Some trannies have grease, while others have pads

• Some trannies have grease, while others have pads
  Posted by alrightspider on 15 Jun, 2014 03:07
• 
  Hi Folks,
  
  I have to remove a big heat sink from 7 power transistors on an audio amplifier. 
  
  The 4 medium transistors are attached with grease, but the two big ones and one small one are attached by pads . . .
  
  I have to assume this is by design, but all I have is grease.  Can I still use grease for all? 
  
  Do y'all know why some transistors are using pads while others are using grease?
  
  I wonder if is because 3 of them need to be electrically insulated from the sink?
  
  [picture below]
  
  THANKS!!
• #1 Reply
  Posted by sleemanj on 15 Jun, 2014 03:17
• Quote from: alrightspider on 15 Jun, 2014 03:07

  
  I wonder if is because 3 of them need to be electrically insulated from the sink?
  
  

  
  Yes, this is the most likely reason.
• #2 Reply
  Posted by pickle9000 on 15 Jun, 2014 03:23
• What's the mounting hardware like?
  
  Yes, it's probably isolated.
• #3 Reply
  Posted by vk6zgo on 15 Jun, 2014 03:27
• I think you are right in your conjecture.
  
  If some have "pads"(I think you mean "washers"),they are ones that need insulation.
  Manufacturers seldom include parts that are not absolutely necessary.
  
  No,I don't really know why we call them "washers",as they are pretty far from the normal understanding of that term.---More like gaskets,in fact!
  
  
• #4 Reply
  Posted by alrightspider on 15 Jun, 2014 15:40
• 
  Thanks guys.
  
  Pickle9000 -- the mounting hardware is a bolt with a washer on both sides.
• #5 Reply
  Posted by KJDS on 15 Jun, 2014 16:41
• Quote from: alrightspider on 15 Jun, 2014 15:40

  
  Thanks guys.
  
  Pickle9000 -- the mounting hardware is a bolt with a washer on both sides.
  

  
  If it is a metal bolt and metal washer then the pads aren't there to provide electrical insulation
• #6 Reply
  Posted by N2IXK on 15 Jun, 2014 17:31
• Are all the components transistors?
  
  It is quite common to mount many common TO-220 package regulator ICs with just a smear of grease, because the tabs are often grounded anyway, being the common terminal (and also connected to the center pin).
  
  The center pin and tab of a transistor is the collector/drain, which almost always needs to be isolated from ground/circuit common.
  
  
  
  
• #7 Reply
  Posted by alrightspider on 15 Jun, 2014 19:53
• 
  OK - I think I got it - pads definitely seem to be insulating the collectors from the heat sinks.
  
  I have discovered that the ones with pads have a shiny metal plate on the back which shorts to the middle pin.
  
  I'm going to buy a bunch of Berquist Sil-Pad 400s.
  
  Thanks for all your replies. 
  
   * * *
  
  In case you're still curious:
  
  (heat sinked transistors for 2 audio amplifiers on one PCP: 
  
  NPN 2SD1047 - TO-3P - PAD
  - Application:  power supply
  - High power NPN epitaxial planar bipolar transistor
  
  NPN MJE340 - TO-126 - PAD
  - High Voltage General Purpose Applications
  - Suitable for Transformer
  - Epitaxial Silicon Transistor
  - Thermopad Construction Provides High Power Dissipation Rating for High
  Reliability
  
  PNP 2SB817 - TO-3PB - PAD
  - 140V/12A AF 60W Output Applications
  - Epitaxial Silicon Transistor
  - Capable of being mounted easily because of one-point fixing type plastic molded package (Interchangeable with TO-3)
  
  Airpax 67F070 - TO-220 - grease
  - 2 Pins bimetal thermostat
  - VDE   approved   with   “P”   bracket   option
  
  (2X) NPN 2SD1264 - TO-220F - grease: 
  - Silicon NPN triple diffusion planar type
  - For low-frequency power amplification
  - Full-pack package which can be installed to the heat sink with one screw
  
  PNP 2SB940A - TO-220F-A1 - GREASE
  - Complementary to 2SD1264 (above)
  - Silicon PNP epitaxial planar type
  - For power amplification
  - Full-pack package which can be installed to the heat sink with one screw