT that supplies 13.5 volts varies dependent on the current into the pumps.

# of Pumps
VO
I Startup
I Steady State
0
13.5
0
0
1
12.4
4.5
1.0
2
11.5
9.0
5.5
Table: Voltage regulation and Steady State Current

 

It can be noted that as you go from using no pumps to one pump the supply voltage drops 1 volt. The supply voltage also drops by one volt when two pumps are switched on from one pump operation.

Two Transformer Implementation

Initially our design used one transformer.When two pumps were running the current rating of the transformer was being exceeded. One transformer was rated 25 volts AC @ 5A. The current measured at the output was 5.5A.To ensure the necessary current could be supplied safely another transformer was added in parallel to increase the output current rating from five to ten amps.

LM338

In our initial design approach to create 12 volts DC, we were going to use two LM338 voltage resistor set voltage regulators. Each regulator was rated to supply 5 amps with a peak startup current of 12 amps each. After implementing and testing this configuration it was determined that the output voltage from the regulators was very dependent on the load current. Even with a minimum load of two hundred milliamps. The current draw of two pumps lowered the output voltage to 10.2 V. This drastically reduced the flow rate of the two pumps relative to the flow rate of one pump operation

 

 

We also attempted an alternate configuration using 3 regulators with a LM741 operational amplifier similar to the circuit in the figure bellow. This configuration is capable of supplying up to 15 amps; however, after implementing this configuration it was also determined to rely too heavily on the load current and the results were similar to that of the configuration above.

 

BJT

Implementing the power BJT (2N3055) required a worst case of nine amps current when two pumps are running. This current initially overheated the first BJT used, as there was no heat sink to cool the BJT. After utilizing some thermal compound and an old CPU heat sink and an alternate BJT this issue was resolved.

Control Logic

The first idea for implementing the control logic was to split the 8 data bits from the parallel port into two sets of four. This would use one of the bits to enable the chip and then the other 3 pins would be de-multiplexed. This would allow for the capability to pump 16 different pumps, with at most two pumps pumping at the same time. Another variation on this was to use 2 data pins to pick one of four de-multiplexers to turn on. This would let us control 32 pumps but only one pump at a time. These two solutions weren't exactly what we wanted for the control. The design criterion was that we wanted the ability to turn on more than one pump at a time and we wanted to have a reasonably large number of pumps hooked up to the system.

The Final solution to this used part of the second idea of using to data bits from the parallel port to enable tri-state buffers. This gave us 24 different control signals with the ability to turn on up to six pumps at a time. This was limited to 2 pumps only due power limitations.

Pull-down Resistor

After initial software integration it was noticed that pumps were not turning off after the tri-state buffers were disabled. It was determined that this was due to the capacitance of the MOSFET gate storing a charge. This charge was larger that then 0.6 volts required turning on the MOSFET, this was causing the MOSFET to not turn off properly. Adding a pull-down resistor to the gate of the MOSFET dissipated the stored charge easily solving this issue.

Conclusion

It was determined that by using 2 bits to select one of four 6 bit buses, we where able to control up to 24 pumps with up to 6 pumps pumping at the same time. The two lowest bits on the printer port (D0, D1) were used through a de-multiplexer to select one of four tri-state buffers. The first three tri-state buffer where connected to 5 pumps each, where the last tri-state buffer was connected to 5 pumps and the solenoid for the carbonator. They system is limited to running only 2 pumps simultaneously from any one of the tri-state buffers due to the large current requirements of the pumps.

Schematics


AC-TO- DC

SWITCHED POWER TO SOLENOID -- FOR SODA

POWER FOR PUMPS

CMOS POWER

CONTROL LOGIC

ONE PUMP CONTROL

 

PARRALEL PORT PINOUT