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(Topic ID: 252890)

IC’s operating temperature


By oldschoolbob

1 year ago



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  • 44 posts
  • 9 Pinsiders participating
  • Latest reply 1 year ago by mbwalker
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    #23 1 year ago

    Bob,

    The problem you're going to encounter is manufacturer's rate their parts (operating, not storage) for the die temperature, not the case temperature. Now, this is where it gets complicated: To determine the die temperature, you need to determine the power dissipated in the part, multiply that by the thermal resistance of the part, do the same for the thermal resistance of the part to the PCB board too, and also figure in the ambient temperature. Yikes! I know that's not the answer you are looking for. Point is, there's a lot a viables to factor in here and as Quench eluded too...some parts are better thermally than others.

    In the end, if you think a part is operating correctly (i.e. dissipating the correct amount of power) and everything is working OK but just warm (maybe from a poor design), you can always try adding a thermal pad under the part which will help wick away the heat by lowering the 'case to PWB' thermal resistance. Look up 'sil pad' or 'gap pad'. Not the ideal solution, but it may help.

    Sorry for the long winded answer.

    #29 1 year ago
    Quoted from oldschoolbob:

    Not much shop time again today. Tonight I installed C5. Made no difference in the voltage. Then I moved my 5 volt clip to the left side of L2 (that would be the same as J4 pin 16 and 17). Still no difference.
    Then I inspected the alligator clip and wire. Andrew was right. Small wire and large insulation - plus the wire looks like it's just bent down and crimped with the insulation. Crappy test leads. I may try to solder the wire to the clip but that won't fix the wire gauge. And I can't take the banana plug apart to inspect that end.
    Any suggestions on where to get a decent set of test leads. I think I got these from Jameco.
    https://www.jameco.com/z/TLM3-Velleman-Set-of-3-Alligator-Clip-Test-Leads-with-Booted-Banana-Plugs_2212293.html
    Thanks
    Bob[quoted image]

    Just make your own!

    #31 1 year ago

    I'd just google 'test lead wire'. I'm sure Amazon has something appropriate. If you're just measuring voltages (i.e. no current), then a thinner gauge (18 or 16 gauge, flexible) would be OK since it's a high impedance measurement. For higher current measurements, you'll need thicker, short wire to minimize voltage drop in the test leads. Solder the connections too. I typically make high current measurements with a semi-custom set of wires. Short as possible if a critical measurement.

    One other neat thing you could do is use a clamp on current probe for your scope. That lets you look at current in the time domain - very useful! The voltage drop due to the current meter and leads are no longer an issue. I added a SniptIt of a time domain current measurement that you couldn't measure w/a meter.
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    #33 1 year ago
    Quoted from HHaase:

    I always thought test lead wire was like that on purpose. The extra-thick insulation is there for safety with high voltage circuits measured in the thousands of volts, and the thin conductor acts like like a fusable link to protect against huge amperage spikes.
    -Hans

    The silicone insulation is great for both high voltage and high temperature, plus it's fairly flexible. I worry more about a soldering iron inadvertently touching the insulation than high voltage.

    Never heard of the 'fusible link' comment. If that was a concern, I would think the leads would incorporate a real fuse. You definitely don't want thinner leads on a current measurement since it would result in a voltage drop. A fair amount of meters incorporate an internal fuse for the current measurements.

    #39 1 year ago

    Hi Bob,

    Still sorting out the posts, sorry if I missed something.

    Mind putting your scope on the 5V line when you are connected to the board and drawing current? You would be making sure there's nothing funny going on w/the power supply 'folding back', and that the VDC is nice and consistent (i.e. a flat line on the scope, no ripple, etc.).

    If the supply is good, then the issue would be resistance somewhere between the power supply and the board under test.

    Don't forget the ground wire too. The same current going out the +5V wire is also being returned in the ground wire. So any resistance there will also show up as a voltage drop too.

    Lastly, looking at your picture...starting at the power supply: Is the voltage meter the first thing, then the current meter? Ideally, the voltage meter would be attached at the output side of the current meter, not the input side. Then you can compensate for any voltage drop due to the current meter.

    I think you need to move the voltage meter red wire *IF* the big fat red wire on the current meter is the output. Like this:
    pasted_image (resized).png

    #44 1 year ago
    Quoted from oldschoolbob:

    What a difference a good set of test leads made.

    Way to go! Case closed.

    BTW, you 5 V meter ground lead is in the right place...after the current meter. Good.

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