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Two unrelated questions

Discussion in 'General Electronics Discussion' started by Martaine2005, Nov 12, 2019.

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  1. Martaine2005

    Martaine2005

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    May 12, 2015
    Hi guys/girls,
    I have two unrelated questions that I thought I would post here.

    The first is with regard to thermal paste vs thermal pads. Are there advantages or disadvantages to using one or the other?. I will be using one or the other for output transistors and regulators to heat sinks.
    Thermal pads are very cheap per sheet and paste/compound can be quite costly.
    Are there any reasons to use one or the other?.

    Secondly, completely unrelated, does anybody know what sort of plastic is used for digital displays?. Digital clocks, frequency counters etc.
    I have called a couple of plastic suppliers and have two types both of which are unsuitable. Too thick (3mm) upwards or too thin like lighting filters (par cans) etc.
    Thanks in advance.

    Martin
     
  2. AnalogKid

    AnalogKid

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    Jun 10, 2015
    When applied correctly to properly prepared surfaces, thermal paste is a far better thermal conductor than the pads. It is also lower weight, but that is important only in a very small number of applications. The pads are much better at gap filling (and some are designed specifically for this), and are a reliable voltage insulator. If you need voltage insulation and you don't need super thermal conductivity, then a silpad is almost always a better choice than grease plus mica.

    When you say "display", do you mean the display component (the plastic box that has the LEDs in it) or the overall display (faceplate and bezel)?

    ak
     
  3. Martaine2005

    Martaine2005

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    May 12, 2015
    Thanks AK, very informative. I am looking more at thermal conductivity than voltage insulation, so I will continue using paste.

    Re the ‘display’. What I mean is the actual front transparent(ish) red, green or blue plastic diffuser in which the 7 segment LEDs can be seen through.
    Thanks again.

    Martin
     
  4. dave9

    dave9

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    Mar 5, 2017
    The front plate is often acrylic, though I suspect this is mostly due to optical clarity, cost and ease to mold it, but that shouldn't stop you from using some other material if you need to, for example polycarbonate if you need impact resistance, but then watch out for UV yellowing if it will be outside, though some polycarb has a UV protective layer applied but it will still eventually yellow and haze like seen on vehicle headlight housings. Due to much higher flexibility you can get away with using a much thinner sheet of polycarbonate than you could acrylic, unless you need impact resistance for what is behind it.

    There is also the machining difference, polycarb can be machined with similar tools you would use in woodworking to cut it, while acrylic is relatively brittle, treated more like glass where you score and break it to size. It can be sanded too but that would be a tedious way to remove much material, can't sand at a high rate or it may get too hot and soften and embed sanding grains, even melt the surface.

    When comparing the price of thermal pads vs grease, are you comparing high end grease to low end/generic thermal pads? Normally grease is much cheaper per application, if you need to do more than a very small # of components, if you compare the old standard silicone oil & zinc oxide formulation opposed to the exotic formulas marketed for computer CPU use.

    Many of the exotic formulas do have one thing going for them (besides a low single-digit temperature difference) which is less susceptibility to oil separation which can be important over several years of heat cycling, but mostly becomes a problem on very high power density chips like the old Athlons or Pentium 3's which didn't have heat spreaders (permanently integrated from the factory) on them which lowers the thermal density at the grease interface.
     
    Last edited: Nov 12, 2019
  5. Martaine2005

    Martaine2005

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    May 12, 2015
    Thanks Dave for the info. I can’t get acrylic less than 2mm. I may have these sizes confused because of many phone calls and inadequate notes taken.
    I can’t even remember all the different plastics!.
    But acrylic, poly carbonate, vinyl etc were either too thick or paper thin. I just want the plastic front (difuser) often seen on 1980s digital clocks or clock radios. I have one in front of me now. It is transparent red and .5mm thick.

    Martin
     
  6. dave9

    dave9

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    Mar 5, 2017
    ^ Is 2mm a problem? It should still be optically good enough for most purposes and if the edge is a little exposed it can be rounded and polished down to a transparent state.

    Regardless I see a couple polycarbonate on ebay 1mm or thinner and there may be more.

    https://www.ebay.co.uk/itm/CLEAR-PO...-UNBREAKABLE-MARLON-PALRAM-PANEL/360813914102

    https://www.ebay.co.uk/itm/Clear-Po...A3-Size-0-25mm-0-50mm-0-75mm-1mm/200971993233

    https://www.ebay.co.uk/sch/i.html?_...arbonate.TRS0&_nkw=1mm+polycarbonate&_sacat=0

    Acrylic https://www.ebay.co.uk/sch/i.html?_...0.Xacrylic+1mm.TRS0&_nkw=acrylic+1mm&_sacat=0
     
  7. Martaine2005

    Martaine2005

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    May 12, 2015
    Yes, thank you Dave.
    I can find clear. I was looking for red or green. These seem to be a problem without the correct search term.

    Martin
     
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    Perhaps try model aircraft suppliers.
     
    Martaine2005 likes this.
  9. Martaine2005

    Martaine2005

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    May 12, 2015
    Now why didn’t I think about that!..
    My RC helicopters (Heim, Hirobo & Kyosho) all have fuselage windows. Albeit smoked and dark in colour. But certainly worth a try. My main two shops have closed down, so a little research on my behalf is needed. Thanks @Bluejets .

    Martin
     
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