Digital Storage Oscilloscopes
It has been stated that a picture is worth a thousand words, this is perhaps an understatement when working on a vehicle. In order to properly diagnose the vehicle systems a picture into the electronics is a necessity. Modern vehicle systems are based on electronic controls over mechanical systems. To diagnose failures within these systems we must first separate these systems from one another by analyzing the electronic control system. We need to look at the vehicles circuits for voltage change over time. This will be accomplished with an oscilloscope.
What is an oscilloscope? Webster’s Dictionary states; an oscilloscope is an instrument in which the variation in fluctuating electrical quantity appears temporarily as a visible waveform on a fluorescent screen of a cathode-ray tube. What is important with this statement is “variations in a fluctuating electrical quantity” or “variations in fluctuating voltage”.
So one would ask, what is voltage? Voltage is a unit of measurement based on an electrical force or the electrical pressure difference between two points of a conductor. What this indicates is the scope or any voltage measuring instrument is only displaying the difference of electrical pressure. So if both the positive and negative leads are connected to the positive post of the battery, the display would show “0” volts. On a charged vehicle battery both leads would have 12.6 volts on them, so in this case there would not be a voltage difference between the leads, thus display “0” volts. If the negative lead was then moved to the negative post of the battery the display would now show the source voltage of the battery, or “12.6” volts. Be aware the measuring device only shows the difference of voltage between the two leads. Where the leads are placed is the test!
When first connecting a voltage measuring instrument to the vehicle it will be important to connect the negative lead to the negative post of the battery, this will insure that you are on the lowest point of potential of the vehicle, thus providing you with accurate test data. The oscilloscope is a tool used to enter into another dimension of time. In our dimension of time you can move from one point to another point fairly quickly. For instance a Top Fuel Dragster can move through the ¼ mile in under 4 seconds. This takes a nitromethane supercharged engine that produces over 10,000 horsepower. Electricity, however, can go around the earth 7.5 times in just 1 second. This is far faster than the human brain can recognize. Therefore this electrical movement must be altered so we can recognize or see it. This is done by having the force that is pushing the electrons (voltage) leave a trace behind it as it changes over time as shown here.
This trace that is left behind is now in our dimension of time. This allows us to physically see the voltage movement in time, and is what will be displayed on the screen of the oscilloscope. It is important to understand what the voltage is indicating to you. Since voltage is only present to push electrons through resistance, if the voltage changes then the resistance within the circuit is also changed. This can be demonstrated by analyzing a good circuit from the point where the source voltage is present on the power side of the circuit to the point of the load or resistance. As soon as you pass the point of the load or resistance on the ground side there is very little voltage remaining. Voltage in a circuit is only present with resistance within that circuit or if the circuit is open. In the case of an open circuit source voltage is present to the point where the circuit is open. Each voltage change displayed on the scope screen will be indicative of what is occurring in the circuit. The oscilloscope display is shown in Figure 2.
As can be seen the screen is divided into two planes, that of a horizontal plane (X axis), and that of a vertical plane (Y axis). The vertical plane or up and down plane will display the voltage level. The horizontal plane or side to side plane will display time. These planes will be divided into 10 grids. These grids are referred to as graticules. A graticule is a network of lines representing meridians and parallels, on which a map can be represented. The voltage changes over time will be mapped on the oscilloscope display. Each line will represent a value; those vertical division lines will show voltage and those horizontal division lines will show time. These divisions can be adjusted by the controls on the oscilloscope. To adjust the scope settings, simply watch the waveform on the display and make changes to the voltage and time settings until the waveform is clear on the scope display. This is very similar to taking a picture with your camera. In order to take a good picture with your camera you adjust the lens while viewing the object. This lens movement will focus the light on the object and make a clear view of the object. It will not be necessary to know exactly where you will need to adjust the camera lens. Once the lens is adjusted the settings can be looked at to see where they are. This is similar to the scope settings, you do not need to know exactly where the scope needs to be adjusted to, just get a clear picture and then look at the setting to determine what the voltage and time of the signal being viewed is.
A scope is very simple tool to set up, don’t over complicate it. Another adjustment of the oscilloscope is how the data will be displayed. This can be setup with a trigger mode, roll mode, or non-trigger mode. The trigger mode is used to stabilize the waveform being viewed. These setting are adjusted by the channel, voltage level, voltage slope, and display position. The channel will represent which channel will be used for the trigger. The voltage level or trigger level will represent the voltage needed to break a threshold that will allow the screen sweep to occur. This is usually set to half of the voltage of the waveform being viewed. In other words if the voltage is moving from 12 volts to 0 volts the level would be set at 6 volts. The voltage slope will be set for a failing edge or a rising edge. In other words if the slope is set to a failing edge the trigger voltage that is set will have to be at a higher level and drop to a lower value in order for a screen sweep to occur. If the voltage slope is set for a rising edge the trigger voltage that is set will have to be at a lower level and rise to a higher value in order for a screen sweep to occur. The display position is the point on the display the voltage trigger level will start from. It is important to understand that a trigger can hide a failing circuit. This occurs when the set voltage level is not broken, and thus a screen sweep will not occur.
On all oscilloscopes, the last triggered event will remain on the scope display. Let’s say the waveform is present on the scope display and is active. If the circuit driver fails intermittently the trigger level is not broken and therefore does not make a screen sweep. However, the last screen sweep remains on the scope display. This will hide the failing circuit. This is why it is best to avoid the use of triggers. It will be best to use the strip chart roll mode. The strip chart roll mode acquires all of the data and displays it on the scope screen. The difference between the triggered mode and strip chart roll mode can best be understood by comparing a single shot camera with a video camera. If you are moving on a highway at 200 mph and take a single picture of the buildings passing by, then reset the camera and take yet another picture of the buildings it would be clear that there would be missing data between the points that the pictures were taken. However, if this same scenario took place but a video camera was used then no data would be missing between the buildings. All of the data would be present for your review. This is why it is best to use the scope in a roll mode and then go back through the acquired data to determine what caused the failure. It is important to have an understanding of what you are connecting to on the vehicle. The vehicle has many physical events that will need to be monitored in order to control a system. In order to monitor these events electrical sensors are used. An electrical sensor converts a physical quantity into an electrical output. The microprocessor can then use these electrical outputs from the sensors to control the system. Electronics are about timing. In order to see where the timing events are you will need more than one channel. One example would be using the oscilloscope to see the ignition coil fire. With only one channel, you could see the coil fire but could not tell where in space this occurred. For instance, if the coil fired at BDC the engine would not start, but with no other reference you would not be able to see where this event occurred, only that it had occurred. The oscilloscope takes the voltage and displays this in a graph format. In order to take advantage of this graphing format multiple channels will be used. These channels will then be compared to one another in order to determine if the events occurred at the correct time and in the correct sequence. When using the oscilloscope it is important to have an expectation of what will be displayed. By looking at a wiring diagram of the circuit under test you can determine what the circuit is most likely going to do. In Figure 3, if the positive lead of the measuring instrument is at the red dot and the ground lead is at the battery ground, you can anticipate what the voltage reading will be.
With the switch in position 1, which is an open circuit, the circuit will have source voltage on it. An open circuit will always have source voltage to the point where the circuit is open, and source voltage in this case is 5 volts. When the switch is moved to position 2, which is a closed circuit, the circuit will have 0 volts on it. When the switch is closed (Position 2) the test point is on the ground leg of the circuit, thus the voltage will be very close to 0 volts. If this circuit has a Negative Positive Negative (NPN) Transistor, which is one of the two types of bipolar transistors, consisting of a layer of P-doped semiconductor (the "base") between two N-doped layers, the collector and emitter. A voltage increase on the base is amplified to produce a large collector and emitter current. This NPN will have two circuit states open, and closed. Additionally the NPN can have a third state where it is partially on. This would depend on the base voltage level applied to the transistor. In Figure 4, if the positive lead of the measuring instrument is at the red dot and the ground lead is at the battery ground, you can anticipate what the voltage reading will be.
With the switch in position 1, which is an open circuit, the circuit will have 0 voltage on it. When the switch is moved to position 2, which is a closed circuit, the circuit will have 5 volts on it. When the switch is closed (Position 2) the test point is on the power leg of the circuit, thus the voltage will be very close to 5 volts. If this circuit has a Positive Negative Positive (PNP) Transistor, which is one of the two types of bipolar transistors, consisting of a layer of N-doped semiconductor (the "base") between two P-doped layers, the collector and emitter. A voltage decrease on the base is amplified to produce a large collector and emitter current. This PNP will have two circuit states open, and closed. Additionally the PNP can have a third state where it is partially on. This would depend on the base voltage level applied to the transistor. In Figure 5, the circuit is based on a voltage divider.
A voltage divider is a circuit which is based on two resistances in a series circuit. The voltage will be consumed or used as it pushes the electrons through each resistor. The amount of voltage used will depend on the size of the resistors. If the resistors are equal the voltage will divide equally between them. With 5 volts supplied to the circuit, if the positive lead of the measuring instrument is at the red dot and the ground lead is at the battery ground, you will read 2.5 volts on the circuit. If the first resistor is larger than the second resistor, the voltage will be smaller than 2.5 volts. If the first resistor is smaller than the second resistor the voltage will be greater than 2.5 volts. This circuit will produce an analog voltage output, or a voltage signal that continuously changes over time. The circuit is based on a potentiometer Figure 6.
A potentiometer is a variable resistance with a third adjustable terminal. When placed between a power source and ground source, the resistor will consume voltage. The third terminal is then moved on the variable resistor, thus producing an analog voltage output based on where the third terminal is positioned on the resistor. If the third terminal is closer to the power source the voltage output will be higher, if the third terminal is closer to the ground source the voltage output will be lower, and if the third terminal is centered on the resistor the voltage will be divided equally. The circuit is based on induction Figure 7.
Induction is based on the production of an electric current in a conductor by varying the magnetic field applied to the conductor. This is a non-contact sensing device that works through magnetic intensity. When a magnetic field is moving across a conductor it releases electrons, thus producing voltage which in turn produces current. Generally the sensor is made with a magnet and is wrapped with winds of a conductive wire. As the trigger wheel rotates, being made of a ferrous (magnetic) metal, the magnetic field moves. This magnetic field in motion produces voltage which pushes electrons through the circuit. This is used to produce an analog voltage that is proportional to the shaft rotation. As the shaft rotates each tooth or target produces a voltage output. This is used to calculate the shafts velocity. If an index tooth (e.g. missing tooth) is used, then the shaft position can be determined. Let’s look at Figure 8.
The circuit is based on magnetic intensity. This is accomplished by using a voltage regulator, a thin rectangular piece of indium arsenic, and a magnetic field. The magnetic flux imparts a force on the conductor (indium arsenic) which causes the voltage or holes (positive force) to drift to one edge while the electrons (negative force) drift to the opposite edge. The force that is exerted on the current flow is called the Lorentz Force. While the magnetic force is applied to the conductor, the carriers will stay at opposite sides. This sets up a voltage drop across the conductor. This voltage differential that is created is the Hall voltage. This Hall voltage is used to turn on or off a transistor which in turn produces a digital signal. Thus, this signal will be in one of two discrete states; off/on, 0/ 1, false/true. This circuit is one that operates like the circuit shown in Figure 3 above. When the transistor is off an open circuit is created so the voltage is that of source voltage, which in this case is 5 volts. When the transistor is turned on, the circuit is pulled to ground. Thus, this sensor produces a 0 to 5 volt signal. As the shaft rotates, each tooth or target changes the voltage of the circuit. This is used to calculate the shafts velocity. If an index tooth (e.g. missing tooth) is used then the shaft position can be determined. In Figure 9, the circuit is based on an inductor.
An inductor is also known as a coil or reactor. The inductor resists changes in electrical current. This occurs due to the nature of the current passing through a conductor which creates a magnetic field. If the current increases the energy contained within the magnetic field also increases, absorbing some of the current and thus stabilizing the current. If the current decreases the magnetic fields energy is put back into the conductor thus stabilizing the current within the circuit. When current is flowing in an inductor a magnetic field is produced that is proportional to the current flowing through the circuit. This magnetic field is used to move or lift a pintle off of its seat. This pintle is used to seal the solenoid or to open the solenoid. When the circuit is open the voltage will be that of source voltage. When the circuit is closed the voltage will drop very close to ground. When the circuit closes, current flows through the circuit creating a magnetic field. This magnetic field lifts the pintle off of it seat so the solenoid can establish flow through it. When the circuit is opened the mechanical spring pushes the pintle back onto its seat, thus closing it. Additionally when the circuit is opened the current is shut off. The stored magnetic field around the conductor windings falls back into the conductor to stabilize the current. This magnetic field in motion crosses the inductor’s windings which induces voltage into the conductor. This produces a high voltage spike which is referred to as fly back voltage.
Learning to use an oscilloscope is just like anything that you want to do well, repetition makes you proficient at it. It is important to use your oscilloscope every day. Have an idea of what the circuit you are connecting to is going to do. Once you start to see what the picture on your scope is telling you, you will be able to speak the language of electronics. This language will teach you many things and soon you will wonder how you ever repaired a vehicle without knowing this hidden language.
Bernie, I haven't had a chance to read the whole article, but there is something not right with how the PNP transistor schematic is drawn... hope it will be easy to correct.