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SMPS repair series #9: Zener diode and power transistor without …

SMPS repair series #9: Zener diode and power transistor without ...

Today we will look into the behavior or a basic linear regulator circuit based first of a Zener diode, after which we extend the circuit with a power transistor. The goal is to understand how do these devices behave in a simple configuration, and how are they used in practice. For this, we will build simple circuits on a prototyping board. Also, we will look at the regulated output voltage as a function of the input voltage and comment on the relative stability of the regulator circuits.

Although Zener diodes offer a relatively stable output voltage, the Zener voltage will slowly drift because of two major effects: the Zener diode will heat up, changing the characteristics of the device, and also the avalanche current will gradually increase as we increase the input voltage. In the 1980s thermally compensated Zeners were still used in liner power supplies. However, due to this drift of the output voltage, Zener diodes are rarely used in modern circuits. In fact, in switch mode...


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  1. Date: December 5, 2018 at 22:38
    Author: William Otis

    These are great videos. I learned electronics initially in the U.S. Navy, took some additional electronics after I got out, and worked many years as a bench tech, and later a field service tech. These are a great refresher for me. Of course I'm retired now, but still interested in electronics, and I have repaired a vintage Kenwood receiver that was sitting around for 20 years collecting dust. It works great now. I'm also a computer "geek." It's just so much fun to watch these videos and renew my skill.

  2. Date: December 6, 2018 at 00:01
    Author: YouGenom

    After watching your videos, I feel like I fully understand the topic you are covering. Great series!

  3. Date: December 6, 2018 at 00:04
    Author: Ahmed Ali

    i have been watching your videos ftom a long time ago

  4. Date: December 6, 2018 at 06:47
    Author: Oerjan Snell

    Oh man, I don't know if I got the energy to wait for the next one :p

    Sooo good series and I hope this one will not be cut as the others I've seen.

  5. Date: December 6, 2018 at 15:39
    Author: Jo Marcha

    Many Thanks, a pleasure to watch & learn best regards

  6. Date: December 6, 2018 at 20:45
    Author: j5p332

    Mr. Donkey, have you given any consideration for starting a patreon aimed at helping people who want to repair SMPS with practical examples and such? I, for one, would happily contribute

  7. Date: December 7, 2018 at 18:21
    Author: Rick Smyser

    Great at explaining things as usual. Thanks again for your invaluable resource and contributions .

  8. Date: December 9, 2018 at 15:34

    are you going to show us the error amp in a future video? also a TL431 will be nice to substitute the zener diode and show us the measurements. i hope you will end up showing us a high amp linear power supply with a transistor and maybe an LDO/tl431 for setting the voltage we like !
    if you can i would like to know why the emmiter "tries" to follow the base voltage.
    thank you!

  9. Date: December 11, 2018 at 16:53
    Author: Eli Tech

    you have nice channel too, about patreon it you can create ties which buy reaching amount you will record something special and so on. Just read it, there a lot info about it.
    But of course you can create without any pushings for you

  10. Date: December 22, 2018 at 02:20
    Author: Kirk Barron

    Great series, looking forward to the next. Thanks for your time and efforts.

  11. Date: January 5, 2019 at 04:42
    Author: John Conrad

    i THINK YOUR SCHEMATIC HAS AN ERROR AT 0:55 as well as at the end. IT SHOULD BE NEGATIVE FEEDBACK. THE REFERENCE MUST GO TO THE + INPUT OF THE AMPLIFIER. The way it is now, if the output goes up the EA output also goes up and the p/s output will also go up, so no control. probably you missed that.

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