How to Use a Multimeter Better?
For readers and related designers, measuring the pins of a triode with a multimeter is no longer a difficult task. However, in fact, a multimeter can not only measure the triode but also determine its working state. This article explains in detail the relevant issues of using a multimeter to detect the working state of a triode.
For example, if a power amplifier has no output and the voltage between the base and emitter of one of the triodes measured with a multimeter is 0V (measured on the circuit board), does this mean the triode is damaged? Based on the given conditions, it is impossible to judge that the triode is broken based on the existing data. Two points need to be noted here: Whether the power amplifier output ensures that this triode acts as an amplifier or a switch. Additionally, if it acts as an amplifier, the bias circuit of the tube should first be checked (if there is no bias circuit, the base of the tube should be negative during normal operation).
So, can a multimeter determine the 好坏 (good or bad) of a triode on a circuit board? What are the voltages at each pole when the triode is working in different stages such as amplification, saturation, and cutoff? Obviously, this method can only be used as a reference. It is also necessary to detect the online resistance with the power off and even remove the triode for re-detection with a multimeter. As for the voltages at each pole of the triode:
In the amplification state: Uc》Ub》Ue (PNP) or (Ue》Ub》Uc (NPN). In other words, the emitter junction is forward-biased, and the collector junction is reverse-biased.
In the saturation state: The emitter junction is forward-biased; the collector junction is forward-biased.
In the cutoff state: The emitter junction is reverse-biased; the collector junction is reverse-biased.
The specific voltages are determined according to the actual situation but must always meet the above conditions.
When measuring the voltage between the collector and emitter and it is between approximately 0.3~0.6V, the triode is in the saturation state.
When the measured voltage between the collector and emitter is almost equal to the power supply voltage, the triode is in the cutoff state.
When the measured voltage between the collector and emitter is between 1.0V~(power supply voltage -1)V, the triode is in various amplification states.
The voltage between the base and emitter is 0, but it cannot be definitely concluded that the triode is damaged at this time. It needs to be analyzed according to the specific situation.
