Partisan Repair – Part Two: Diodes and Transistors, by DN

See Part One – Resistors and Capacitors


Diodes

Normally diodes only conduct in one direction, zener diodes will throw you for a loop but for now I wont go over those. Like how resistors typically go open when bad; diodes typically look like a closed circuit when bad or a short circuit….or they just explode. There is a bunch of special conditions that people will argue about but no one ever encounters them so whatever?

Meter is set to diode mode? “I think this diode is bad”? No flip your leads around!

There you go! Now it looks good! So most diodes you will be concerned with for now will be general purpose, rectifier diodes and power diodes. The side with the stripe is called the cathode or the (-). The other end is the anode or the (+). Like polarized caps if you put these in backwards bad things happen. Most diode we are concerned with will measure between 0.100-0.750 volts when good. The value we are measuring is the diodes forward voltage drop. For now check diodes in both directions; some people would be mad if I told you I typically don’t.

So here I am checking a double diode. It is 2 diodes in one package. It looks bad on the meter; but a lot of times these are in parallel with a transformer in a switching circuit. So you can de-solder the center leg to remove it from the circuit or pull it out.

Look how poorly I crop and resize my photos. But now out of circuit she is looking pretty good! Now check the other diode in the pair.

Other side looks good!

Transistors

There are a lot of different transistors out there. I am only going over the most common types I find bad, the kind that switches. Also time and opportunity has limited me. MOSFET, IGBTs are the most common types I find to go bad. You will find bad fet and Bi-polar transistors etc. But I will just go over MOSFET and IGBT, because checking them is generally the same. In circuit many times you may find them shorted but for now if you do remove them from circuit and check them.

N-channel MOSFET. Set meter to diode mode! Not shown in photo is you first short gate and drain pins to make sure the device is turned off. Here I have my black probe on the source and the red on the drain to show the device turned off.

Now my black probe is on the source and red on the gate. Good still looks like an open circuit.

Now something magic happened when I moved my red probe from the gate to the drain, is I can see the device is turned on and has a voltage drop of 0.428 volts. Personally  I still think this is all witchcraft but I cannot prove that with science. This happens because the meter produces a voltage in diode mode. Some meters cannot gate MOS devices!

Since we turned on the device we need to turn it back off. But some idiot will try doing this in a live circuit so use a screw driver or something conductive to short between the gate and drain. Just pretend my fingers are an alligator clip or a screw driver. FYI your fingers will also shut the device off! But don’t you use your fingers!

So for a P-channel MOSFET it is the same order but put your red probe on the source. You know all you got to do is flip them around.

IGBT? same way testing as the MOSFET, but this one is from a 3 phase inverter so it is a little more tricky. I am showing this because a lot of power electronics are going to these transistor modules and if you can understand this the rest becomes easy. Here are some schematics I borrowed from Infineon. But right now my black probe is on terminal 7,8 (that looks like the emitter from a bi-polar transistor) and my red probe is on 27 (what looks like the gate of a MOSFET

Now my black probe is still on terminal 7,8 and my red is now on terminal 30,31 (this terminal is connected to what looks like the all the collectors of the top row of transistors) and now the IGBT has turned on in this transistor module. So from the collector to emitter there is a voltage drop of 0.312

So now just repeat this process 5 more time to the rest of them. But just like the MOSFET example make sure all those IBGT’s turn off. Also notice there are diodes across each IGBT, these are usually the first thing to go bad so if this one diode across this IGBT were bad you would most likely see a short between terminals 7,8 and 30,31. Also in this device there is a 3 phase rectifier (on the left of the picture) and since you already know how to check diodes check them all individually; But as you can see if anyone component is bad you need to replace the whole thing. NOT SERVICEABLE!

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About the Author: Patriotman

Patriotman currently ekes out a survivalist lifestyle in a suburban northeastern state as best as he can. He has varied experience in political science, public policy, biological sciences, and higher education. Proudly Catholic and an Eagle Scout, he has no military experience and thus offers a relatable perspective for the average suburban prepper who is preparing for troubled times on the horizon with less than ideal teams and in less than ideal locations. Brushbeater Store Page: http://bit.ly/BrushbeaterStore

4 Comments

  1. Freewheeler February 2, 2022 at 07:44

    I have owned a Fluke 87 for about 25 years. It has served me well! Thanks for the articles!

  2. Taxpayer February 2, 2022 at 09:23

    Multimeters and Component testers (TC1 or one of the cheaper bare board ones) make great Gifts for kids.

    Besides getting up to speed with component testing NOW is a good time to get resoldering gear.

  3. Walt Westbrook February 2, 2022 at 16:48

    Oh yea, another reason to buy more tools…like I needed the prompt. Thanks for the continuing series. Please release a crib note when you are done!

  4. JB February 2, 2022 at 22:05

    As an adjunct to this series a list of relevant blogs are listed here:

    https://www.eevblog.com/wiki/index.php?title=Electronics_Related_Blogs

    I might add that one should understand the fundamentals of DC and AC electric circuits to troubleshoot circuits beyond that of testing individual parts. High school math will suffice for DC calculations and most AC circuits. At minimum Ohms Law is essential. Algebra and trigonometry will be needed for AC circuit analysis where reactances such as Capacitors and Inductors are present. Understanding how to measure voltage drops can minimize desoldering and potentially damaging the PCB.
    But soldering is a subject to itself. Todays electronics components are tiny and the use of magnifiers is essential as well as special solders and fluxes.

    Electrical safety. Low cost Chinese meters and test leads can be killers. They can and have blown up when used on typical AC line voltages of 220 volts. Buy UL listed meters.

    Some info here:
    https://www.eevblog.com/forum/testgear/a-list-of-multimeters-that-do-not-appear-to-meet-their-claimed-safety-specs/

    I have been working in this field for over 50 years on systems using microvolts to kilovolts, from audio to radio to radar and utility scale power systems. One will find an adventure chasing electrons and good paying employment. From electrical linemen to power plant technicians, jobs are calling. In my day the US Navy had the best electronics schools of the services. Today its the internet.

    The authors of this series deserve a gold star in stimulating interest in electronics by AP’s readers.

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