Discharging a capacitor with constant current

Post by RC, Sep 28, 2016.
Capacitor Discharge

Since the beginning of my engineering career l have always put a lot of effort to design a "perfect" product, one that will work as designed right after the first release. I do my due diligence and calculate all the analog circuits and run simulations on my circuits assuming different environment. Recently I have designed a WiFi temperature sensor that runs on a battery or an AC power and can monitor and report temperatures to a cloud application. Everything seemed to work fine. The unit takes temperature readings connects to our local access point and from there to our cloud application. Sleep mode current is under 20uA - great. Everything was working just fine until we tried to reset the unit by disconnecting all the power. Since unit consumes only 18uA in the sleep mode it takes forever for the unit to reset. The reason is the total amount of capacitance on my board. I have estimated that the total capacitance on the board is about 740 μF.

The amount of current in a capacitor can be determined from:

The time it will take to discharge capacitors is:

The electrical circut parameters are:

It will take approximately 123 seconds to discharge my capacitors down to 0.3V. From 3.3V to 0.3V:

Ouch, that hurt my reputation!!!