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I planned to make a DIY power meter with automatic switch for current range, usually towards low (uA maybe) currents.

I came up with the design below to switch between various current-sense resistors. The MOSFETs will be chosen to have very low Rds with provided gate voltage, and the ADC has differential inputs and gain control so each pair of ADC input measures exactly the voltage drop on resistors alone. While theoretically it seems OK, it's also quite simple which makes me think there would be an unseen problem with this design since some other similar devices have more complexity (like this one from Elektor).

So would this design work as expected?

Current sense switching mechanism

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  • \$\begingroup\$ It's hard to tell what your expectations are for this circuit and I don't see any similarity with the Elector device either. There is a shadow of impenetrability hanging over your question it seems. \$\endgroup\$
    – Andy aka
    Commented Feb 27 at 21:17
  • \$\begingroup\$ You needn't put "question about..." in the title. \$\endgroup\$
    – JYelton
    Commented Feb 27 at 21:51
  • \$\begingroup\$ I could not find the schematic in the Elektor article, can you add it? I assume you want to measure only positive currents? \$\endgroup\$
    – BitLauncher
    Commented Feb 27 at 22:05
  • \$\begingroup\$ Do you measure the voltage between V_SENSE and ground? Then UDS of Q1/2/3 * (R8 + R9) / R9 will be added (wrongly) to your measured voltage value. Otherwise I would tie R9 to J1 pin2 instead of ground - and measure between V_SENSE and ADC+. And you measure the current between ADC+ and ADC-1/2/3? PS: SI2302 has some parasite capacitance between drain and gate (and source and gate) what reduces the bandwidth of your current signal, here a similar MOSFET PMV20XNE. \$\endgroup\$
    – BitLauncher
    Commented Feb 27 at 22:28
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    \$\begingroup\$ I haven't looked really closely at this schematic, but I should point out that it's very common to put a current-sense signal into an amplifier... this way you don't load the circuit being measured with a voltage drop big enough to measure. (Ideal ammeter has zero series resistance and zero voltage drop, the closer you get to ideal the better). Once you have an amplifier stage, you can look at controlling its gain instead of switching the current-sense resistors. \$\endgroup\$
    – Ben Voigt
    Commented Feb 27 at 23:15

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