Why is it that when I use a multimeter to measure a good LED bead, it is not bright?
LED lamp beads, also known as light-emitting diodes, belong to one kind of diode, when using a digital multimeter to measure the LED lamp beads, generally use its diode gear measurement, the red pen connected to the positive pole of the LED lamp beads, the black pen connected to the negative pole of the LED lamp beads, and observe the brightness of the LED lamp beads (slightly bright) to judge by the naked eye. Sometimes, why is a good LED bead measured with a multimeter but not bright? There are two main reasons.
1, digital multimeter diode file no-load voltage is generally 2.8V ± 0.2V or so.
Recall that we often use the multimeter beeping gear to locate faults, such as measuring the wire through, etc., when the two test points straight through, the multimeter will beep. The buzzer gear measures the resistance between the two points, the resistance value is less than 70Ω or so when the multimeter will beep. Many multimeter diode gear and buzzer gear in a gear, through a key switch, in fact, its measurement principle is similar, a reality of the specific voltage value, a beep.
The principle of the multimeter diode file is the internal constant current source, the constant current source of the no-load output voltage of about 2.8V, different models of the multimeter the voltage value is slightly different, generally in the 2.6V ~ 3V between the constant current source output current of about 1mA or so, when measuring ordinary diodes, the red meter output current is about 1mA.
When measuring the ordinary diode, the red pen connected to the diode positive, black pen connected to the negative, the multimeter shows the voltage value of the diode voltage drop, silicon material diode is generally 0.7V or so, germanium material diode 0.3V or so.
2, LED lamp beads forward voltage drop 1.8V ~ 3.5V or so
Different colours and models of LED lamp beads with different conduction voltage drop, generally used as a power indicator light-emitting diodes in red, green and yellow, its conduction voltage drop of about 1.8V ~ 2.2V or so.
White and blue lamp beads its conduction voltage drop of about 2.7V ~ 3.3V or so, home lighting LED lamp beads are generally white LED lamp beads, its operating voltage of about 3V, 2.8V, 3.3V, 3.5V, etc..
Comprehensive analysis: understand the principle of the multimeter diode gear and the working voltage of the LED lamp beads can be seen, the multimeter diode gear maximum test voltage of 2.8V ± 0.2V or so, the measurement of the LED lamp bead working voltage within the voltage value, 1mA of the working current will make the LED light, but when the measured LED lamp bead working voltage is higher, for example, more than 3V, due to the output of the multimeter diode gear Voltage is not enough, the LED beads will not be slightly bright. This is why the LED beads are normal, but not bright when measured with a multimeter.
