Advanced Automotive Fault Diagnosis
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LÔØI NOÙI ÑAÀU, ledklok
- Điều hướng trang này:
- 3.1.5 Logic probe This device is a useful way of testing logic cir- cuits but it is also useful for testing some types of Tools and equipment 27
- 3.2 Oscilloscopes 3.2.1 Introduction
| Meter resistance 10 M
⍀ With parallel combined value of 10 M⍀ and 100 k⍀ ⫽ 99 k⍀ the voltage drop in the circuit across this would be: 99/(100 ⫹ 99) ⫻ 12 ⫽ 5.97 V This is an error of about 0.5%. Note: This ‘invasive measurement’ error is in addition to the basic accuracy of the meter. Figure 3.3 shows a digital multimeter. Now that this is recognised, only two further skills are required – where to put the probes and what the reading you get actually means! 3.1.5 Logic probe This device is a useful way of testing logic cir- cuits but it is also useful for testing some types of Tools and equipment 27 V 100 k⍀ 100 k⍀ 12V Figure 3.2 Loading effect of a test meter Figure 3.3 Multimeter and accessories sensor. Figure 3.4 shows a typical logic probe. Most types consist of two power supply wires and a metal ‘probe’. The display consists of two or three LEDs labelled ‘high’, ‘low’ and on some devices ‘pulse’. These LEDs light up, together with an audible signal in some cases, when the probe touches either a high, low or pulsing voltage. Above or below 2.5 V is often used to determine high or low on a 5 V circuit. 3.2 Oscilloscopes 3.2.1 Introduction Two types of oscilloscope are available: analogue or digital. Figure 3.5 shows the basic operation of an analogue oscilloscope. Heating a wire creates a source of electrons, which are then accelerated by suitable voltages and focused into a beam. This beam is directed towards a fluorescent screen where it causes light to be given off. This is the basic cathode ray tube. The plates as shown in the figure are known as X and Y plates as they make the electron beam draw a ‘graph’ of a voltage signal. The X plates are sup- plied with a saw tooth signal, which causes the beam to move across the screen from left to right and then to ‘fly back’ and start again. Being attracted towards whichever plate has a positive potential the beam moves simply. The Y plates can now be used to show voltage variations of the signal under test. The frequency of the saw tooth signal, known as the time base, can be adjusted either automatically as is the case with many analysers or manually on a stand alone oscilloscope. The signal from the item under test can either be amplified or attenuated (reduced), much like changing the scale on a voltmeter. The trigger, in other words when the trace across the screen starts, can be caused internally or externally. In the case of the engine analyser triggering is often external, each time an individual spark fires or each time number one spark plug fires. A digital oscilloscope has much the same end result as the analogue type but the signal can be thought of as being plotted rather than drawn on the screen. The test signal is A/D converted and the time base is a simple timer or counter circuit. Because the signal is plotted digitally on a screen from data in memory, the ‘picture’ can be saved, frozen or even printed. The speed of data conver- sion and the sampling rate as well as the reso- lution of the screen are very important in ensuring accurate results. This technique is becoming the norm as including scales and notes or superim- posing two or more traces for comparison can enhance the display. A very useful piece of equipment becoming very popular is the PC based system as shown in Figure 3.6. This is a digital oscilloscope, which allows data to be displayed on a PC. The Scope tải về 7.07 Mb. Chia sẻ với bạn bè của bạn:
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