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GeoffT

Member Since: March 22, 2010

Country: United States

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EE with over 30 years experience designing opamp circuits, sensors, amplifiers, servos and safeties.

  • I used a certain pulse generator capable of giving me an On-Off-On signal to control a high current PMOS FET switching a high current battery source. The high currents can also flow from an adjustable supply with an output cap >> the DUT input cap. I adjusted the On and Off times looking for switcher misbehavior. Things to look for include output voltage overshoot and high slew rate of the input cap, indicating over-current. These stresses result from millisecond Off times, but only exist for microseconds. It is quite possible for the DUT to survive wounded, only to become a failure later. At least one previous employer mandates this test as part of new product DVT.

    It would make better sense to use a micro-controller with a level shifter to drive the PMOS, so you could even sequence the times while watching or frame-sequence-storing the behavior.

    The series-resistor option for input spike-taming (a separate problem than the On-Off-On issue) has some notes: – It obviously must be rated for the RMS input current maximum of the system. – The current spike connecting to a battery can be double-digit Amps, so R must be pulse-rated. Tiny metal films might erode. Composition types are more robust. – The R value depends on the system wiring inductance and input capacitance. I’ve recommended 1 ohm for applications in the upper half of the input rating and had good results, but optimal for this or other systems may vary some. Sometimes I test for the minimal R capable of limiting to a safe spike peak, with the worst case maximum wire length and the source and return leads separated.

  • Although the described circuit adequately clamps the Vin spike, it requires 3 parts, 2 of them semi-conductors. Also, the scope shows 3,000,000 V/sec ramp rate on a 10uF cap, indicating a current of 30 Amps that the FET must divert, assuming the cap ESR is near zero. In some applications, the same spike remediation can be achieved with a small resistor of an ohm or even less. The resistor can be either inserted in series with Vin, at a small loss in run-time efficiency, or only in series with the 10uF cap, costing some input voltage ripple due to switcher current draw. Another subtle hazard when plugging a switcher to a battery is the risk of certain make-break-make contact bounce intervals that can partially charge the switcher output cap, stall during the break, and re-commence charging at the next make. If the soft-start function does not re-engage, the final charge can be violent enough to cause over-current damage to the switcher or voltage over-shoot damage to the powered components. Not all switcher types are susceptible, but it’s worth checking for in design, especially as this cause is hard to diagnose on returned units.

  • Primary protection against electric shock is provided by the resistors in series with the arm and leg leads. SF should confirm that those resistors also are rated for >36V insulation resistance and that the clearances of the leads are sufficient for that voltage. Although SF customers are generally savvy, my rule for design of such devices is that they should not conduct even if an idiot user touches 120V and the device is earth-grounded.

  • I must disagree with other posters' enthusiasm for McGuckin Hardware. My mistreatment at the hands of their security, when I had stolen nothing, was not just unprofessional, it was un-American! As a former store detective, I say so from experience. SF is blameless for associating with them, but I will not attend.

  • A PCB was being calibrated on a pogo-pin vacuum tester, but was not repeatable and passed PCBs failed in system. It turned out unequal forces on the (large) board could generate piezoelectric currents in a high-impedance node, sufficient to swamp the cal. This could show up today testing designs using modern accelerometers or cap sensors. (The solution at the time was foam pads under the unsupported areas).

  • An EKG connection is one of low resistance to the human body, close to the heart. A fault current much less than a milliamp can kill, if the frequency and phase are unlucky. No one should modify anything not designed for this purpose for this purpose!

    That said, you can make a homebrew EKG, just design from the ground up for safety. You would want battery power and optical isolation for the human-contact circuit’s interface to anything wall-powered. There are alternative instrumentation amplifiers in friendly DIP packages. You may also find some packaged circuits prewired for the EKG application.

  • The TL084 opamp has a 3MHz gain-bandwidth product. The 150K resistors in the circuit are, for this topology, about 10X the value for which stability would be assured. Users should check IC2 pins 7 and 8 for signs of ringing or oscillations at about 1MHz. If so at pin 8, add a few pF in parallel with R11. If oscillation is seen at pin 7, try a few pF in parallel with R8 and R9, but check for non-linear oscillation due to interaction with the diodes.

    Also, the TL084 only has proper specifications for +/- 5V or greater supplies.

  • The 3.0V threshold is for only 250uA drain current, and increases typically 100mV at 0degC, so 3.1V. A 3.3V supply at -5% tolerance is only 3.135V. Even if the driving device had zero drop from supply to logic output HIGH, the small divider at the FET gate reduces applied voltage to 3.104V, leaving essentially no extra voltage to enhance the current beyond 250uA. Compounding the issue, the e-textile application often uses thin wires having more resistance, so drop in the GND wiring further subtracts from Vg-s.

    The saving grace is that the “typical” Vg-s(th) is only 1.7V, so the worst-case scenario is rather unlikely. This circuit would be flagged in a design review for a critical application.

  • All CMOS devices are type HC, not HCT, so 3.3V control signals would be near the minimum spec value for recognition as a logic “1”. It would probably work, but temperature and unit-unit variations could cause mysterious errors over a population.

  • Probably not. The MOSFET is only rated for 30V and the resistor divider would slow the switching, causing extra power dissipation.

No public wish lists :(