Jump to content
Electronics-Lab.com Community


  • Posts

  • Joined

  • Last visited


Posts posted by maksar

  1. Hi audioguru!

    He, he. The way you show cutouts in the heatsink to fit the shoulder of the transistor, you would need to cut the heatsink down the middle for them to be assembled. He, he. ;D ;D

    I think you didn't understand me.  :P If the transistors won't fit the heatsink in ambient temperature, I'll cut a little one of it's sides. The cutouts near the "shoulders" of the heatsink are only for providing the room for the transistor and heatsink to enlarge on high temperatures.

    5) With the total of 1.95 degrees C/W and a dissipation of 60W in the transistor, its chip temp is 147 degrees C. Not bad since its max rated temp is 200 degrees C. ;D

    First of all, I don't understand why we have to refer to 200 degrees temperature? This is the max temp. for 0W dissipation. According to the datasheet of 2n3055 there is the max temp. for each value up to 115W. So for ~60W we have to keep transistor only at ~100 degrees, and not at 147C. Am I right?
    If so, 3 transistors for 5A project, for example, is not enough.

    For 4 transistors:
    Dissipation for each 2n3055 is 190W/4=47.5W.
    If total resistance is 2C/W (I am complitely agree with your calculations) the temperature is 125C, while the max for 47.5W according to the datasheet is a little above 125C.

    For 5 transistors we will get much better situation. So I think that the best option for 5A project is 5 2n3055 transistors.  :D

    I think heatsinks are rated with max convection airflow. So their fins must be vertical and open at top and bottom. Therefore the heatsinks should be the sides or back of the case. ;D

    :-\ I want to have only one but long piece of this heatsink on back wall, so that it's lengh will be the weidth of the case.
    What is it "natural convection"? And what do you mean by "max convection airflow"?
    Do you think it is critical if the fins won't be vertical (in the case of back wall of the case)?
    I think the hot air will go up by itself...  ???

  2. Hi audioguru!

    Interesting point, I didn't think about this!  :D  But I don't think that this is a big problem. You can make the "shoulders" the shape you want them to be with a dremel. Also you can enlarge the space near the bottom surface of the heatsink, so that 2n3055 will match. Or you can use dremel to match the transistors to the heatsink  :)
    By the way there is other heatsink with mach more space: http://www.semikron.com/internet/ds.jsp?file=649.html
    But due to it's dimensions, I think, it should be the top wall of the psu box.


    P.S.  Paint rulez  ;D


  3. Hi!

    Why not to say simply, that we need 4 transistors for 3A project and 5 for 5A project? But still a large heatsink is needed (I probably will buy this one: http://www.semikron.com/internet/ds.jsp?file=648.html) with fan.
    Or it is posible to use more powerfull transistors with thermal resistence ~0.5 C/W (maybe there are even better transistors) and not 1.5 C/W as 2n3055 has. It will be enough to use 2-3 of better transistors).
    Also the best solution may be to do teramal control based on termal sensor (sample from Maxim semi, for example) and opamp. So the fan won't work all the time, but only when it is needed (rarely).


  4. Hi audioguru!

    As you can see the standard lengths are 15cm(5.8") and 29cm(11.5"). So it isn't so long and can be easily matched to the case dimensions (case's height and width are fixed and length may be variable for individual needs). Also I don't see problems to mount it as a rear wall, there are several options how to do this.

    The heatsink has 0.8 C/W/3", so it is ~0.5 C/W/5.8". With fans (200LFM): ~0.2 C/W/5.8". Am I right?

    The question is: Is this heatsink in length 15cm suitable for 3A project without fans and for a 5A project with 1 or 2 fans on the side wall of the case (and holes on the other side wall)?
    I think we need to require for 3A project as well as for 5A, that the temperature won't rise above 100C. And of course we sould use termal grease with insulator pads. Am I right?


  5. Hi!

    What about this heatsink?


    Does this one need fan too? (for 5A project?)

    I think the upside of the heatsink in the picture should be outside of the case.



    Thanks for this explanation. I didn't think that it is so simple to calculate and determine the needed heatsink  :o  (After your post I got some reading on this topic).

    It seems like I'll have to use fan. But does an airflow has big influence on the effective heatsink resistance? What will be the temperature of 2n3055 if I will put the 80mm computer fan on the 1.4 C/W heatsink?
    Maybe I'll do termal control of this fan (or fans..) by using Maxim's temperature sensor.

    By the way, should I make aluminium case for psu? It has low termal resistance and is
    easy to work with, but it won't too soft?


  7. Hi!

    Ok, thanks audioguru. The problem is that I don't have these diodes and my local electronics store don't have them too. So I tried  ;) ;D ;D ;D  But it seems that I'll need to search BZX79C5V6 in the other stores  :-\

    By the way, I understand nothing in planes, however, your model doesn't look ugly at all. Pretty, small plane... But I believe to expert's word  :P ;D ;D


  8. Hi All!

    I'm going to build this PSU (0-30V/5A), but I want to do in addition digital 7-segment volt- and ampermeter, +-5V/1.5A and +-12V/1.5A. Here I thought about 2 ways to do this:

    1. to power these additional voltages from the big trafo of 0-30V PSU. As I understand I need two windings (of 15V each) to get negative and positive voltages <35VDC (the requirement of most 7800s), and then I'll have the virtual ground - in the middle of the secondary winding. However, there will be some problems with this way:
      a. the current of 5V and 12V supplys will cause changs in the trafo voltage for the main   
          PSU. Will it be critical for 0-30V PSU? Also this additional load won't be simetrical and
          +5V supply and -5V, for example, will influence on each other.  ???
      b. I'll need bigger and more expensive trafo. I guess something around 11-12A insteade of
      c. I'll have to measure voltage and amperage of the 0-30V PSU driven from the same
          source. It may cause some problems, were said earlyer.
      d. I'll will not be able to use the main PSU and +-5V, for example. in the same circuit,
          because there will be difference between their ground. Am I right?

    2. to power these additional voltages from the other smaller trafo, taken from the PC power supply. There are two types of trafo: toroidal and "ragular" for switching power supply (yellow in most cases). Which of them I can use for my purpose, and can I connect it directly to the 220VAC? What current this trafo can give for +-5V and +-12V?

    By the way, should I also do +-3.3V for home lab? Or I won't need it?

    That's all  ;D  I will be very gratefull to everyone who will answer.
    After getting answers on these questions I believe,  I'll be able to begin building this project. I'll put the final schematic on the forum.


    P.S. Sorry for my english.  I'm working on it  :P

    P.P.S. Thanks for your answer, Audioguru!!!  ;) ;D  You are doing amazing job on this forum!

  • Create New...