Lesson Plan

Teacher: mentor, faculty, or senior electrical student Prerequisites: None Objective: Understand Voltage, Current, Resistance, Multimeters, and how they all relate to robots Materials required for every 2 students:

Additional materials (only one of each needed)

  1. Batteries
    • Briefly discuss redox reactions (ask who has taken chemistry)
    • Batteries as redox reactions with cell voltage determined by types of chemicals
    • Cells can be added together to increase voltage (e.g. 9v battery is actually 6x 1.5v cells)
    • All batteries have positive (red or +) and negative (black or -) terminals (show them on 9v battery)
  1. Voltage
    • Explain voltage as a measure of electrical pressure - how hard one side of the battery is trying to pull in electrons and the other side is trying to push them out.
    • Voltage is a potential, like water behind a dam - it isn't doing anything until it starts flowing
  1. Measuring Voltage
    • Turn on multimeters, black wire in COM, red wire in V, rotate knob to DCV (draw line+dashed line symbol)
    • Measure 9v battery
    • Measure other batteries
    • Test courage: be a multimeter and check 9v battery by tasting it
    • Explain safety as relates to voltage: voltage below around 17-40v is usually relatively safe with respect to electrocution because the body doesn't conduct it well. Higher voltages cause breakdown and allow the body to conduct well which is very dangerous. 12v batteries used on robots aren't dangerous because of electrocution but because they store a lot of power and when that power is released quickly, it can generate a lot of heat and cause severe burns.
    • SUPERVISED: measure 12v battery voltage, making sure meter probes don't touch each other.
  1. Current
    • Explain current as the flow of electrons analogous to the flow of water through a pipe
    • When current is flowing, work is being done, like allowing the water behind a dam or from a river to pass over a water wheel
    • Display the alligator jumper wires and discuss the fact that wire is made of metal (usually copper) which conducts electricity well because the metal atoms have many loosely held electrons and the battery can easily pull electrons off one end of the wire as long as it inserts them in the other end - creating a circuit.
    • Examine brake light bulbs; point out the two filaments.
      • Explain that filaments are wires
      • Explain how thin the filament wire is (double coiled)
      • Discuss brake light function: turn on headlights and the red lights turn on dimly, press brakes and light turns on bright.
      • Explain that one end of each filament connects to the shiny outer body of the bulb and that the other end connects to one of the gray terminals on the bottom of the bulb.
    • DEMONSTRATE: with two students:
      • one student uses an alligator jumper wire to connect one of the battery terminals to the outer shiny outer body of the bulb (holds connection in place)
      • the other student uses the other alligator jumper wire to connect the other battery terminal to one of the dull gray terminals on the bottom of the bulb
      • bulb lights up
      • try connecting to other terminal to see which filament is brighter
    • EXPLAIN:
      • Ask students what happened when the bulb was lit (they will observe that it got hot).
      • Point out that if they'd kept it lit for a while, even the wires start to get warm.
      • Because the filament is very thin, it presents a lot of resistance to the flow of electrons that resistance results in heat.
      • The wires are thicker than the filament so they get warm but not nearly as hot.
  1. Measuring Current
    • DEMONSTRATE: Move red multimeter wire to 10A hole and rotate knob to A
    • Explain that to measure current, the current must go through the multimeter so it can be measured
    • DRAW on whiteboard how to insert the multimeter in series with the bulb circuit:
      • alligator jumper from battery + terminal to multimeter red wire
      • multimeter black wire to bulb (e.g. a gray terminal on bottom)
      • alligator jumper from other blub terminal (e.g. shiny outer body) back to battery - terminal
    • Point out the circuit that allows electrons to flow from battery through meter, bulb, back to battery
    • Measure current drawn when each filament is lit
      • Students should note current drawn through bright filament (around 1.1 Amps)
      • Students should note current drawn through dimmer filament (around 0.3 Amps)
    • Explain that more current is needed to do more work (e.g. make a brighter light)
  1. Motors
    • Use tape to create a "flag" on the motor shaft so you can see which way it is turning
    • replace the bulb in the circuit with a DC motor
      • connect the black multimeter wire to one motor terminal
      • connect the other motor terminal to the wire that returns to the battery
    • Observe the direction the motor is spinning
    • Reverse the connections on the battery (or the motor)
    • Observe that the direction of spin reverses
    • Discuss that the direction a motor spins is determined by the direction the current flows
    • Point out that one of the roles of the Talon motor controllers on the robot is to control the direction of current flow so the robot motors can go forward and backward. The other role is to control how much current is flowing.
    • Measure the current flowing through the motor when it is spinning freely (~0.08 A)
    • Use fingers to slow down the motor (gently squeeze the spinning shaft) and observe current increase
    • Discuss that as the motor does more work and slows down it draws more current.
    • Show that if motor is briefly stalled it can draw as much as 1.5A
    • Explain that if the motor is kept stalled it will overheat and burn out
  1. Power
    • Explain Watts as the measure of power
    • Watts = Volts x Amps
    • Examples: 100W light bulb at ~100V draws around 1A of current
    • Note how hot the brake light bulb got when drawing ~1A at 9v
  1. Resistance
    • Show thick (6 gauge) battery/main breaker wire, thinner (12-14 gauge) CIM motor wire, thin (18-22 gauge) signal wire (switches, CAN bus, etc.)
    • Explain that the thickness of the wire is mandated by FIRST for safety: the more current the wire must carry, the thicker the wire. Since the wire from the battery and main breaker must carry all of the current used by the entire robot, it is thickest while the wire to any one motor must carry no more than 40A and so is thinner and the wire used for the CAN bus or limit switches/sensors where hardly any current is needed (because they are not doing any work) is thinnest.
    • Measure resistance:
      • Move multimeter red wire to hole with Omega symbol
      • Turn selector knob to Omega/Ohms? setting
      • Measure student resistance (hold probes tightly)
      • Measure resistance of an alligator jumper wire (~1 ohm)
      • Measure resistance of a thick battery/breaker wire (less than 1 ohm)
    • Make a resistor
      • Draw a very thick line on paper (put a lot of graphite down on the paper with a back and forth motion of pencil)
      • Measure resistance of line with multimeter
      • Slide probes closer together/further apart on line and observe change in resistance
      • Observe that the more graphite the electrons must path through, the greater the resistance
      • Draw a potentiometer on the whiteboard and explain that this is what is inside
    • Ohms law
      • Explain that voltage, current, and resistance have a fixed relationship: Voltage = Current * Resistance
      • If you know 2, you can determine the other; for example, if a 12v battery is connected through 12 ohms of resistance, 1 Amp of current will flow

Bringing it back to robots

  1. Robot Motors
    • Show CIM motor
    • Discuss free current (2.7A), typical max operating current (40A), and stall current (133A)
    • Recall how much heat the bulb generated with ~9W, calculate Watts at CIM stall current
    • Observe thicker motor wires to allow greater current without overheating
    • Observe that with a 12v robot battery, a stalled CIM would draw so much current that the wires would melt and could catch fire
    • Explain importance of not stalling a robot motor and why limit switches to prevent this are crucial

  1. Overcurrent protection
    • Show glass fuse (pass it around) and point out thin filament designed to melt if too much current passes through
    • Discuss circuit breakers: how different metals expand at different rates when heated and breaker is like a mousetrap with two different metals such that it will spring open when heated by too much current.
    • Discuss circuit breakers at home and purpose (protect house from burning down, not to protect people from electric shock)
    • Show 120A breaker and explain purpose
    • Show PDP and 40A snap fuses (one per motor) and explain purpose
Last modified 4 years ago Last modified on Nov 12, 2017, 2:49:49 PM