Ward

Member Since: August 21, 2008

Country: United States

  • News - Little Box Stores | about 2 years ago

    Funny, when I started my post below yours hadn’t hit the comment stream yet. Great minds…

  • News - Little Box Stores | about 2 years ago

    Surplus Gizmos (http://surplusgizmos.com) in the Portland, OR area. They’re nominally electromechanical surplus, but they do carry kits.

  • Tutorial - Nest Teardown | about 2 years ago

    Take another look at the RF connectors…they’re not actually u.fl, they’re switch jacks. In normal operation, the RF signal passes through them to the antennas, but when a coaxial probe is inserted into the socket it breaks the circuit and the RF energy is directed down the coax instead. It’s an and easy way to do RF test, a bit more expensive than a coaxial testpoint but the offsets are more consistent because the test gear isn’t in parallel with the antenna.

    On the battery, the third wire is a temperature feedback line for charging. Many lithium chemistry chargers have provision for this, as an added safeguard in the event of cell failure.

  • News - A Fasten-ating New Produc… | about 3 years ago

    Sooo…it’s a $35 tachometer. Meh.

  • Product DEV-10182 | about 3 years ago

    Indeed, that’s a fine solution!

  • Product DEV-10182 | about 3 years ago

    Thanks for pointing me to the datasheet…you’re totally right, it’s reverse voltage protection. I should’ve started there rather than spouting off based solely on the schematic. The FET/diode/resistor combo is in the application schematic in Figure 32, and section 3.1 addresses reverse polarity protection approaches. Cap negative to the H-bridge ground, as you point out, and it should be fine. Need a different FET though, as Abs. Max VDS for the FDD8580 is 20V.
    All that leaves is the LED1/R20 path…what’s LED1’s reverse breakdown voltage (often 5V-ish) and what happens when you run 120mA backwards through him? Tying his cathode to the bridge common would solve that problem too. Finally, even in normal operation, at 41V all of the LEDs will be subjected to ~120mA forward current, likely at least double their rated capacity. Bigger resistors (intensity issues at lower Vccs?) or some Zener magic (cost?) are in order here. Rev. B? ;)

  • Product DEV-10182 | about 3 years ago

    41V max? Those LEDs may not like 120mA of forward current.

  • Product DEV-10182 | about 3 years ago

    Hard to tell. One might guess reverse polarity protection, but the only thing they’d be protecting is U1 and U2. C2 and C5 (polarized electrolytic caps) would still be exposed to the reverse voltage and would likely fail catastrophically. Vcc undervoltage? Maybe…below 3V or so the FETs would be off. That doesn’t really matter, though, since the FET drains (rather than the sources) are connected to ground. All that happens when the FET is off is an extra diode drop, the FET’s body diode, is inserted between U1/2 ground and system ground. Overvoltage? I don’t see a viable mechanism for that. Puzzling indeed. D1 is a good addition, to prevent VGS overstress when Vcc > 20V.

  • Product DEV-10585 | about 3 years ago

    Neat product, but what on earth is that potentiometer (R2) doing there? I worry about its power rating…there are scant few details on the schematic, but the biggest, baddest SMT pot I can find on Digi-Key is rated at 0.5W. Assuming the resistance of the pot at the close end of its wiper range is at least 1 ohm, and I bet it is, drawing the USB spec max of 500mA you’ve likely exceeded the power rating of the device. Here’s hoping that the IOIO doesn’t allow the peripheral to enumerate as a high-power device. That would afford you 2.5 ohms on R2 before you violate the USB bus voltage spec. My vote: replace R2 with a resettable fuse.

  • Product PRT-10007 | about 3 years ago

    One word of warning: these are not .025" square-pin headers. The pins are rectangular in profile and have significantly less mechanical strength than the .1" pin headers most of us are used to. I’ve had some continuity issues with the headers when mated to the sockets on the Netduino, especially if the pins have any inward or outward angle to them where they meet the socket.
    Edit: Just pulled out the calipers: 0.022" by 0.012".

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