When is a transistor bad

The body diode will show cathode at the drain for an N-channel device and at the source for P-channel device. The problem is that testing a FET with a modern multimeter alone becomes problematic.

The reason is most power FETs require a gate-source voltage bias of at least V to switch on. Logic-Level FETs can be switched on at voltages of between 0. A simple circuit shown here for an N-channel FET helps detect whether the device works correctly as a switch.

The multimeter should read a voltage between points 2 and 4 that is quite low. With points 1 and 2 shorted together, measure the voltage between point 2 and point 4, then short point 3 to point 4. You should see the voltage vary from low in first test to the actual battery voltage applied usually 9 V. To measure the nominal V gs threshold starting-to-turn-on voltage of the FET, short point 2 and point 3 and then measure the voltage between point 2 and point 4 as before.

All multimeter probing polarities will be reversed, but the same procedure still applies. Now consider JFETs. Checking a JFET as a diode gate-to-channel junction with an ohmmeter should indicate low resistance between gate and source with one polarity and a high resistance between gate and source with meter polarity reversed. If the meter indicates high resistance with both the polarities, the gate junction is open. On the other hand, if the ohmmeter indicates low resistance with both polarities, the gate junction is shorted.

Now consider testing continuity through the drain-source channel. Without this step, any meter reading of continuity through the channel will be unpredictable because a charge may or may not be stored by the gate-channel junction.

A good strategy to follow is to insert the JFET pins into anti-static foam prior to testing. The conductivity of the foam makes a resistive connection between all JFET terminals. This connection ensures all residual charge built up across the gate-channel PN junction dissipates, thus opening the channel for an accurate test of source-to-drain continuity. Because the JFET channel is a single, uninterrupted piece of semiconductor material, there is usually no difference between the source and drain terminals.

A resistance check from source-to-drain should yield the same value as a check from drain to source. This resistance should be relatively low below a few hundred ohms when the gate-source PN junction voltage is zero. Applying a reverse-bias voltage between gate and source should pinch-off the channel and produce a higher resistance reading on the meter. This brings us to bipolar transistors. The floating leads provide two test points, and the connected leads are a third center-tap test point.

These two diodes will not function as a real transistor because the center-tap connection is not a semiconducting junction, and the two-diode model does not have three separate silicon layers as in a transistor. However, the hookup demonstrates the basic concept involved in transistor testing and identifying the terminals. To test a transistor using a multimeter in the diode-test mode, insert the black probe into Common and the red probe into Diode Test or Ohms. Most manufacturers connect red to the positive terminal of the internal battery but this can vary, so it is best to check the polarity using a second multimeter in dc Volts mode.

The usual test voltage is 3 V. Read times. Posts: Total likes: 0. The bad tube thread was pretty interesting and got me thinking about a transistor thread. So I was wondering how you experts determine bad transistors while they're in circuits, are there any tricks to use multimeters to find issues?

Posts: Total likes: 45 Eddy M. I can usually spot bad transistors by the skull tattoos and dragon bandanas. If they're smoking, you can bet it ain't tobacco All children left unattended will be given a mocha and a puppy. Posts: Total likes: 3. Here is an article by R. Paul Marossy Awesome! Posts: Total likes: Just Another Guitarhead. However, this can also make bad transistor symptoms slightly easier to determine: if a circuit is failing to function properly due to either a shortage or excess of current, it is possible that a transistor has failed and should be tested.

Potentially faulty transistors can be tested with a digital multimeter, but the type of transistor will determine the type of test used. To begin, ensure that the circuit is disconnected from a power source, and then use a pair of needle-nose pliers to remove the transistor from the circuit. Next, twist one lead from the first resistor to the drain terminal on the transistor. Twist one lead from the second resistor to the source terminal on the transistor.

Twist the free leads from both resistors together with the gate terminal on the transistor. Wait 30 seconds, and then remove the resistors from the transistor terminals. For a p-channel JFET, place the red multimeter probe on the drain terminal and place the black probe on the gate terminal.

Check the multimeter display. If you need to test a bipolar junction transistor, you can follow similar steps -- but you will not need resistors.

For a PNP transistor, place the black multimeter probe on the base terminal, and place the red probe on the collector terminal. David Sandoval holds a degree in microelectronics, and has several years of experience in the technology field.