Week Three Blog

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Week Three Blog:

1. Compare the calculated and measured equivalent resistance values between the nodes A and B for three circuit configurations given below.

For the three circuits below we found the equivalent resistance value by calculating the resistance value and comparing our results with the measured reading from the DMM. Our resistors for the circuit are:

        R1: 100 Ω

        R2: 220 Ω

        R3: 120 Ω

        R4: 41 Ω  

The three circuits we used are:

Our results were:

2. Apply 5V on a 120 Ω resistor. Measure the current by putting the multimeter in series and parallel. Why are they different?


When measuring currents over a resistor you must connect the leads in series with the resistor. If you connect the leads in parallel the measured current will be zero because the current follows the easiest path which is through the resistor, completely surpassing the DMM. We proved this experimentally. When we measured the current in series, it came out to 40.7mA. When we measured the current in parallel, there was no current.

3. Apply 5V to two resistors (47Ω and 120Ω) that are in series. Compare the measured and calculated values of voltage and current values on each resistor.









4. Apply 5V to two resistors (47Ω and 120Ω) that are in parallel. Compare the measured and calculated values of voltage and current values on each resistor.

5. Compare the calculated and measured values of the voltages of the following current and voltage for the circuit below:

       a. Current on 2kΩ resistor,

       b. Voltage across both 1.2 kΩ resistors.

The picture below is the actual circuit that the diagram above is showing.

a) 

 b)

6. What would be the equivalent resistance value of the circuit above (between the power supply nodes)?

The above circuit has a measured equivalent resistance of 0.563kΩ.    

7. Measure the equivalent resistance with and without the 5V power supply. Are the different? Why?

Without the power supply, the DMM reads that the equivalent resistance is 0.563kΩ. But with the power supply adding 5V to the circuit, the DMM reads 0M for the equivalent resistance. This difference comes from how the DMM measures the resistance. The DMM supplies a small current to the circuit and measures the resulting voltage. If you apply an outside voltage to the circuit it throws off the measurement, and if you add 5V like we did, the reading is 0 because the voltage being applied to the circuit by the power supply is greater than any resulting voltage the DMM would have used.

8. Explain the operation of a potentiometer by measuring the resistance values between the terminals (there are 3 terminals, so there would be 3 combinations).

The potentiometer operates by spinning the dial to increase or decrease the resistance value. This only works if the two connections you have to the potentiometer has one being the center lead. Think of a 10kΩ potentiometer as a horse shoe with a total resistance of 10kΩ, at one end of shoe is lead 1 and at the other is lead 3. Lead 2 is attached to the dial and the piece the moves. Say for example you attach the DMM to leads 1 and 2, the resistance value will increase when you move lead 2 away from lead 1 and decrease when you move it closer.
Below is the measured resistance values for each pin combination and at three dial placements:

Below is a video explaining how we measured the resistance values on each lead of the potentiometer. Notice that if you add the resistance value from lead 1 to 2 and 2 to 3 together you get an overall resistance value that is around 10kΩ.

9.  What would the minimum and maximum voltage that can be obtained at V1 by changing the knob position of the 5kΩ pot? Explain.

The minimum and maximum voltage values obtained in V1 are both 5V because the voltage drop across the resistance should be equal to the input voltage. When there is only one resistor, no matter its size, it will have a voltage drop value equal to the voltage input. Since the input voltage is always 5V, the voltage drop across any resistance value of the potentiometer is 5V.

10. How are V1 and V2 (voltages are defined with respect to ground) related and how do they change with the position of the knob of the pot?

V1 and V2 will always equal the input voltage when added together, in this case the input voltage is 5V so V1 + V2 = 5V. When the position of the knob of the pot is changed so that the potentiometer has a higher resistance value the voltage of V2 increases and the voltage of V1 decreases respectively. Likewise when the potentiometer has a lower resistance value the voltage across V1 increases and the voltage across V2 decreases respectively. Below is a video showing this happening.

11. For the circuit shown in the video below, YOU SHOULD NOT turn down the potentiometer all the way down to reach 0Ω. Why?

Because when you turn down the potentiometer that far you are effectively shorting the voltage source creating a very high current which will destroy your potentiometer.

12. For the circuit shown in the video, how are current values of 1 kΩ resistor and 5 kΩ pot related and how do they change with the position of knob of the pot?

The current value for 1kΩ will be 5mA, this value will not change because the voltage across the resistor does not change and the resistor's resistance does not change. The current across the 10kΩ pot will vary based on the resistance value of the potentiometer at that time. The currents across the 1kΩ resistor and 10kΩ will equal the current measured at the voltage source, but even that current will change based on the resistance value of the potentiometer changing the equivalent resistance of the circuit.

13. Explain what a voltage divider is and how it works based on your experiments.

A voltage divider consists of two or more resistors in series to separate the input voltage into different controlled voltages. The voltage over each resistor is a fraction of the input voltage. If all of the voltages are added together, they are to equal the input voltage.

14. Explain what a current divider is and how it works based on your experiments.

A current divider consists of two or more resistors in parallel to separate the current through each resistor. The current through each resistor is a fraction of the input current. If the measured current through each resistor is added together, it will be equal to the input current.

Week Two Blog

The A/B switch does not switch between voltage sources. It switches between which voltage source is to be measured on the meter. If the switch is on A, source A is shown on the meter. If the switch is on B, source B is shown on the meter. Even if the switch is turned to B, source A will still give an output.

The current specification for the power supply is either 0.5 A or 4 A, which means that the channels can give a maximum current of either 0.5 A or 4 A. The fixed channel has a maximum current of 4 A while A and B have a maximum current of 0.5 A.

The power supply has two main operation modes, tracking and independent. The independent operation mode keeps all three channels separate and three different voltage values can be put out at one time. The tracking operation mode allows the user to wire the A and B channels together, either in series or parallel. Wiring in series will double the voltage. Wiring in parallel will double the current. Below is a video of the different ways you can wire a power supply.

You can generate a 30 V output by wiring the two channels in series and setting the voltage to 15. When connecting the leads you connect the positive to positive and negative to negative.

To generate a -30 V output, the wiring remains the same except for the fact that the positive of the DMM is to connect to the negative of the power supply and the negative of the DMM is to connect to the positive of the power supply.

You can generate both a positive and equally negative voltage at the same time if you ground the positive of B, connect positive of DMM1 to negative B, connect positive B to negative DMM1 and negative DMM2, and lastly connecting positive A to positive DMM2. Basically, the entire power source is wired in series. 

When 5 V are applied to a 100 ohm resistor, both the power supply and the DMM read 50 mA. When adjusting the current knob, the LED lights on at just under the left horizontal. If straight downwards is set to be 0 degrees and the knob moves clockwise, the LED would light on at a little less than 90 degrees. If the value of current limit is decreased with the current knob, both the voltage and current decrease linearly.

The fuses for both the DMM and the power supply are located on the back of the mechanisms, under the power cables. Fuses are used on this equipment to ensure that the amount of current flowing through the machine is not too high to cause damage to the machine. A fuse will blow and stop the current from flowing if it becomes a dangerous amount.

When taking resistor measurements, there is a 2 wire option and a 4 wire option. The 2 wire function reads the resistance directly on an ohmmeter. The 4 wire function reads voltage with 2 wires and current with 2 wires, then uses ohm's law to calculate the resistance. 4 wire seems to be more accurate on a smaller resistance scale.

You should work safely when measuring a large amount of current, as it can be very dangerous. To measure current on a DMM you should break the loop to allow the current to flow through the DMM and back through the circuit. 

Week One Blog

Monday:

The lab is formatted so that every week begins with a pre-quiz. After the pre-quiz is completed there is to be a quick lab introduction to introduce ideas to know for the following week. The remaining time involves students completing said lab. Students discuss blog entries on Friday and end the week with a post quiz.

The important safety tips for the classroom are:

• Fire Extinguisher is located in the corner of the room by the right door.
• First Aid kit is located by the left exit door, behind the podium.
• The telephone in the classroom is located on the podium.
• If there is an emergency call 911.
• If equipment seems defective, let the instructor know before continuing use if deemed safe.
• Keep all work areas clean and clear of all unnecessary parts.
• Make sure power is off when working on circuits.
• Always ground your circuit properly.
• Do not attempt to work on a circuit with wet hands.

These safety tips are very important because current can kill you if it exceeds 100 mA.


Reading resistors is a simple task as long as you know the process used to read them. Below is a video explaining how to read both five band resistors and four band resistors.

The tolerance of a resistor is the determined range of error between the resistance value printed on the resistor and the actual resistance value. For example, none of these resistors have the exact resistance value that is printed, but they are all within the tolerance.

Wednesday:

When measuring current using a DMM you have to break the loop to get the current to flow through the DMM. If not, the current will remain in the circuit because that path is easier to take. With measuring voltage, the loop must remain intact to allow a current to flow. If the circuit is broken, no current will flow causing there to be no voltage. 

At any given time you can have three different voltages being supplied by the power supply. One is fixed at 5V while the other two can vary from 0V to 25V.

5.04V 47.7mA

Measuring Voltage with a handheld Digital Multi Meter. When measuring voltage, the circuit must create an independent loop in order for the voltage to be anything but zero.

Measuring Current with a handheld Digital Multi Meter. When measuring current, the circuit must be broken before the reading is taken otherwise the measured value will be zero.

Ohm's Law states that V=I*R or Voltage = Current * Resistor, and can be proven by measuring the current through the same resistor using multiple voltages. These two values can be plotted on a graph to show a linear correlation between them. The slope of the resultant line is the value of the resistor.

Here is an experiment proving Ohm's Law with two different resistors:

Below is our first Rube Goldberg circuit using a photo-resistor and DC Motor.

Friday:

This is the circuit diagram for the above Rube Goldberg design.

This circuit can be used for different types of mechanical functions. We decided to make a quick Rube Golberg design in which a flashlight is used to make the motor/fan spin at a faster rate. The fast spinning motor is attached to a string, which will lift a door and the marble will begin to move downwards on a ramp. At the bottom of the ramp is a basket sitting on a pressure sensor. When the marble lands in the basket, the pressure sensor will engage and allow power to flow to a fan, which will keep us all cool!