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EEVblog #1336 – DT71 LCR Tweezer Destructive Teardown + Lab Update

Destructive teardown o nthe Miniware DT71 LCR meter Tweezers Plus a lab move update. Forum: ...


  1. These slots in PCB look like a thermal barrier. The design is pretty compact and the separation helps to reduce heat transfer from the hot, power part of the board to the signal part. With the DAC built from discrete resistors, having gradient of temperature across the circuit might cause a serious thermal drift.

  2. Looks like this weird shield is directing the airflow to make sure most of it passes by the transformer.

  3. Around these parts, namely Providence, RI (Region: New England) it’s 125VAC. I know, I’ve tested.

    But odd that they do 220 only when they use a standard power connector.

  4. Korad KA3005P PSU, SRA Soldering Products (via Amazon) $99.99 USD, TRIO Smartcal A$199.00 (approximately the same in USD these days). Pffft…

    The non programmable KA3005D version via SRA Soldering Products is ten bucks less.



  5. Wow the Amazon offer is good! but you must include the $60,- shipping and don’t forget the local taxes is you unlucky. So that will count up and the triosmartcal offer As$199,- is close then. Also if you inlcude if you have a problem and need to ship back.

    For less then that price you will have a ATTEN PPS3005S with RSR232 in the EU. (€136,- incl. vat )

    If you like to have a realy good PPS look at the new ATTEN ATM8811 http://www.atten.eu/index.php/power-supply/atten-atm8811-programmable-power-supply.html

  6. The punch on the back looks more like a GPIB sized hole rather than a DB25.

    The bodge cap on the USB board is cute.

    I like the power board too. I’ve used that arrangement of heatsink-device-board sandwich before, and it works nicely.

    The ‘isolation’ slots may be hedges against thermal expansion issues.

    All of those HC595s and resistors gets one a 24 bit R2R DAC a lot cheaper than a ‘real’ unit, depending on the cost structure. I’ve done similar things. With reasonably accurate resistors, it’s a great way to cheat the budget with something like an ATTINY or PIC12.

  7. Just noticed: never even turned it on 🙂

    It would be interesting to see a basic testing side by side with some comparable Agilent power supply, like U8002.

  8. Hi Dave !

    I think the encoder is not optical, just cheap mechanical. But a bit hw and sw debouncing, no problem at all, except lifespan.

    Oh, nd turn it on, I like to see gloving those 7segs.

  9. Hi Dave, just took a sneak peek at eevblog #315

    Like Marlon Brando would say ” the horror “

  10. Maybe the controller is a NXP LPC11xx? they come in LQFP48 (seems to be the package), have 32 bit words and NO EEPROM.

    Maybe babe.

  11. ah, the Quarz seems to be 12 MHz ?? That would fit, LPCs use 12 MHz oscillators and beef them up with a 4x PLL.

  12. To me if I use hahaha 🙂

  13. Hole in the back is CLEARLY a GPIB hole and not db25. You are getting lazy Dave.

  14. AM I the only one who saw the ground wire, soldered on capacitor and choke on the USB board and thought “quick and dirty EMC fixes”?

  15. On main board you got nouvoton 8051:
    http://obrazki.elektroda.pl/8204593300_1327392137.jpg and on usb board you have some cortex m3 nouvoton: http://obrazki.elektroda.pl/9513294100_1334172237.jpg

  16. In my opinion, a good digitally-controlled power supply design should always default the output voltage to 0 if a fault is detected. And the measurement circuitry should always be separate from the control circuitry so that even in a situation where the control circuit fails the output voltage and current reading will remain accurate.

    I was shocked to see This PS outputs 50+ when it went haywire. It certainly gives you a lot of confidence.

  17. The idea to add solder to a copper trace to add current capability is not very clever. The conductance of solder is much worse than copper so most of the current will flow in the copper trace anyway (Kirchhoff’s current law). At the same time the solder will increase the thermal resistance of the solder covered copper trace and lower its maximum current capacity.

    Increased copper thickness is a better way to go but thicker base copper also increases the minimum trace spacing. 0.2 mm was the limit with 70 um copper with one supplier we used. The option to plate extra copper on top of thinner etched copper was not good enough as the plating was thinner on the wider tracks, those who really needed extra plating, than on the thinner signal traces.

    • Niklas, you totally beat me to that, except you explained it quite a bit clearer than I would have. Kudos. In fact, because they are so dissimilar in heat conductivity it actually increases the heat transfer resistance compared to a trace exposed to air and hinders cooling.

    • Niklas, I think you are mistaken. Whether adding solder to a copper trace improves current handling capability or not depends on how much solder you add (cross-sectional area of the solder compared to the cross-sectional area of the copper trace).

      For example, at room temperature a 1oz/sqft copper trace (0.034mm thick) will have a resistance that is 1.34 times greater than 63% tin, 37% lead solder tinning that is a semi-circle in cross section with a diameter equal to the trace width. This is regardless of trace length and/or width.

      This factor doubles for 0.5 Oz/sqft traces (better), and halves for 2.0 Oz/sqft traces (worse). So for 2.0 Oz/sqft, the tinning actually has a higher resistance than the copper trace.

      This assumes the resistivity of copper is 1.68X10-5 Ohms*mm, the resistivity of 63/37 solder is 1.44X10-4 Ohms*mm and the density of copper is 8.96 g/cm^3.

      As for heat dissipation, I have not seen a similar analysis of this so-far (interesting exercise though). But it should be noted that the thermal conductivity of copper is approximately ten times greater than that of 63/37 Sn/Pb solder [401 W/(m*k) for cu vs. 41 W/(m*k) for 63/37 Sn/Pb].

      I did this analysis some time ago (citing from lab notebook). Perhaps someone can check my findings.

      I do not have resistivity and thermal conductivity figures for RoHS compliant solder.

      Regards, David in Jakarta

  18. Hey Dave,

    Just got this PS from Amazon in the US (for $99.99 inc shipping). Pretty good deal I thought.

    The version I got as a double main tap on the transformer and a switch on the back to select one or both wiring (2x110V).
    Thought some of the readers might be interested to know that.

    And unfortunately the uC markings have been erased pretty hard on mine too…

    Thanks for a great video (again)!

  19. You can see the chips whose marking were erased here: http://translate.google.com/translate?hl=en&sl=pl&tl=en&u=http%3A%2F%2Fwww.elektroda.pl%2Frtvforum%2Ftopic2164853-90.html
    These are made by Nuvoton (both on control and USB boards).

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