Advanced Automotive Fault Diagnosis
tải về 7.07 Mb. Chế độ xem pdf
|
LÔØI NOÙI ÑAÀU, ledklok
- Điều hướng trang này:
- Meter resistance 1 M
| 3.1.4 Multimeters
An essential tool for working on vehicle elec- trical and electronic systems is a good digital multimeter. Digital meters are most suitable for accuracy of reading as well as available facilities. The following list of functions, broadly in order from essential to desirable, should be considered. Function Range Accuracy DC voltage 500 V 0.3% DC current 10 A 1.0% Resistance 0 to 10 M⍀ 0.5% AC voltage 500 V 2.5% AC current 10 A 2.5% Dwell 3,4,5,6,8 cylinders 2.0% RPM 10 000 rev/min 0.2% Duty cycle % on/off 0.2% (kHz) Frequency 100 kHz 0.01% Temperature ⬎900°C 0.3% ⫹ 3°C High current clamp 1000 A (DC) Depends on conditions Pressure 3 bar 10.0% of standard scale A way of determining the quality of a meter, as well as by the facilities provided, is to con- sider the following: ● accuracy; ● loading effect of the meter; ● protection circuits. The loading effect is a consideration for any form of measurement. With a multimeter this relates to the internal resistance of the meter. It is rec- ommended that the internal resistance of a meter should be a minimum of 10 M⍀. This not only ensures greater accuracy but also prevents the meter from damaging sensitive circuits. Figure 3.2 shows two equal resistors connected in series across a 12 V supply. The voltage across each resistor should be 6 V. However, the internal resistance of the meter will affect the circuit con- ditions and change the voltage reading. If the resistor values were 100 k⍀ the effect of meter internal resistance would be as follows. Meter resistance 1 M ⍀ With a parallel combined value of 1 M⍀ and 100 k⍀ ⫽ 91 k⍀ the voltage drop in the circuit across this would be: 91/(100 ⫹ 91) ⫻ 12 ⫽ 5.71 V This is an error of about 5%. tải về 7.07 Mb. Chia sẻ với bạn bè của bạn:
|