0-30v 0-3A. One 2N3055 pass really enough

Discussion in 'The Projects Forum' started by jimmiegin, Apr 6, 2014.

  1. jimmiegin

    Thread Starter Member

    Apr 4, 2014
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    Hi all.
    I Have been looking at this:
    http://diyfan.blogspot.dk/2013/03/ad...-take-two.html
    As the title of my post suggests, I am not comfortable about running this circuit with only one 2N3055. Now I may have this wrong but at max dissipation this pass transistor would have to waist 90W of power. (If I am mistaken please correct me) And although the designer states that a "huge heat sink" is needed and "forced air cooling", I am not convinced that is sufficient. so something is telling me I should be using about three times 2n3055 in parallel. On a comment from one builder of this supply he says that he burned three transistors but a reply to this says that is normal with a short. I know very little of what I am speaking and I am just starting out with my hobby. But this tells me there is no short circuit protection, but I though a variable current limiting was there to insure the limiting of current and therefore large current would not be drawn if a short happens, so nothing should be burned up. Until I have studied enough to design and build my own supply, I would like to go with this schematic. Please could I get guidance for any changes or additions that need to be made. Thank you. Jimmiegin
     
  2. wayneh

    Expert

    Sep 9, 2010
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    That link doesn't work. I guess you meant this?

    I don't see a 2N3055 indicated on the schematic there. Which one are you looking at?

    Stepping farther back: What are you trying to do? One way to get started in this hobby is to just use an old computer PSU on your lab bench. No reason to build one.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I have to agree with Wayne, this is a fairly complicated power suply for someone who is starting out. Being able to buy a pc board would make a huge difference.

    Beyond that, yes, a single 2N3055 will in fact dissipate 90 W all day long if you have a big enough heatsink and forced air. That's why it still is widely used after 50 years. Calculating the heatsink and fan sizes are not difficult.

    IC1a is the current limit amplifier/comparator. If the circuit is built correctly, then a short shouldn't directly destroy the transistor. Howevr, as you've noted, excessive heat could.

    ak
     
  4. jimmiegin

    Thread Starter Member

    Apr 4, 2014
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    Well when I say just starting out... I have already designed and built a valve amp and a pre amp pedal for my guitar. I have designed and built a couple of solid state pedals but op amp based and so all was pretty simple. now I am moving onto transistors and although I can follow instruction for a lab procedure list to design ac amplifier circuits, im still not fully memorising how different transistor configurations work, even though it is all similar to triodes. I have actually only started looking into transistors for the past few days. I am experienced enough to build from this schematic as understanding symbols and soldering is not exactly demanding, and I understand roughly how it works in the sense of looking at it from a block diagram point of view, and the zener regulator? reference point amplified by a transistor and the whole pass transistor thing, I am kind of familiar with now. but I have ran no numbers on it myself. Just looking for a little guidance on the number of parallel pass transistors i should include. So far I have put together 5 parts of my single unit power supply(all in one case)....2x 6.3v @2A... 2x 12v @2A... 350vdc output and now I am working on 45v @300mA and (the link that didn't work but yes you got the right one) for my variable supply 0-30v @3A. All of this means I can power E valve heaters and U valve heaters and the 350vdc for the B+ and the 0-30v variable for anything else I may want to play with (like hybrid amps). That aside, thank you for the reply, I do have huge heat sinks and many large fans...but I thought max disp for the 3055 is 115W? Do you not think that is a little bit too close to max disp? Did I get the 90W thing Right? Could you please provide a link for calculating heatsinks please, and do I really need to work out all the thermal resistance stuff cos I have a box of big heat sinks taken from some 300 watt amps that had STK463 stereo amps bolted to them. Oh my days, sorry that was looong!
     
    Last edited: Apr 6, 2014
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    Do need to go down to 0 volt? Maybe a LM723 (thats an IC) based supply can be a good and cheap first project power supply
     
  6. jimmiegin

    Thread Starter Member

    Apr 4, 2014
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    I would like to use the schematic i have decided on and i will change all surface mounts to through hole and redesign the pcb. I am not happy to start looking for a lm723 schematic as i will eventually design my own based around the lm723 or design a discrete version variable supply. I just don't know enough on the subject yet. This is just a power supply to power my learning till i know enough to design my own.
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    My 2 cents worth.
    It doesn't sound like you need 0 volts. Maybe 1.25 for the lowest would be fine?
    I would use 2 or 3 3055s in parallel as the 115 watts is if you can keep the case at 25C.
    What is your input voltage? This will determine the highest voltage as well as the maximum power. The reason I ask is 30 volt transformers aren't common.
     
  8. #12

    Expert

    Nov 30, 2010
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    A 24 volt winding could run a 30 volt power supply if the power line is steady at the input voltage the transformer is rated for.
    24 x 1.414 = 33.94V
    Use Shottky rectifiers, lose 2V for ripple, and you have most of a volt left over for collector to emitter voltage when it needs at least .3 volts, and it's just about possible.

    The 2N3055 in a TO-3 case has 1.52 C/W heat resistance. Add about .1 for the thermal bond to the heat sink and 200C for the high limit and you get

    math math math... 171 Centigrade if the heat sink is infinite.
    .324 C/W heat sink required to run this at the ragged edge of smoking (200 C at the junction).

    Nobody in their right mind would do that and expect it to last more than a year.
    I'd say, 2 transistors, at least. The LM317 datasheets have examples of how to do this if you can settle for not getting to exactly zero volts. But then, I don't know of anything that can run on zero volts. :D
     
  9. jimmiegin

    Thread Starter Member

    Apr 4, 2014
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    Hi. what I have is either a massive transformer from an solid state 300 watt amp 30.0.30 (60v) so I can use one side and the centre for to get 30v that would give 42v rectified minus the diode drop so I may get away with using that one but I have no idea what adverse effect there would be on the lm723/337 at so close to max voltage?. Or a 14.0.14 (28v). I also have plenty other transformers around that would be hard to source new. I do have the luck of living opposite a recycle yard whose manager and I have a deal. I give just over scrap value so I end up with lots of hard to find items for a few pounds :). I will find something suitable.
    And yes you are right, I can not see me ever needing 0 volts. Thank you for you reply, may I ask why 3 parallel 2N3055? I mean obviously you have said because of the 115W disp but what is the best percentage of max to get away with please?
     
    Last edited: Apr 6, 2014
  10. #12

    Expert

    Nov 30, 2010
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    30 radical 2 = 42.4
    minus 1.9V for rectifiers, minus 5 volts/2 for ripple
    assuming 3 amps at 1 volt out = 111 watts
    Definitely time for using more than one pass transistor.
     
  11. jimmiegin

    Thread Starter Member

    Apr 4, 2014
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    I read this over at diy audio, I was inclined to believe it:

    I worked in the engineering lab at Fairchild Semiconductor and talked to a power device test engineer regarding the 2N3055. He said the 2N3055 specifications designed to allow just about any power transistor meet the specifications. I didn’t understand until he told me the new parking lot asphalt was mixed with several tons of transistor rejects that functioned but did not meet specifications. The low performance specifications of the 2N3055 allowed the company to sell all functional rejects as a 2N3055.

    Just thought it was something you may not read every day.
     
  12. jimmiegin

    Thread Starter Member

    Apr 4, 2014
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    111W! so three or four times pass transistor?
     
    Last edited: Apr 6, 2014
  13. jimmiegin

    Thread Starter Member

    Apr 4, 2014
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    Just been looking at the datasheet for the lm317 application notes, cc/cv with the op amp controlling the adjust pin and mj4502 series pass. I have ordered a stack of tl081 op amps and wondered what I would need to do to sub out the lm301 for a 081 and so like most things I am unsure of, I Googled it and found this:

    "You can easily replace 301 opamp with any other opamp including the TL081 (C4 and D2 are for frequency compensation and will not be required with other opamps) ..
    Just connect -Vcc to -6 and and +Vcc to max 30Vdc (TL081 is rated for +/-18V =36V) ..
    If you replace D1 with a Schottky diode (Vf≈0.3V) and connect it directly to the opamp's output, increase the value of R4 to >3kΩ and insted of connecting it to the LM317 output wire it directly to your +Vcc supply, you will be able to supply the whole circuit without any need for negative (in this case -6V) voltages"

    Any thoughts from those who don't think it a good idea to use a 2N3055 to keep warm in the winter! [​IMG]
     
  14. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Based on the 2N3055 data sheet provided, the crossover case temperature at 90 W is +63C, which is 38C above ambient, or 2.4degC/W. This is very easy to achieve with a medium sized heatsink and a small fan.

    Here is the fan equation:

    http://www.nmbtc.com/fans/engineering/airflow-formula.html

    Plugging in 90 W and 38 degrees, you need only 4 CFM *net* airflow. Now the adjustments come in. 4 CFM net means that it takes only 4 CFM *if* 100% of the air is in contact with 100% of the heat generating surfaces. While you can use that to design a heatsink, the real solution is to go through heatsink catalogs for a part that has a thermal impedance of less than 2.4 degC/W, note the airflow needed for that performance, and buy a fan to do that.

    All of that has to be adjusted for the headroom above 30 V coming from the filter caps, and I completely agree that you should use 2 3055s. Rule of thumb from the military arena for high reliability designs: operate things like power transistors at 50% of everything - max. voltage, max. current, max. temperature.

    ak
     
  15. jimmiegin

    Thread Starter Member

    Apr 4, 2014
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    HI. when you say medium size heat sink what kind of dimensions would you be thinking of a 'medium' please? I have lots of heat sinks in a box from thing like active speaker systems, so although I am not sure how, or even if one would be able to tell the thermal resistance unless I was to note the device that's on it and maybe look at the power ratting on the back of the unit? then go through the data sheet and work out what size sink would be needed and if it matches roughly what I have in my hand then I know roughly? But that all seems a bit long to me. so is there a rough guide to sizing heat sinks by dimensions or are there specific variations of ally gradients?

    I failed to mention before that this is a junkbox project but all links and posts are very useful to aid my learning so many thanks to all
     
  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I went here:

    http://www.aavid.com/product-group/extrusions/search

    and started fishing around. I put in 2" high, 4" wide, and 2 degC/W. That got me here:

    http://www.aavid.com/products/extrusion-heatsinks/62720

    The lower right chart shows a thermal impedance of 0.75 degC/W at 200 LFM.
    2" H x 4" W = 8 sq in - 0.055 sq ft.
    200 x 0.055 = 11.1 CFM

    That's not much airflow, even for a net number, but you have to figure that without ducting to improve thermal efficiency you'll need 4x that.

    I, too, have a selection of various undocumented heatsinks acquired from projects over the years, and this is the general technique I use. Find a vendor with a heatsink that is close to what you own, run the numbers based on that part, adjust for the real world.

    ak
     
  17. #12

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    Nov 30, 2010
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  18. jimmiegin

    Thread Starter Member

    Apr 4, 2014
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    Thanks for all your help guys. I have all but a few resistor values for this project now so its off to maplin to spend a few pounds and my almost junk box project will be ready for the build. Is there somewhere on this site that I may post my project pic's when it is done? I have an old amp housing with a lovely aluminium front and some digital panel meters to throw in there too so I would like to show it off when it is done.
    It will be a complete power supply for bread boarding valve amps and effects pedals so maybe there will be some that would be interested in it.
    350vdc regulated B+ (with a little bread board for voltage divider)
    12v/6.3V heater supply
    45v heater supply
    0-30 0-3A variable
    with volt/ammeter for the low voltage supplies and dmm points for the 350v
    Thanks again for all your help
     
  19. #12

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    Nov 30, 2010
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    It's called the Completed Projects Collection.
    Read the instructions. Partial documentation isn't allowed.
     
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