Anatomy of a Breadboard:

Intro to Circuits Lab

Anatomy of a Breadboard:

The breadboard is where you will be assembling your circuits. The breadboard is composed of rows and columns of metal clips. These clips are housed in a plastic covering with holes that allow for pin connections.

Circuit components whose pins are in the same row set, are electrically connected. Therefore, in the figure to the left, components 1 and 2 are electrically connected. Current flowing through component 1 will flow through component 2 as well.

However, Components 3 and 4 are not placed in the same row set, therefore current flowing through component 3 will not flow through component 4.

Likewise, components 5 and 6 are placed in the same column, so they are electrically connected, while component 7 is placed in an isolated column and is not electrically connected to components 5 or 6.

If however, a wire is placed between components 3 and 4, an electric connection is made between the two isolated rows of pins, and now components 3 and 4 are electrically connected.

Powering up your circuit

In order to provide power to your circuit, connections to the power supply should be made. Banana plug cables can be used to form connections between the power supply and the three voltage source connections at the far end of the breadboard, as seen in the illustration on the bottom right. The three necessary connections are V+ and V- which provide positive and negative voltage levels to the breadboard, as well as ground () which provides a common reference for the voltage potential.

Note that according to convention, leads used to connect to ground are black, while those used to supply a +/- voltage from the power supply are red.

In order to supply these voltages to the entire breadboard, wires should be used to connect the source connection at the far end of the breadboard, to the sequence of columns of vertically connected pins. This is indicated in the breadboard schematic below. With this, Hi and Lo voltage levels are easily accessible along the entire breadboard.

Different Lead Types

Depending on the type of connection that needs to be made between lab equipment and the protoboard, different leads can be used. Always use the appropriate lead when making connections. The following is an illustration of the various types:

The Bioinstrumentation Lab does not use Oscilloscope Probes. BNC cables can carry two signals. The center pin is used to carry a signal of interest (such as a waveform from the breadboard) and is surrounded by an outer sheath that is at ground voltage (the sheath is covered by wire insulation except at the ends where the metal connectors are exposed).Basic Circuit Components

Resistors A resistor is an electronic component that resists electric current by producing a voltage drop between its terminals. The value of the resistance is equal to the voltage drop across the resistor divided by the current through the resistor. This is also known as Ohms Law, given by:

Usually, resistors are color-coded to represent their value and tolerance. The first two bands are a numerical value, the third band is a power of ten multiplier and the fourth band indicates the tolerance within which the actual resistance is given.

For example, a resistor with color code red, violet, yellow gold is a resistor with a value of 270k(( and a 5% tolerance. Thus the actual value of the resistor is between 256.5k( and 283.5k(. More expensive resistors have lower tolerance.Designing and building simple circuits requires full understanding of the properties of resistors when placed in series or parallel with each other.

Series and Parallel Resistor Circuits:

Resistors in parallel have the same voltage across them. To find the equivalent value of several parallel resistors the following expression is used:

A shortcut for two resistors in parallel is:

Unlike resistors in parallel, current through resistors in series stays the same though the voltage across each resistor can be different. However, the voltage drop across all of the resistors is equal to the sum of the voltage drops across each of the resistors.

CapacitorsA capacitor is an electrical device that stores the energy in the electric field between a pair of conductive plates. Charges of equal magnitude but opposite polarity build up on these plates when a voltage is applied to the capacitor. Therefore, capacitors are used as energy-storage devices in circuits. Capacitors are also useful in making electronic filters. Simple filter designs will be discussed later.

Series and Parallel Capacitor Circuits:

Note that capacitors in series are summed up in the same manner as resistors in parallel, while the expression for capacitors in parallel is similar to that for resistors in series.

Capacitors in parallel each have the same voltage drop across them, therefore total capacitance for several capacitors in parallel is given by the following expression:

Meanwhile, each capacitor in a series set up has a different voltage across it. Therefore the current through series capacitors stays the same. Total capacitance for a set of series capacitors is given by:Capacitor Behavior as a Function of Signal FrequencyCapacitor behavior is related to the frequency at which the signal flows through. For very high-frequency alternating current, the capacitor behaves as a wire or short. On the other hand, for very low frequency alternating currents, the capacitor approximately behaves as an open circuit. This property of capacitors is what makes them a fundamental component of filter design. Further details on the impact of these characteristics on filter type will be discussed in a later section. Diode

A diode is an electronic component that restricts the direction of movement of charge. Therefore, it allows current to flow in only one direction, and blocks it in the opposite direction.

A first order model for diode behavior is illustrated in the plot below.

If the voltage across the diode is less than a threshold voltage VF, which is usually 0.7V for silicon junction diodes, then the diode is considered off and no current flows through it. In the event that the voltage across the diode reaches Vf, the diode turns on and current flows through the diode in the specified direction (A ( C). Note that the magnitude of the current is not defined by the diode, but rather by the rest of the circuit, which the diode is a part of. Also, since current can only flow in one direction, the current can only be zero or any positive value, but not negative. Finally, because an unlimited amount of current could flow though the diode, the circuit the diode is a part of cannot cause VAC to become larger than Vf. Thus, the diode clamps VAC to VF and no higher.

A unique type of diodes made from materials other than silicon, and that typically have a VF of 1.0 to 2.0V, dissipate power in the form of light. These diodes are known as Light Emitting Diodes, or LEDs, and are available in red, yellow, green and blue. These diodes are useful to use as indicators in electronic circuits.

Making Measurements

Multimeters:

A multimeter is a device that is able to measure current (ammeter), potential difference between two locations (voltmeter), and resistance (ohmmeter) amongst other things. The purpose for which you are using the multimeter (i.e to measure current, voltage, or resistance) dictates the method in which you connect it to the rest of your circuit.

AmmeterTo measure current, the multimeter should be connected in series with the rest of the components in your circuit. This allows the current flowing through the circuit to pass through the ammeter as well. However, meters should not alter the behavior of the circuit whose current they are measuring, and thus, to avoid causing a voltage drop across them, ammeter should have very low resistance.

Voltmeter In order to measure voltage across a given component in your circuit, the voltmeter is connected in parallel to that component. Because the voltmeter provides a parallel pathway, it should pass as little current as possible, so as not to short circuit the component across which it is measuring. That being said, a voltmeter should have very high resistance.

OhmmeterUnlike ammeters and voltmeters, ohmmeters cannot function if the circuit is connected to a power supply. In order to measure the resistance of a given circuit component, it must be removed from the circuit and probed independently. The ohmmeter then passes a small current through the circuit component of interest and subsequently measures the voltage produced, and using principles based on Ohms law, displays the resistance of the component. Probing a powered circuit with an ohmmeter will likely damage the meter.

Generally multimeters have a central knob with various positions to which it can be rotated. Where you position the knob will be dependent on the purpose for which you will use it. If you circuit is operating from a constant voltage source such as a battery, current flow will always be in the same direction, and thus it is referred to as direct current or DC. In this case, you could set the meter to 10V DC, and with this the maximum voltage that can be measured at this setting is 10V. If you know that the measurement you will be making is in the millivolt range, then you will achieve more accuracy if you set the multimeter to make measurements in the 10V to 100mV range. If the current you have flowing through the circuit periodically switches direction from positive to negative, then you are dealing with an alternating current or AC power supply, and you should adjust your multimeter knob to take AC measurements. The multimeter will display the RMS, or root mean square, voltage of the AC signal.Measurements Practice:

For the following 3 steps, use a 12 V DC source voltage from the power supply, then repeat with a 12 V AC source voltage from the function generator.

Voltage Divider In electronics, the voltage divider rule is a very useful design technique used to supply a voltage that is a fraction of that provided by the power supply.

Build the following circuit and use the following equations to determine the voltage across Resistor 2, for some power supply voltage V1.

Note that if the voltage divider is supplying a load resistance RL, the output voltage you detect would be different than a setup in which there is no load hooked up to the divider circuit.

Current Divider

If two or more impedances are in parallel, the current that enters them will be split between them in inverse proportion to their resistance. As we have seen, if the impedances that are in parallel are equal, then the incoming current will be evenly split across each resistor.

Build the following circuit and determine the current across each of the three resistors:

Note that since R1 is half of R3, the current through R1 will be twice that through R3.

Since the voltage across all the components in a parallel circuit are the same, we can calculate the current through each branch using Ohms law (I=V/R).

Then, knowing that in parallel circuits, branch currents add up to equal the total current, you can determine the total current for the circuit using the branch currents previously calculated.

Finally, knowing the power supply voltage, and having just determined the total current, the total resistance for this circuit can be determined, again using Ohms law.

Use the following table to help guide you through the above calculations:

R1R2R3Total

VV

IA

R((

Remember:

Current through any resistor: I//=V///R//

Voltage in parallel circuit: Vtotal=V//=ItotalRtotal

So, substituting ItotalRtotal for V// in the first equation gives the current through any parallel resistor:

I//=Itotal(Rtotal/R//)

This equation is the current divider formula.

Since Rtotal/R// will always be a number less than one, you can see that this parallel circuit is able to proportion, or divide, the total current into fractional parts through the parallel resistors, hence the term current divider.

Review:

Voltage divider: V//=Vtotal(R///Rtotal)

Current divider: I//=Itotal(Rtotal/R//)

Practice with Resistors in Series and Parallel

Remember that in a series circuit, the current flowing is the same all throughout. Build the following circuit and verify the values of voltage and current across each resistor.

Important expressions to remember: Rtotal = R1 + R2

Here, Rtotal = 1+1=2k(Using Ohms Law, I=V/R, the current I flowing through this circuit, given a power supply providing 6V is, I=6V/2 k( = 3mA. This current flows through each of the two resistors.

With this, the voltage across R1 is, V1=IR1= (3mA)(1k() = 3V

Since R2=R1, the voltage across R2 is also 3V. Note that the voltage supplied to the circuit is equal to the sum of the voltages across the two components of the circuit. This follows in line with the law of conservation of energy.

Now consider this circuit with resistors in parallel:

Probe this circuit and verify the voltages and currents across the two resistors.

Calculate the total resistance in this circuit using the expression for Rtotal for parallel resistors.

Using this value, compute the total current for this circuit using Ohms Law.

Note that the current calculated for this parallel resistor set up is greater than that for the series circuit because in this case, connecting the resistors in parallel provides alternative pathways for the current and makes its flow easier. The current that goes through each resistor is equal since they have the same resistance value. Therefore, the Itotal you computed gets evenly split between the two resistors.

Finally, compute the voltage across each resistor. This voltage value should be equal to the voltage provided by the power supply. Conceptually, this makes sense since the top of R1 is connected to the positive terminal of the battery while the bottom end of the resistor is connected to the negative terminal, with no other circuit components in the way. Following the same logic, the voltage across R2 is equal to that across R1 and to that provided by the power supply.

Remember that components in parallel have the same voltage across them.

Consider the following circuit with both series and parallel parts.

First, determine the total resistance of the entire circuit by adding the combined resistance of the parallel setup to the resistor R1 in series. Then, determine the current through this Rtotal. This current is the I that flows through R1. Since R2 and R3 have the same magnitude, this Itotal value is evenly split between the two resistors. Use Ohms law to determine the voltage across R1. The difference between the power supply voltage and the voltage drop across R1 gives the voltage difference across R2, which is the same as that across R3, thus Vtotal-V1= V23.

Pinholes are electrically connected in these rows

Each row of 5 pinholes is electrically isolated from all other rows.

Pinholes are electrically connected in these columns

Each column of pin holes is electrically isolated from all other columns.

2

1

3

4

5

6

7

4

3

PAGE

Spring 2007Page 2 of 17