Transistor Heat Sink Question?

Discussion in 'Homebrew and Kit Projects' started by VK4HAT, Nov 22, 2020.

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  1. VK4HAT

    VK4HAT Ham Member QRZ Page


    On the above image the tab soldered to the heatsink is also the collector pin for the transistor. Wont the heatsinks radiate somewhat?
  2. KD8DEY

    KD8DEY Ham Member QRZ Page

    Dunno. No Idea if the circuit is @ AF, RF, or DC....
    My big question would be WTH would they solder the tab to the heat sink to begin with instead of the traditional screw arrangement.....
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    Because the copper is too soft & will distort with the clamping force?

  4. G4COE

    G4COE Ham Member QRZ Page

    Depends on the circuit and frequency because it *could* easily cause instability due to coupling with other circuits.... and then there's the capacitance coupling to worry about.

    I noted the collector is not isolated and I assume this is at a +ve supply potential, I suspect one could argue if this is a good or bad thing.... again depends on the circuit and frequency in question so I'll leave this point out for the time being.

  5. VK4HAT

    VK4HAT Ham Member QRZ Page

    Sorry bubba, kind of assumed RF, thats a 5w class c common collector power amp. 2sc5706 has an ft of 450mhz. Its kind of a useful part. Being at vcc i can deal with using plastidip, making a giant heat sink antenna radiating into an always on receiver, i dont want.

    Normally i use the smd version and heatsink the board, but i ordered the wrong parts.
    Last edited: Nov 22, 2020
  6. WO1X

    WO1X Platinum Subscriber Platinum Subscriber QRZ Page

    May I ask a question? Do the transistors plug into a socket on the board? It looks like you could tug on the heat sinks and pluck the transistors right out of what appears to be sockets.
  7. CX3CP

    CX3CP Ham Member QRZ Page

    Hi, because the transistor does not have a hole in the TAB for screwing it, they must have soldered to the copper heatsink.
    C5706 Nice Transistor to be in mind for RF applications.
    here the datasheet EN6912-D.PDF (

    N7EKU likes this.
  8. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, they will radiate somewhat, e-field. More correctly stated they will tend to capacitively couple to circuits and parts nearby at the frequencies discussed. Whether that’s a problem in your application depends on many factors.

    It’s the same problem if folded over and soldered to a square of copper on the board. In that case the geometry is less favorable for coupling to other parts - but worse for coupling to any traces underneath or nearby.
    W7UUU, VK4HAT and N7EKU like this.
  9. AG6QR

    AG6QR Premium Subscriber QRZ Page

    The difference between soldering and clamping with mica insulators is that soldering provides DC connectivity. That DC connectivity is not important at RF.

    Any means of thermal coupling to a metal heatsink is going to at least capacitively couple to that heatsink. Capacitors, whether created intentionally or accidentally, are quite capable of conducting RF.
  10. N5HXR

    N5HXR Ham Member QRZ Page

    Not exactly the same situation, but the last time you guys mentioned heatsinks at rf potential, I ended up reading through this article from EMC. I'm certainly still a swaddling babe in terms of RF stuff, but some of it even made sense to me!

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