Recorder Drain

conclusion from the future: voltage buffering OpAmp works flawlessly!

Problem

  • harvest source is weak, so it should be connected with low leakage

  • current drain is a Mosfet, that can pull the source to the desired voltage

  • mosfet is controlled by an inverted opAmp-voltage-buffer, resulting in a PWM-Controller that is smoothed out by a slow RC-Filter

  • RC-Lowpass is delaying (big) voltage-changes by 1 to 1.7 ms

  • second problem: opAmp is oscillating, there must be a better way

Ideas / Approaches

  • JFET

    • pro: larger resistive range than mosfet

    • con: negative voltage required, very little variety

  • PID controlled Mosfet

    • controller can hopefully be reduced to just one OpAmp

    • con: very complex, tuning required

    • TODO: maybe there is just some integrator and a small proportional part needed

  • Bipolar Transistor

    • TODO: can a BJT go into very high impedance? Seems not (small sample check)

  • drain directly into output of OpAmp (voltage buffer)

    • pro: fast, clean, simple

    • con: no high impedance, could feed into a harvest source

  • Switched OpAmp-Sink

    • Mosfet above OpAmp ⇾ would need voltage offset to control mosfet

    • analog switch would be a nicer solution than mosfet

    • con: could still feed into harvest source

  • oneway OpAmp-Sink

    • Diode above OpAmp to stop reverse flow

    • could be included in opAmp-return-path (compensate diode offset)

    • high impedance (over opAmp voltage range) with a low leakage diode

    • con: still no (real) high impedance, needs dual supplied opAmp to reach 0 V

Conclusion

  • combined switched oneway OpAmp-Sink is a proper solution

  • diode-solution should be fine

  • WARNING for diode solution

    • watch out with power-levels mixed with enabled-states

    • if OpAmps VOut settles at V-, the diode will drain harvester below GND

    • V- could be higher than V+ in disabled state (LDO for Pos, BoostInverter for Neg)