Resistors in a Circuit

Steps

1. Draw the schematic diagram and label the components. 

• When labeling your components in a circuit each resistor will be R#, so in this circuit R1, R2, R3, and R4. R1 will typically be the resistor closest to the positive node.

• Your circuit should also have the nominal values of each component annotated on the schematic diagram. With Resistors, you can find this using the Resistor Color Codes. 

• Using what we know about current, you should also label the schematic with the anticipated current flow direction.

2. Find the measured values for each resistor.

• Use your multimeter to check that each resistor is within its tolerance range set by its tolerance band. Set your multimeter to its ohm settings and make sure no power is applied to the circuit- on most multimeters you will break them if power is running through a circuit while measuring resistance. If you are using a multimeter that is not auto-ranging, set it to the closest resistance range that is not lower than your anticipated resistance load.

Questions:

1. Are your percent errors within the ranges of the tolerance band on the resistors?

2. What is the total resistance of this circuit?

3. Is the measured resistance total close to the calculated resistance total using the nominal values?

4. How does current flow in a series circuit?

5. Why is calculating power before building any circuit important?

2. Find the measured values for the resistor.

• Use your multimeter to check that the resistor is within its tolerance range set by its tolerance band. Set your multimeter to its ohm settings and make sure no power is applied to the circuit- on most multimeters you will break them if power is running through a circuit while measuring resistance. If you are using a multimeter that is not auto-ranging, set it to the closest resistance range that is not lower than your anticipated resistance load.

3. Record your power source voltage. 

• Measure it with the multimeter. Just because the battery is a 9V battery does not mean it is exactly 9-volts. 

• Set your multimeter to Direct Current Voltage (DCV). If you are using a Multimeter that is not auto-ranging, set it to the closest Voltage range the is not lower than your voltage source. In this case, we will be using a 9V battery, so most multimeters will have a 10 or 20 VDC range. 

6. Find Current. 

• Measure current in the circuit with your multimeter. To do this, you have to physically disrupt the circuit to insert your multimeter leads. If you are using a multimeter that is not auto-ranging, set it to the closest current range that is not lower than your anticipated current from your calculations. 

7. Estimate resistance of the diodes.

• Diodes have variable resistance determined by the amount of current flowing through them. Since we cannot measure their resistance while current is flowing because we will break most multimeters, the best we can do is calculate them using Voltage drops and Ohm's Law.

• Now that we know the voltage drop over each component and the total current of the series circuit, use Ohm's law to estimate what the resistance of the safety diode and LED.

8. Find the power consumption of the circuit and all individual components.

• This is to make sure the power consumption of a component does not exceed its safety rating. To find total power, you can use total current multiplied by total voltage.

• In a series circuit, every component in the series circuit will have a different voltage drop and therefore consume a different amount of power. The new power formula for individual components becomes: Power equals the voltage drop of a component multiplied by the current through the component. 

• Calculate Percent Error of Power when you compare the total power consumption and the sum of the individual powers.

Questions:

1. Why does each voltage drop of the components have to add up to total voltage? What if they aren't exact? Why would that happen?

2. How do voltage drops affect power over a component?

3. Would the four resistors in series from Exercise One have had a voltage drop over each of them?

4. Does any component in this circuit exceed its power safety rating? (Resistor's Power Rating: .25 Watt, LED's Power Rating: .2 Watt)

1. Draw the schematic diagram and label the components. 

• When labeling your components in a circuit each resistor will be R#, so in this circuit R1, R2, R3, and R4. R1 will typically be the resistor closest to the positive node.

• Your circuit should also have the nominal values of each component annotated on the schematic diagram. With Resistors, you can find this using the Resistor Color Codes. 

• Using what we know about current, you should also label the schematic with the anticipated current flow direction.

2. Find the measured values for each resistor.

•  Use your multimeter to check that each resistor is within its tolerance range set by its tolerance band. Set your multimeter to its ohm settings and make sure no power is applied to the circuit- on most multimeters you will break them if power is running through a circuit while measuring resistance. If you are using a multimeter that is not auto-ranging, set it to the closest resistance range that is not lower than your anticipated resistance load.

Note: Unlike the series circuit from above, you have to measure each resistor with the multimeter while not attached to the rest of the circuit unless you are measuring total resistance. 

Questions:

1. Why do you have to take the resistors out of the bread board to measure individual resistance in a truly parallel circuit?

2. Does total current of the circuit equal the sum of the added individual currents of each leg when using Ohm's Law for both?

3. Why does knowing the total power consumption still important even if it does not exceed the power rating of the circuit?

Let's go back to the breadboard and see how different resistors affect a simple circuit. The resistor and LEDs are both loads. The resistor uses most of the voltage but usually leaves just enough for the LED to work. LEDs have variable resistance depending on the current running through them. Typically, this resistance causes them to use about 2 volts but can vary depending on the total current in a circuit. To see this, we can experiment with what would happen if you changed the resistors on the circuits?

You will measure the voltage used across the resistor and measure the voltage used across the LED. To measure a voltage drop, place the red and black probes of the multimeter on either side of the required component. Polarity matters. The red (positive) probe should be in front of the component, closest to the power source, otherwise, your multimeter will read negative. Using everything we know from the exercises above, you should be able to fill out this entire table through measuring with your multimeter or calculating using Ohm's Law and the Power Formulas. 

Questions:

1. What is the relationship between resistance and voltage? Do you notice a trend forming?

2. Why does the LED get dimmer?

3. Does the amount of power consumed by the resistor and LED have any relationship to their resistances?

4. Using the relationship between voltage, current, and resistance could you find out the resistance of the LEDs? If so, what would be the resistance of the LED in series with the 470-Ohm Resistor?