Jump to content
Electronics-Lab.com Community


  • Posts

  • Joined

  • Last visited


Posts posted by Capricious

  1. See my reply in your other post for a discussion about heatsinks. Maybe you need to add an additional output transistor or two.

    Yet another question to you gurus ::)

    Have a look at the attached image ... is this circuit for the 2nd transistor correct?
    Which values should R23 and R24 have? Should I use one variable resistor?



  2. Capricious,
    The author of this project used a pretty big heatsink. He even used a clip thingy on Q2. He didn't show his huge fan. Is his fan used for cooling, or for blowing away the smoke? (Or for blowing away your hair that you tear-out when the transistors melt)

    Hehe :P
    If the author's heatsink is on this picture:
    I think my heatsink is bigger ;)

    See my reply in your other post for a discussion about heatsinks. Maybe you need to add an additional output transistor or two.

    Ok. I'll see. Maybe I'll use a fan. My trafo has multi secondary. So I could use the 12V secondary to power the fan. Is it for a fan enough to put in 2 diodes?

    What is a real value for my heatsink with 2.4 K/W and the additional thermal resistances? Should I take 3 or maybe 4 K/W for calculations instead?

    An extra pole on the switch will change the range of the output voltage adjustment control. The negative supply will need revision or elimination.

    Do I understand right that I could use the circuit with 12 VAC for lower output voltages without any changes?

  3. Capricious,

    2) Your project has heat problems:
    a) [...] So you need a larger heatsink and/or forced air cooling with a fan.

    I think it is not possible to cool this thingy with passive heat sinks.
    Please answer my question
    about heat sink specification, too ...

  4. I always get notifications about a reply but I have not marked the option to get notifications.
    I've done it in older posts in the same thread but not in the later ones.
    The reply is to one of my posts without this notification option.

    Is this a strange behavior of this board? So to say ... once choosen in a thread always choosen?

  5. Capricious,
    That's good that you finished it and its wiring looks excellent.
    A few questions:
    1) Did you use a 24VAC, 3A transformer?

    I use the 24V secondary of a 75VA trafo.

    2) What is the total supply voltage of U2 and U3 without a load?

    Both 41.5V.
    Specified max. of TL081 is 36V.
    Is there a simple pull out and put in replacement in DIL8 with higher max. ratings?

    3) How hot does Q2 get with 5V or less output at 3A?

    1.75V 3A => 130 deg. C
    4V 4.3A => I switched it off at 175 deg. C
    Should buy an heat sink ...

    4) How hot do the rectifier diodes get at full current output?

    14V 4.3A => 120 deg. C

    The 2N3055 gets really hot, too.
    But it is below the spec. max. of 200 deg. C.
    Nevertheless I should not touch the heat sink at the back of my case. 120 degrees at heat sink are hot enough.

  6. Capricious,
    Are we talking about the same circuit?
    C7 is with the output voltage of this project across it, not R1.

    We are talking about the same circuit ... and you are right but misunderstand me. I've never been talking about voltages across resistors. I've simply measured the output voltage and current of the PSU to know the load.

    Thanks for the hint with R7.

    My thingy is finished ... some technical data:

    It has a max. output power of 73W.
    30V => 2.5A
    24V => 3.0A
    14V => 4.3A => 60W
    The max. current is limited to about 4.3A.
    The max. output voltage is about 34V.

    I try to find someone with an oszi to check the output quality.


  7. Capricious,
    R1 is across the unregulated supply voltage. Maybe you mean R2 or R3 gets hot with 3.7V across it.

    A current meter is connected in series with the load to measure the current that is flowing in the load (and flowing through the current meter). You simply connect a very low value resistor in series with the load and use your meter to measure its voltage drop.

    No, it's R1 and it is the output voltage. But I understand that the problem is not the output power. So the temperature will be constant at about 40 degrees C. I think it is too "warm".

    Ok, I know that a current meter is connected in series. I will connect it in series but in parallel to the "shunt". And the "shunt" is a piece silver wire on the board from circuit. What I want to do is to split the current in two parallel flows. One with max. 2.9999A and the other with max. 0.0001A which is together the max. output current 3A. Then I can put in the A-meter in series to the low current part.
    Now I only need the right resistor value to split it up like that.
    Is this not possible?
    I have the problem that I do not really know the resistance of the wire. It is too low for measurement and I think the "spezifische Widerstand" (specific resistance?) for silver isn't accurate enough to calculate the resistance of the wire in practice.
    But maybe this way is too extreme so there must be a resistor with known value?

  8. Ok, my PSU is nearly finished... and what I've tested works fine.
    I've used a "Lochrasterplatine" (don't know the English word for the boards with many holes).
    Tomorrow I'll use some car lamps to test the maximum current.

    I think I will change the R1 resistor to a 4W version. The 1W version has at 3.7V and 0.28A a temperature about 40 C.

    I have still one question...
    I have a 100uA analog amperemeter. I want to connect it parallel to the circuit. Which resistor I have to use in series to I-meter to get max. deflection at 3A overall current?

  9. Can anyone please list all the parts should be changed to be safe?
    Perhaps with a short description why it should be changed.
    I am a newbie and after reading the 8 pages of this thread I am a bit confused.

    I've read something about exploding C7... Is it only a problem of one guy doing something wrong or must I take glasses if I switch on my PSU?

    I can not find the specified 50V 3300uF C1.
    I have the choice between a 40V 3300uF and 50V 4700uF.
    Would it be a problem using more capacity for C1?
    Can a 24V trafo has more than 28V output?
    My trafo is on the way so I can not check the max. output voltage.
    If the maximum is 28 V the 40V C1 should be enough, shouldn't it?

    Then the 200nF C5. Is 200nF a usual capacity? I can only find 220nF capacitors.


  • Create New...