When is a transistor running too hot?

Discussion in 'General Electronics Chat' started by Damo666, May 21, 2010.

  1. Damo666

    Thread Starter Member

    Aug 25, 2009
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    Hi guys. In general, when is a transistor running too hot? I'm concerned because a little rf amp that i have uses a TO5 transistor that gets quite got. You can feel the heat with your finger but it doesn't burn. However, i can keep my top lip on it for about 5 seconds then it becomes too hot to cope with. Any ideas please guys? All the best. Damian.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It depends upon the transistors' rating.

    What is it's part number?

    Look up the datasheet for it.
     
  3. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    What sarge said. once you find that info post it on here an we can advise weather you need to replace with something different or just add a heatsink....


    B. Morse
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    If you can touch it for a few seconds it is probably quite safe. Transistors can reach more 120degC which is more than the boiling point of water, although they are not normally allowed to get this hot.

    That said, any component that runs hot to touch will (should be) well ventilated and perhaps on a heat sink. Make sure that this ventilation is never obstructed.
     
  5. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    My guess is that is 60-70C. That's fine but if it was installed in a car or somewhere warm it's temp would rise quite a bit. You should be able to get clip on heatsinks which would make quite a difference.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    A common trick to cooling transistors is provide a thermal path to the outside of the case. This can be as simple as an aluminum or steel block from the transistor to the case wall (assuming the case is metal).
     
  7. Damo666

    Thread Starter Member

    Aug 25, 2009
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    Ok guys. Thanks for the informative replies. The transistor is a 40965 npn transistor made by rca in an old 1980's transmitter/receiver. I can't find any info at all on this transistor. Can anybody help at all please?
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Attached two pictures with scans from an old databook on the 40965.

    Bertus
     
    Damo666 likes this.
  9. Damo666

    Thread Starter Member

    Aug 25, 2009
    19
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    Thanks for the info Bertus, but i don't understand most of it as i'm a newbie. If my circuit draws 170ma at 18vdc when i transmit will it be operating within safe parameters. By the way - the 170ma is running a crystal oscillator, multiplier, 1st pa at 100mw and the final pa at approx 500mw rf. The final pa uses the 40965. Sorry about all the questions.
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    According to the table :
    The transistor is an multi emittor, transmittor and end stage transistor with a maximum output power of > 0.5 Watt.
    The Vceo = 14 Volts.
    The Icmax = 200 mA.
    The maximum power dissipation @ 25 °C = 3.5 Watt.

    You say you run at 18 Volts , that is to much according to the table.

    Bertus
     
  11. Damo666

    Thread Starter Member

    Aug 25, 2009
    19
    0
    Thanks for your good input Bertus. When i say 18 volts, i mean 2 x 9v nimh pp3 batteries which is more like 16.8v. Have you got an email address that i could send the schematic to please? I really do appreciate your help. All the best. Damian.
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    We do not want to give email support.
    This breaks the purpose of the forum, sharing information.
    You can post the schematic over here.

    Bertus
     
  13. Damo666

    Thread Starter Member

    Aug 25, 2009
    19
    0
    Ok Bertus. I do all my posting from a mobile phone so that's an impossibility for me. Is it ok to give a link to an host site, as i can upload the pdf document there? All the best, Damian.
     
  14. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    If the documantation carries some copyright there is a problem.
    We do not allow copyrighted stuff over here.

    What does the documentation say about the powersupply range?

    Bertus
     
  15. Damo666

    Thread Starter Member

    Aug 25, 2009
    19
    0
    The unit itself runs off 2 x 8.4v nimh pp3 batteries. When the " push to talk " key is pressed, it applies these 2 batteries in series to the pa stage, ie- 16.8v. The 16.8v + terminal is connected to the collector of the 40965 via an inductor, and the - 16.8v is directly connected to the emitter. Regards. Damian,
     
  16. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    I finaly found a complete datasheet for the 40965.
    There are also some endstages descibed in there.
    Do you think the schematic of your endstage does look alike one of them?

    Is there any heatsink mounted on the transistor?
    It could look like the heatsinks in the attached heatsink datasheet.

    Is the output of the amplifier correctly adjusted to the used frequency?
    A frequency missmatch could lead to reflections that would have the tranistor to deliver more power.

    Bertus
     
    Last edited: May 25, 2010
  17. Damo666

    Thread Starter Member

    Aug 25, 2009
    19
    0
    Hi Bertus. The final PA stage is a replica of the second one shown. There isn't any space for any heatsink, and the transistor isn't using one. Re mismatch; Do you think this could be the issue here, even at such low rf power levels? I was always under the impression that PA stages are at risk only when running at reasonably high power levels.... Ie- approx 2 watts or more. By the way- the final stages of the PA are designed to transmit at approx 466mhz and are untuneable, but are transmitting on 433mhz, albeit at a much reduced power to to out of band operation. Regards. Damian.
     
    Last edited: May 25, 2010
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