how can I size a charging capacitor in a parallel RC circuit?

Thread Starter

Moe Ali Bukhamsin

Joined Oct 2, 2016

I am using a fluke 1503 to induce an instantaneous DC 600V/ 1mA (see across a across the parallel resistor in an RC parallel circuit. I am measuring the resistance between the positive lead and the ground, checking for insulation.

Design goal/requirements
My goal is to simulate the capacitor charging in 8 seconds, the fluke is measuring the resistance. In 8 seconds, aka, the capacitor is charged, the voltage build up goes through the parallel resistor and the Fluke will read the resistor value: 1M ohms. This verify the testing results are good (safe insulation).

DC voltage: 600V (from the fluke)
Input current 1mA (from the fluke)
Wanted Resistor reading: 1 M ohms
Time simulating the capacitor charging build up: 8 sec

- Am I approaching this the right way? Is a parallel RC circuit the right application I need to achieve my goal/objective?

- Are there better approaches with the same design specifications?

- Which equations should I use?

I understand that DC capacitor equations are the needed ones to articulate the circuit set up.
I adhere to these equations

Vb = IR + Q/C

where Q = C*V

i(t) = (V/R1)*e^(-t/(R1*C))

Where R1 is the source resistor and R2 is the parallel resistor
But I am still lost in sizing because when I assume the constant values of current and resistance, I get a huge number for capacitance. Does that make sense? Should it be huge?
If so, how can I protect myself and design the circuit with circuit protection?

see the attached pictures for the base schematic and math.

Any guidance would be appreciated. thx