How to use a pointer multimeter to identify field effect tubes?

The following is a method for testing VMOS tubes.
 1. Determine the gate G. Set the multimeter to R×1k and measure the resistance between the three pins. If the resistance of a pin and its two pins is infinite, and it is still infinite after exchanging the test leads, it proves that this pin is the G pole, because it is insulated from the other two pins.
2. Determine the source S and drain D. As shown in Figure 1, there is a PN junction between the source and the drain. Therefore, the S pole and the D pole can be identified based on the difference in the forward and reverse resistance of the PN junction. Use the exchange test lead method to measure the resistance twice. The one with a lower resistance value (generally several thousand ohms to more than ten thousand ohms) is the forward resistance. At this time, the black test lead is the S pole and the red test lead is connected to the D pole. 3. Measure the drain-source on-state resistance RDS (on) Short-circuit the GS pole, select the R×1 position of the multimeter, connect the black test lead to the S pole, and the red test lead to the D pole. The resistance should be several ohms to more than ten ohms. Due to different test conditions, the measured RDS(on) value is higher than the typical value given in the manual. For example, using a 500-type multimeter with R×1 gear to measure an IRFPC50 VMOS tube, RDS(on) = 3.2W, which is greater than 0.58W (typical value).

 4. Check transconductance. Set the multimeter to R×1k (or R×100), connect the red test lead to the S pole and the black test lead to the D pole. Hold the screwdriver to touch the gate. The needle should have obvious deflection. The greater the deflection, the higher the transconductance of the tube. Note:
(1) VMOS tubes are also divided into N-channel tubes and P-channel tubes, but most products are N-channel tubes. For P-channel tubes, the position of the test leads should be swapped during measurement.
(2) A few VMOS tubes have protection diodes between GS, so items 1 and 2 in this test method are no longer applicable.
(3) There is also a VMOS tube power module on the market, which is specially used for AC motor speed regulators and inverters. For example, the IRFT001 module produced by IR Corporation of the United States has three N-channel tubes and three P-channel tubes inside, forming a three-phase bridge structure.
(4) The VNF series (N-channel) products currently on the market are ultra-high frequency power field effect tubes produced by Supertex Corporation in the United States. Their maximum operating frequency fp=120MHz, IDSM=1A, PDM=30W, and common source small signal low-frequency transconductance gm=2000μS. They are suitable for high-speed switching circuits and broadcast and communication equipment.
(5) When using VMOS tubes, a suitable heat sink must be added. Taking VNF306 as an example, the maximum power can reach 30W after the tube is equipped with a 140×140×4 (mm) heat sink. (
6) After multiple tubes are connected in parallel, the high-frequency characteristics of the amplifier deteriorate due to the corresponding increase in inter-electrode capacitance and distributed capacitance, which can easily cause high-frequency parasitic oscillation of the amplifier through feedback. For this reason, the number of parallel composite tubes is generally no more than 4, and an anti-parasitic oscillation resistor is connected in series on the base or gate of each tube.

Precautions for using field effect tubes (1) In order to use field effect tubes safely, the circuit design must not exceed the limit values ​​of the tube's dissipated power, maximum drain-source voltage, maximum gate-source voltage, and maximum current.
(2) When using various types of field effect tubes, they must be connected to the circuit in strict accordance with the required bias, and the polarity of the field effect tube bias must be observed. For example, the gate, source, and drain of a junction field effect tube are PN junctions, and the gate of an N-channel tube cannot be positively biased; the gate of a P-channel tube cannot be negatively biased, and so on.
(3) Due to the extremely high input impedance of MOS field effect tubes, the lead pins must be short-circuited during transportation and storage, and metal shielding packaging must be used to prevent external induced potential from breaking down the gate. In particular, it should be noted that MOS field effect tubes cannot be placed in plastic boxes. It is best to store them in metal boxes when storing them. At the same time, attention should be paid to moisture-proofing the tubes.
(4) In order to prevent the gate of the field effect tube from being induced and broken down, all test instruments, workbenches, electric soldering irons, and the circuit itself must be well grounded. When soldering the tube pins, solder the source first. Before connecting to the circuit, all the lead ends of the tube should be kept short-circuited to each other, and the short-circuit material should be removed after soldering. When removing the tube from the component rack, the human body should be grounded in an appropriate manner, such as using a grounding ring. Of course, if an advanced gas-heating electric soldering iron can be used, soldering the field effect tube is more convenient and safe. When the power is not turned off, the tube must never be inserted into or removed from the circuit. The above safety measures must be taken into account when using field effect tubes.
(5) When installing the field effect tube, pay attention to the installation position to avoid being close to the heating element as much as possible. In order to prevent the vibration of the tube, it is necessary to tighten the tube shell. When bending the tube pin lead, it should be 5 mm larger than the root size to prevent bending the tube pin and causing air leakage. For power field effect tubes, there must be good heat dissipation conditions. Because power field effect tubes are used under high load conditions, a sufficient heat sink must be designed to ensure that the case temperature does not exceed the rated value so that the device can work stably and reliably for a long time.

In short, to ensure the safe use of field effect tubes, there are many things to pay attention to and various safety measures to be taken. The vast number of professional and technical personnel, especially the vast number of electronics enthusiasts, must proceed from their actual situation and take practical measures to use field effect tubes safely and effectively.