Can the multimeter's diode gear illuminate the diode?
The multimeter has a diode range that can be used to detect diodes and turn them on. However, lighting up is not absolute, mainly due to two reasons: 1) the diode gear voltage of the multimeter is low; 2) The working voltage of the LED is relatively high.
The voltage of the diode gear of a multimeter is generally around 3V, which can light up ordinary light-emitting diodes. When measuring the quality of a diode, a red probe can be used to contact the positive electrode of the LED, and a black probe can be connected to the negative electrode of the LED. If the LED is on, it can be judged that the LED is good. However, some meters with low output voltage cannot light up LEDs or can only light up slightly. I have used a relatively cheap multimeter from Unilever before, but it cannot light up the LED. The diode range is basically non-existent, which is very inconvenient.
LED is a special type of diode with forward conduction voltage drop, and this parameter varies greatly. The conduction voltage drop of light-emitting diodes with different colors varies. Generally speaking, the conduction voltage drop of red light-emitting diodes is the smallest, with a range of approximately (1.5-2) V; Green comes second, around (1.8-2.5) V; The blue color has the highest voltage drop, around (2-3.5) V. So, when measuring LEDs of different colors on the same watch, their brightness is different, generally red is the brightest and blue is the darkest. Even some colors cannot be lit.
How to Check Leakage with a Digital Multimeter
One pen contacts the metal coating on the power socket, and the hand touches the metal part of the other pen. The meter clearly displays a voltage value of 119V! The metal coating on the socket is indeed live. After investigation, it was found that the fixing screw of the socket broke the plastic protective layer of the live wire and came into contact with the californium core. The screw pressed against the metal coating, causing the socket to be electrified. The internal resistance of the voltage range of a digital meter is mostly lOM Ω. The connection between the human body and the ground can be equivalent to a resistor (shoe, wooden floor series resistance) in parallel with a capacitor (distributed capacitance between the human body and the ground). The voltage on the live wire is transmitted to the ground through the DC resistance of the meter, meter, meter 2, human body, wooden board, and the human body distributed capacitance in parallel. The meter displays the internal resistance of the meter at both ends, where UDY is the live wire to ground voltage, RB is the internal resistance of the voltmeter, and Z is the total resistance between the human body and the ground. Obviously, the greater the internal resistance of the meter, the higher the sensitivity of the electrical testing. It must be pointed out that the above electrical testing method requires that there must be an electric current passing through the human body in order for the meter to indicate. Undoubtedly, it should be small enough that the human body cannot sense the presence of this current. The digital multimeter has an internal resistance of lOM Ω. Even when standing barefoot on damp ground, the current flowing through the human body is only 22 when the live wire voltage is 220V μ A. Pointer type multimeter is generally not suitable for electrical testing, as the internal resistance of the voltage range of such meters is much less than lOM Ω. Therefore, the sensitivity of electrical testing is very low. Attempting to use a low voltage range to improve the sensitivity of electrical testing is incorrect, as the low voltage range usually has a small internal resistance (the internal resistance of the voltage range of a pointer multimeter is multiplied by the Ohm value per volt). The sensitivity of the indication is almost not improved, but rather
