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BD139 Alternate replacement or surface mounted? help — Parallax Forums

BD139 Alternate replacement or surface mounted? help

Hello everyone. So recently I've been interested in learning more about inductive coupling. A wireless power circuit I'm trying to build calls for a transistor (BD139 in particular). I am salvaging the parts I need for this project as for I am not able to order or go buy parts atm. My main question is, is there a replacement for the BD139 that would meet the same parameters as this transistor? Providing surface mount equivalent would be better but full size is fine also.

maybe I'm being lazy but in my mind asking is the most logical choice. I read and learn, often I have to break my concentration on my study material so I can go study a related subject before I can continue what I was studying. I have a decent understanding of how transistors work.
thanks in advance


  • Wow, that part is a blast from the past! I use devices in SOT-23 packs that far exceed the capability of the BD139 but don't be too lazy and forget to post the schematic/link to the circuit so don't have to go to the "effort" of finding out that the recommended replacement doesn't work. It may be that a low Vce(sat) NPN may do, or a MOSFET would be even better perhaps.
  • Mike GreenMike Green Posts: 23,031
    edited 2019-01-29 01:25
    Here's a link to ON Semi's page for the BD139. Looks like it comes in a couple of through-hole packages.
  • Thank you so much for your fast response! this forum is great as always. this is an image of the circuit.
    The only reason I want to build this is so I can study it's design and make my own wireless power circuit but I need to fully understand how it works and for the most I do. However not fully and the value of the parts is what I'm more interested in.


    I do so many things in and out of the world of electronics and its all just too much to try and retain so I try and skip remembering things that won't have a repetitive use or reference.

    So im not sure about the working of a mosfet (I wanna say I learned about that years ago). If i remember correctly its a logic switch right?
  • The fact that they use a 9V battery shows have basic and limited this "wireless power circuit" is. The more important aspects of this circuit besides the frequency and drive method is the construction of the inductors and orientation and effective distance etc. Lighting an LED without any series resistance already "indicates" that is circuit will probably only deliver milliamps (they could also have skipped the series diode and used another reverse LED in parallel).

    Since it uses a 9V battery you may as well use a 2N2222 or similar (only the Vceo is important here really). Goodness knows which ark they got this circuit from. Expect the results to be very poor, you will need to have the windings very close together and poor efficiency.

  • Awesome thats what I was looking for, I have an understanding of how it works I think.
    I have been working on building coils using different material and methods but have no way to test.
    So my next question is do you have a good idea of direction to point me in for learning more about this?

    All of the examples I see online either are fake videos (The concept on which they build the circuit is not valid-guessing they make the videos for likes), or nice circuits with lack of explanation and what seems to be the most annoying is people are taking apart the QI standard chargers and putting the parts in other devices. That is not MAIKING your OWN wireless charging circuit, furthermore when I look into the QI chargers build construction and operation I noticed that there must be some handshaking going on and continuously but idk- its not a simple give and take, there definitely seems to be some conformation of information before the circuit kicks into full power. However there is some power present and I know because I tested it with a toothbrush-I don't have ANY lab items to dig deeper.
    Anyway any help or direction would be great, I have no need or want for my circuit to transmit any information. I appreciate any help. thank you
  • These days you'd use a microcontroller as the oscillator, and MOSFET as the power device. Then the transformer
    doesn't need a second primary winding.
    When you have a microcontroller in play there's a lot of scope for monitoring things like current voltage and power and
    doing things like auto-switch-off, adjusting oscillation for maximum efficiency/resonance.

    Using an old BJT as an free-running oscillator is ancient tech these days, kind of archeology :)

    One thing to get right is the quality of the tank circuit capacitor and inductor - they need to be low loss as
    they will be resonant to get good energy transfer to the secondary, which means you want quality plastic film
    capacitors and low resistance in the coils (litz wire is often used). That way you get most of the power into the
    receiving circuit, not heating the capacitor or coils. You have to exclude any iron or steel parts from the
    vicinity of the coils too, they will soak all the power as eddy current losses.
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