Prelab

1. Read the lab exercise and familiarize yourself with the procedure.

2. Print out this document and bring it to the lab session with you, you will record your data and findings on it. 

Purpose

1. To become familiar with the use of a multimeter, the circuit properties it can test, and the correct procedure to do so. 

2. To learn how the DC supplies are used.

3. To learn to identify resistors and read their color codes.

4. To learn how to build a circuit using a protoboard.

Theory

Power Supplies

The DC power supplies in the lab are capable of supplying up to 30V at 500mA, and can be run in constant current or constant voltage modes. In practice you will find a wide variety of supplies available in varying specifications for different applications. They all function in a very similar manner, and skills learned with the models in our lab will translate well to almost all others.

The power supplies typically have a coarse and fine adjustment knob used to select the desired voltage or current.  Most supplies have a voltmeter/ammeter built in, but it is often best to confirm the value with a multimeter for increased accuracy.

We will always use the  Red "+" positive and  Black "-" negative ports on the supplies. If yours had a ground port, you can ignore it for the content in Circuits I.

Resistors

Resistors are among the simplest electronic components and serve to resist the flow of current. They are typically made of very thin, laser cut films. The thin films restrict the flow of electrons in the same way that a narrowing or necking in a pipe would slow the flow of water.  The flow of current through the restriction in the resistor causes them to heat, and careless construction without consideration of the power to be dissipated in the resistor may cause them to overheat and burn. The resistors used in the lab, and those most commonly used in everyday devices are rated to safely handle 1/4W (0.25 Watt).

The resistors used in the lab are typically light blue or tan in color and have two metal leads that run axially through the part. The value is indicated by a series of colored bands that run across the part.  There are many form factors for resistors that range from tiny SMD resistors that are less than a mm, to others as large as a AA battery. Of course, they may be much larger in industrial applications that dissipate a lot of power, in such cases there is no upper bound for their size.

The resistor coding scheme shown below describes how to identify resistors by the colored bands that run across each part. The colors are often difficult to tell apart on a blue background, so pay special attention to red/orange and green/blue bands.

Example

Protoboards

Protoboards, also known as breadboards, are a convenient way to construct non-permanent circuits quickly without tools. They feature a grid of tiny holes which are designed to fit resistor and other electronic component leads. The internal connection structure is shown below:

As shown above, each of the holes in vertical rows  are connected, but not across the centre break. and each of the holes in the horizontal rows are connected.  Occasionally, the horizontal rows will be split halfway across the board. Be careful of this if your circuit occupies a large part of the protoboard.

It's often good practice to build your circuit on the protoboard in a way that resembles the circuit in question. This will make it easy to identify components in your schematic and their counterpart on the protoboard. 

Constructing a circuit is as simple as treating each vertical column of 5 holes as a node or connection point. If two or more components are to be connected you insert a lead from each component you want to connect into the same column. It is important that only one lead from each component be in a column, and that only those components you wish to connect are in the column. Each connection must use a unique column.  Finally, you may use a wire or the alligator clips to connect power to your circuit from the DC supply.

Problem

AP Engineering Inc. is deciding on a new supplier for electrical components and would like to evaluate the precision and consistency of a manufacturer's product to ensure that designs will function as intended when using these components. You will be given a sample of their product and the appropriate test equipment.

When evaluating electrical components, it is often important that the component's actual properties match those reported by the figures printed on them. For resistors, this property is the resistance which is printed on the component using the color coding shown above. Electrical components are typically manufactured by the millions in huge batches and this introduces some manufacturing tolerances and an average 'error' in the actual value of the component relative to the stated value. For example a resistor may be color coded as though it is a 1000 ohm part, when in reality it may be 1007 ohm or 994 ohm. This may be negligible in a design, or may be inappropriate in design where a more consistent value is required.

The manufacturer of the parts chosen for testing has specified 1% tolerance (indicated by the brown final color band)

AP Engineering Inc. has laid out the following requirements for the supplier they will ultimately choose:

• Resistance values must fall within 2% when tested individually.

• Resistance values must fall within 4% when calculated based on in-circuit functionality.

Your group has been tasked to evaluate and report back on your findings based on a series of tests that will be described below.

Requirements

Results

• Create a spreadsheet or table with your lab data and the data obtained from the calculations in the results section.

• Compare the individually measured values and rated tolerance, do the measured values of the resistors fall within the rated tolerance ? 

• Use Ohm's law to calculate the effective resistance from the voltage and current measurement, do the resistors fall within the rated tolerance ? 

• Overall should this manufacturer be considered as a potential supplier as described in the problem statement above?

  • Why or why not giving specific examples.

Questions

1. Why do the measured values of resistance typically not exactly match the resistance indicated by the color code?

2. Create a power balance comparing power absorbed on all resistors and the power supplied by the source. Is there any discrepancy, and if so why?

   • Note that the directions for voltage and current arrows were chosen arbitrarily and may not reflect actual behavior.

3. What other property or value of a resistor (aside from resistance) might be important to consider in a circuit design?