Arduino capacitance meter

Question

I am trying to measure capacitances using an Arduino UNO.

The C pin charges the capactitor through a 10K resistor (I have also tried 100K and 220), and the G pin discharges it.

V_A and V_B are measurement points across the 10K resistor.

The program does the following:

discharge_capacitor()
while v_b < 4.0V:
    t1 = get_time()
    pin C = HIGH
    dv_1 = v_b - v_a
    wait_a_bit()
    dv_2 = v_b - v_a
    pin C = LOW
    t2 = get_time()
    t = t2 - t1
    calc. q (q = I*t; I = V/R; V is the avg. of dv1 and dv2)
    total_charge += q
capacitance = total_charge / last_measured__v_b
print(capacitance)

I.e. it calculates the charge that goes into the capacitor.

It can measure capacitances of 1 and 3 uF with pretty high accuracy (for my purposes), but it outputs 0 nF when I'm trying to measure 470 uF caps. What could be the problem? Am I doing everything completely wrong?

Answer 1

A 470 uF will be likely an electrolytic type and this will have a leakage current in the order of 1 to 10 uA per microfarad, Assuming 1 uA is the figure then the total leakage will be 470 uA and this will cause a volt drop (irrespective of capacitance value) across the 10 kohm resistor of 4.7 volts and your algorithm will probably never really perform properly.

Answer 2

You are testing at v_b < 4V, so I assume you are using an NFET with Vt<1V.

Vt is often defined as the 1mA conduction threshold, when you want the top FET to conduct much faster, its gate voltage should be greater than supply or drop Vd by 2 diode drops and test at a lower threshold.

Or better yet, change statement to test from 0 to 2.5 to 3V

  while v_b < 2.5V:

Use different Rs values for auto scaling to get measurement counts in an appropriate range with selectable CMOS switches.

An automatic auto-scaling method might use a constant dt method by adjusting RdsOn so get a constant d(v_b)/dt by differentiating it to get say dV/dt= 50m~100mV with a closed loop to control Vg using a PFET with a much smaller Rs.

then C=Ic / constant dV/dt then average this.