ELEC 243 Lab

Experiment 7.1

Motor Drive

Components

The low output current of the D/A converter and the 741 op amp has restricted the physical processes we are able to control. An op amp with higher output current will allow us to drive more power hungry devices, such as motors.

The TCA0372 has a maximum output current of 1 A (which is also the limit of the power supply). It is a dual op amp (i.e. two op amps in the same package) so obviously it can't have the same pinout as the 741. We will only use one half of the package, so here are the pins we will use:

\includegraphics[scale=0.640000]{ckt8.2.5.ps}
Fig. 8.2: TCA0372

So far we have always used dual power supplies (+15 V and -15 V) with our op amp circuits. This has allowed us to produce outputs which can be either positive or negative. Many devices (e.g. light bulbs, blower motors) only require inputs of a single polarity (typically positive). In this case the negative power supply is an unnecessary additional expense. Many op amps, including the TCA0372, are capable of being used with a single power supply.

\includegraphics[scale=0.640000]{ckt8.2.1.ps}
Fig. 8.3: Single Supply Operation

Part 1: Power Op Amp

When operating with a single power supply (positive in our case), we need to be sure that no negative voltages are required or would be produced in the operation of the circuit. Inverting amplifiers require careful consideration, so we will use the non-inverting configuration.
\includegraphics[scale=0.500000]{ckt8.2.3.ps}


Wiring:

 Connect $V_{CC}$ (+15 V) and ground as shown in Fig. 8.3. Then wire up a non-inverting configuration as shown above. We want a gain of a bit less than 1.5, so use a 2.2 kΩ resistor for $R_1$ and 1 kΩ for $R_2$ .

Warning
The TCA0372 is NOT pin compatible with the 741. If you try to wire it like a 741, you CAN blow it up.


Testing:

 Set the function generator to produce a 6 V p-p, 100 Hz sine wave with a 5 V offset (i.e. a minimum value of 2 V and a maximum of 8 V). Connect the function generator output to $v_{in}$ and monitor $v_{out}$ on the oscilloscope. Verify that $v_{out}$ has the expected value.

Observations:

 Measure the actual gain of the circuit. Increase the input amplitude until clipping of both the top and bottom of the waveform occurs. What are the minimum and maximum achievable output voltages?

Part 2: Motor Driver

The device we are going to control is a small cooling fan like the ones used in desktop PCs. Just watching a fan spin is not very exciting, so we will incorporate it into a more complex system in the next Experiment. For this reason, the fan has been combined with additional components into a high-tech fan assembly.


Wiring:

 Disconnect the power amplifier input from the function generator and connect it to D/A output channel 1. Plug the appropriate end of a phone plug patch cable. into J2-2. Plug the white, 3-pin plug on the fan motor into the connector on the other end of the cable, The red wire on the fan cable should be on the same side as the white stripe on the patch cord connector.

Via the interface connector strip, connect the positive (red) terminal of the motor to the power amplifier output and the negative (black) terminal to ground. When you are finished, your system should look like this:

\includegraphics[scale=0.500000]{ckt8.2.4.ps}


Testing:

 Restart the Lab 8 VI. On the tab labeled Manual increase the value of the slider labeled Motor. The fan should begin to turn, and run faster as the slider value is increased.

Although an ordinary DC motor will run in reverse if the polarity is reversed, the fan motors are brushless motors, which won't run at all if the polarity is incorrect. If nothing happens, verify that $v_m$ is in fact positive.

Design Challenge:

 Devise a method for measuring the rotational speed of the fan, using components that you have worked with in the lab.