Section Electrical Circuit Theory Introduction The first step in being able to effectively diagnose and repair automotive electrical circuit problems is to have a fundamental understanding of basic electrical principles, as well as identify High Voltage and SRS circuit wiring that should not be repaired We’ll begin with basic electrical circuit theory, defining the terms of electricity: • Voltage • Current • Resistance Then we will introduce Ohm's Law and explain how it can be applied when diagnosing electrical circuit faults At the conclusion of this section you should be able to: • Define the terms Voltage, Current, and Resistance • Identify the variable values of electricity • Explain the principles of Ohm’s Law G-W G-W (USA) (USA) G J 8(A), J 9(B) Junction Connector C A A G G G A A G G J1 Junction Connector G (Canada) 13 Tail 2 L2 License Plate Light RH L1 License Plate Light LH R11 Rear Combination Light RH Tail 1A F4 Front Parking Light RH Tail C9 Combination Meter (USA) 18 R9 Rear Combination Light LH G 15 G D2 Daytime Running Light Relay T F3 Front Parking Light LH G-B (USA) TAIL G-W C A G G 14 100A ALT G (USA) (Canada) G-W G B B ID C A IB (Canada) IC W IL G 1C B B TAIL Relay 15A TAIL G G IH 10 W-B W-B W-B Tail Head C12 Combination SW (Canada) B Light Control SW OFF G A A W-B W-B 18 4B W-B 11 4B W-B W-B FL MAIN 0L (USA) (Canada) A A A A J 8(A) Junction Connector W-B W-B 21 4B Battery A IE J6 Junction Connector W-B W-B ED EA BI Figure 1-1 T502f101 BH Figure 1-1 T502f101c Body Electrical Diagnosis and Repair Course T502 1-1 Section Voltage Voltage is electrical pressure, a potential force or difference in electrical charge between two points, one point positively charged and one point negatively charged A chemical reaction in the battery creates a difference in potential between the positive and negative terminals in an automotive application The pressure differential provided by the positive and negative terminals of the battery causes the electrons in a conductor to move when the two terminals are connected This movement or flow of electrons is used to perform useful work Whenever work is done, pressure is used up We can measure where work is done in an electrical circuit by measuring where voltage is used up Figure 1-2 T502f102c When working with automotive electrical systems, there are three applicable voltage measurements; source voltage, available voltage, and voltage drop Voltage drop being the most useful, because it is a measure of exactly how much voltage is used up in any part of a circuit Source voltage is typically the battery voltage, available voltage is the amount of voltage available to operate a circuit, and voltage drop is a measure of voltage used Voltage measurements will be explained in much more detail in the sections ahead 1-2 TOYOTA Collision Repair and Refinish Electrical Circuit Theory Current Current is the term used to describe the flow of electrons through the circuit It is this flow of electrons that does the “work” in the circuit The unit for measuring the amount of current flow is the Ampere or Amp (A) One Amp is equal to billions and billions of electrons flowing through a circuit per second Current will only flow if there is a complete circuit (continuity) between a source of higher voltage (power) and a lower voltage (ground) Voltage is the pressure that pushes the electrons through the circuit and Amperes is a measure of the number of electrons flowing Figure 1-3 T502f103c Resistance Electrical resistance describes how much something opposes current flow This opposition to current is measured in Ohms (Ω) or in thousands of Ohms (kΩ) In every circuit, resistance regulates current In a "perfect" circuit, the only resistance would be the load being operated like a bulb or a motor But even the best conductors (circuit wiring) have a certain amount of electrical resistance Figure 1-4 T502f104c Body Electrical Diagnosis and Repair Course T502 1-3 Section Ohm’s Law A simple relationship exists between voltage, current, and resistance in an electrical circuit; understanding this relationship is important for accurate electrical diagnosis Ohm’s Law states that current in a circuit is directly proportional to the applied voltage and inversely proportional to the amount of resistance This means that if voltage goes up, the current flow will go up and vice versa Also, as the resistance goes up, the current goes down Mathematically, we can always determine what electricity is going to in a circuit, as long as we know what any two of the three values are So, for example, if you know the voltage and resistance values of a circuit, you can easily calculate how much current there should be Ohm’s Law Equation The important fact about Ohm’s law is that electrical systems and electrical problems are predictable There is no magic to how electricity works! Voltage = Amperage x Resistance Amperage = Voltage Resisitance Resistance = Voltage Amperage Amps Resistance Amps Resistance When Resistance goes up, Current Flow goes down When Resistance goes down, Current Flow goes up Figure 1-5 T502f105 1-4 TOYOTA Collision Repair and Refinish Electrical Circuit Theory Note: In most modern publications on electricity, Voltage is represented by the letter “E”, Current by the letter “I”, and Resistance by “R” However to simplify the principle for this training we will be using the letters V and A, and the symbol Ω V – Electromotive Force (Volts) A – Intensity of Current Flow (Amperes) Ω – Resistance to Electrical Flow (Ohms) Basic Principles of Series/Parallel Circuits Series Resistance Ohm's Law can be used to calculate resistance values in circuits This can prove to be very helpful during troubleshooting by determining what should be as opposed to what is Series circuit resistance is probably the easiest calculation to understand because it is simply adding the sum of resistances to find total resistance Figure 1-6 illustrates this method Figure 1-6 T502f106c Body Electrical Diagnosis and Repair Course T502 1-5 Section Parallel Resistance Total resistance of a parallel-series circuit is calculated slightly different than series resistance, and will effect the amount of amperage or current that can flow through the circuit In figure 1-7, we have a parallel resistance rule at work The total circuit resistance will always be less than the smallest resistance To find total resistance you must treat each branch as an individual series circuit 12V divided by 2Ω equals 6A current per branch Adding the branch currents, 6A plus 6A equals 12A total current Dividing 12V by 12A equals a total parallel circuit resistance of 1Ω Notice that this number is smaller than the smallest branch resistance Figure 1-7 T502f107c Series/Parallel Resistance Series/parallel circuit resistance can be a more challenging calculation Figure 1-8 T502f108c The first step is to calculate the total resistance of the parallel portion of the circuit The formula is: 1 Ω1 + Ω2 Using values from the example: 1 + = 25 + 25 = = 2Ω Adding the total resistance of the parallel portion, 2Ω, to the 2Ω from the series portion equals 4Ω total circuit resistance 1-6 TOYOTA Collision Repair and Refinish Electrical Circuit Theory Why Does It Matter? Knowing the formulas which arise out of Ohm’s Law is not necessarily helpful for repairing automobiles A person can know theory, but not know how to apply it in the real world However, a knowledge of the relationships between these elements is essential to a Technician You need to be able to predict what should be as opposed to what is in a problem vehicle We are going to use the concept of voltage drop in this class to make this kind of diagnosis quicker and easier Later, as you become more familiar with electrical theory and concepts, there may be an opportunity to more advanced calculations like the ones we have discussed here On-Vehicle The “math” side of Ohm’s Law is important if we are designing a circuit Applications But because we are in the business of repairing electrical problems, of Ohm’s Law what we need to know about Ohm’s Law can be summarized Since most circuits (and basically all body electrical circuits) work on what is called 12V power (which is actually more like 12.6 – 13.6V) we will look at Ohm’s Law with the voltage held constant Assuming this fixed voltage, we can summarize Ohm's Law as: Circuit electrical values are predictable; "When resistance goes up, current goes down When resistance goes down, current goes up.” This is the heart of Ohm’s Laws when it comes to servicing a vehicle Knowing the principle of Ohm’s Law plays an important role when diagnosing an electrical problem However, a Technician will rarely use a calculator to fix an electrical problem The importance of Ohm’s Law is that it provides the foundation for being able to understand and predict how an electrical circuit will respond • If a circuit is inoperative, and there is no current in the circuit, it means that there is an infinite amount of resistance or an opening somewhere in the circuit • If the circuit is partially working (such as when a bulb is dim), and the load is not receiving full battery voltage, it is probable that there is excessive resistance in the circuit The excessive resistance can be located either in the load itself or in series somewhere in the circuit Or, the battery may not be delivering 12V Body Electrical Diagnosis and Repair Course T502 1-7 Section hint: If the battery voltage is low, more than one circuit will be affected • If a 20A fuse is blown, we know that a lot of current must have been flowing in the circuit In order to get a 20A fuse to blow at 12V, there must be very little resistance (0.6Ω) in the circuit This condition is caused by too many loads connected to the fuse, aftermarket accessories or a possible short-to-ground Shorts-toground provide either no or a very low resistance path to ground before the load, allowing current to flow unregulated In this case, the resistance of the load has been removed Because of Ohm’s Law the actual circuit does not need to operate in order for us to know how much current (if any) there will be or where it is going to flow Ohm’s Law makes current predictable So instead of checking every component, connector or wire on the vehicle, a technician can use the circuit wiring diagram to determine where current flow should be, and which specific area could be causing the problem This type of diagnosis eliminates unnecessary checks during the repair process It saves the Technician time 1-8 TOYOTA Collision Repair and Refinish Electrical Circuit Theory Body Electrical Diagnosis and Repair Course T502 1-9 ... voltage used Voltage measurements will be explained in much more detail in the sections ahead 1-2 TOYOTA Collision Repair and Refinish Electrical Circuit Theory Current Current is the term used... Current Flow goes down When Resistance goes down, Current Flow goes up Figure 1-5 T502f105 1-4 TOYOTA Collision Repair and Refinish Electrical Circuit Theory Note: In most modern publications... “E”, Current by the letter “I”, and Resistance by “R” However to simplify the principle for this training we will be using the letters V and A, and the symbol Ω V – Electromotive Force (Volts)