1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Heat Sink is Positive ground, what does this mean?

Discussion in 'General Electronics Chat' started by Billy Mayo, Jun 22, 2013.

  1. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    At work , the heatsinks to all the power transistors, etc. is positive ground, mostly I have just seen heatsinks that are negative ground. My manger said that i should be careful when touching the heatsink with my hands plus it's easy to short out when the heatsink is positive ground.

    Why would a designer use a positive ground for a heatsink?

    Any reasons why the heatsink is positive ground?
     
  2. blueroomelectronics

    blueroomelectronics Well-Known Member

    Joined:
    Jan 21, 2007
    Messages:
    12,536
    Likes:
    168
    Location:
    Toronto, Canada
    Because the tab or flange of the device is not ground.
     
  3. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    Most power transistors, power diodes, power FETs' I have seen are ground

    How do you know if the tab or flange on the device is not ground? any test I can do with my DVM meter or the datasheets will tell?

    Any reason why a designer would choose components or devices that was not grounded or needed positive ground?
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. blueroomelectronics

    blueroomelectronics Well-Known Member

    Joined:
    Jan 21, 2007
    Messages:
    12,536
    Likes:
    168
    Location:
    Toronto, Canada

    Datasheets always tell you. Sometimes heat sinks are electrically isolated but if there is no real need to most engineers will not bother.
     
  6. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    So why didn't the designer just choose the negative grounded devices?

    I don't see positive grounded devices much or a heat sink that is positive grounded? what is a positive grounded heat sink used for?

    Any advantage of using a positive heat sink?
     
  7. crutschow

    crutschow Well-Known Member Most Helpful Member

    Joined:
    Mar 14, 2008
    Messages:
    10,592
    Likes:
    477
    Location:
    L.A., USA Zulu -8
    What do you mean by positive and negative ground? :confused: Generally there's only one ground and it has no polarity.

    The heat-sink tab of a power transistor is connected to the drain/collector (not source/emitter) for MOSFETs/BJTs, unless the package is specifically isolated. Thus if you connect the transistor to a heat sink without an insulating washer, then the heatsink is at the same voltage and polarity as the transistor drain/collector. So depending upon the transistor polarity, the heatsink could be either at ground or at the positive or negative supply voltage, depending upon the circuit design. The polarity of the heat sink is immaterial as long as it's isolated from the chassis and it is not something a designer is usually concerned about.

    Try not to generalize from a limited number of observations. Perhaps the transistors, diodes, and FETs you've seen have one terminal connected to ground but there are many circuits where they aren't. :rolleyes:
     
  8. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    I have only seen heatsinks that is at ground zero volts

    When is the heatsink at a positive voltage or negative supply voltage other than zero volts?

    Isn't it dangerous to put your hands or work around a heatsink that is other than zero volts ground?
     
  9. dougy83

    dougy83 Well-Known Member

    Joined:
    May 18, 2008
    Messages:
    2,672
    Likes:
    215
    Location:
    Brisbane, Australia
    Crutschow answered this above. Read his comment.
    It would be dangerous to you if the voltage magnitude is high enough. It would be bad for your circuit if you touch a conductor between the heatsink and some other potential (e.g. short it out by touching a screwdriver between the heatsink and the chassis/ground).
     
  10. ChrisP58

    ChrisP58 Well-Known Member

    Joined:
    Apr 27, 2012
    Messages:
    1,081
    Likes:
    131
    Location:
    Provo, Utah, USA
    I have never seen a transistor or diode that had a ground connection as it came out of the box. How a component is connected is purely a matter of what the design engineer wants it to do in his circuit.

    As for how an engineer uses those components tend to be dictated by how components that he needs are commonly built by the manufacturers.

    Mosfets and BJTs that come in TO-220 and similar packages usually have their drain terminal connected to the tab. (See crutschows post #6) For an N polarity device, that means the positive part of the circuit needs to be connected to the tab. If the tab is directly connected to the heatsink, the heatsink will be positive.

    The next power component likely to be on a heatsink will be the rectifier. Again, most rectifiers in tabbed packages have the cathode on the tab. Since the cathode is going to be the positive output of the rectifier, the heatsink, if not insulated from the tab, will be positive.

    But one thing that you need to be aware of, is that it really doesn't matter which polarity is connected to the heatsink in a high voltage circuit. A shock hazard is present anytime you have high voltage in a circuit. Even if there are no heatsinks. This is especially true if it is the front end of an off-line switching power supply. Even if the heatsinks were at 'ground' (the negative terminal of the bulk capacitor) it would still be at a lethal voltage with respect to earth ground.


    Billy, some of the questions you have asked have been of the "why is a product built this way", or "why component X was chosen". There are no universal answers to those questions. If you give the same product spec to 10 different engineers, you will end up with 10 different designs. Granted, 8 or 9 of them may be similar as there are 'common' ways to approach many tasks. But they won't all be the same down to each individual component.

    Also, engineering is always about trade-offs. Product safety, cost, size, efficiency, component count, ease of assembly, components already in use in other products, etc. All of these affect each other in sometimes very complex ways. An engineer will take lots of factors into account when working through a design task.
     
    • Like Like x 2
  11. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    My manager was saying that a lot of times Heatsinks are FLOATING because they want to separate and isolate low voltages and high voltages or digital grounds from analog grounds

    Why would a designer or circuit want a bunch of heatsinks to be Floating?

    My manager said that the Heatsinks that are Floating can either be negative ground or positive ground

    Any other reasons why you would want a heatsink to be floating?
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

    Joined:
    Jan 4, 2007
    Messages:
    21,180
    Likes:
    644
    Location:
    Ex Yorks' Hants UK
    ONLINE
    hi,

    Higher voltage isolation can be achieved by isolating the heat sink from external voltages.

    For grounded/common heat sinks the isolation of the semiconductor from its heat sink is limited to what the thermal/isolation pads on the semiconductors can withstand
     
  13. atferrari

    atferrari Well-Known Member

    Joined:
    Oct 8, 2003
    Messages:
    2,810
    Likes:
    121
    Location:
    Buenos Aires - Argentina
    Actually I do not care why you insist in using those expressions but what is the meaning of them? Ground is just ground AFAIK.
     
  14. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    If the heatsinks aren't floating or isolated from each other what will happen? it will cause what?
     
  15. arunb

    arunb Member

    Joined:
    Oct 11, 2006
    Messages:
    250
    Likes:
    0
    most laser diodes are positive grounded, which means there will be some voltage between the body and the ground (-ve).
     
  16. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

    Joined:
    Jan 4, 2007
    Messages:
    21,180
    Likes:
    644
    Location:
    Ex Yorks' Hants UK
    ONLINE
    hi,
    Surely you can imagine what would happen IF the devices on the heat sinks required isolation and the isolation was not present.??

    Its not rocket science.:rolleyes:

    Tell us what you think would happen in the above case.?

    E.
     
  17. WTP Pepper

    WTP Pepper Active Member

    Joined:
    Jan 24, 2012
    Messages:
    648
    Likes:
    41
    Location:
    UK
    One reason is that in mass production, mica isolating washers cost money and if there is no reason to isolate a heatsink then why bother.
    Sure it maybe sat at a high voltage, but that's what cases are for.
    Last time I designed a MOSFET power amp for home I left the heatsinks floating voltage wise as there was no reason to do otherwise.
    Heat flows through the air, not the ground connection.

    Also an incorrectly grounded heatsing could cause unwanted feedback into sensitive input stages to an amp as the high power output(s) could be capacitively coupled back via the mica washer.

    As for positive and negative grounds...try questioning your manager or get him to post here....
     
  18. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    I have no idea what will happen or cause

    maybe it will cause hum , noises, oscillations?
     
  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,912
    Likes:
    1,095
    Let's use the pos/neg ground concept correctly and appropriately. I'll let you use the conceps of your heat sinks being above groud, below ground, floating or grounded, but not positive ground UNLESS all voltages are referenced to this heat sink.

    Pos ground is a term used in older automobiles where the pos terminal of the battery was connected to the car chassis.

    The concept is not even used in telephony. TIP and Ring are the concepts. TIP is at -48 VDC relative to ground. Why? Because corrosion is less.

    It does seem like "ground" is a misunderstood concept.
     
  20. Billy Mayo

    Billy Mayo Member

    Joined:
    Mar 24, 2013
    Messages:
    825
    Likes:
    2
    Why would u want a heatsink above ground?
    Is a floating heatsink above ground? Why would u want a heatsink to be floating and not grounded?
     
  21. kubeek

    kubeek Well-Known Member

    Joined:
    Mar 11, 2006
    Messages:
    1,486
    Likes:
    185
    Location:
    Prague, Czechia (not Chechnya)
    A floating heatsink is floating wherever it´s pushed by electric fields, so it could be basically anywhere.
     

Share This Page