Microcontroller driven circuit that opens a charging circuit and closes a resistive circuit simultaneously from the same I/O pin

Question

I'm working on a project that charges a capacitor with an external 5V DC power supply and then discharges it across a very low resistance. Since I don't want to close the circuit while the power supply is in-line with the load, I'm trying to design a circuit that would simultaneously close the loop between the capacitor and the load while opening the loop between the capacitors and the power supply. I'm working with a microcontroller with a max voltage output of 3.3V. I've been trying some different designs, and fundamentally it should be made up of the following components:

1. An inverter circuit to invert the signal from the IO pin using S8050 or similar
2. A Darlington array (ULN2003 in this case) to drive a 5V signal to trigger the MOSFET and charge the capacitors
3. A power MOSFET (IRLB3034) as a switch to open or close the resistive circuit

But I'm having an extremely hard time getting all of the components to work together since I'm a novice. Which resistors to use, what considerations to take in when using multiple power sources in order to avoid damaging the microcontroller, if I need any additional components are holding me up from progressing with this, admittedly, simple circuit. How should I design a circuit like this and can I accomplish it with the components listed above?

EDIT:

I’m working on an art project that displays a sequence on a CRT that is powered by a Raspberry Pi. The RPi is also controlling a number of components (e.g., power LEDs, fans, spinning HDD) stored in an old desktop computer case to make it appear as if the sequence being displayed is coming from an old computer. The RPi is controlling an ATX power supply that is driving all of these components. At the very end of the sequence, the computer abruptly shuts off with a loud popping noise from a speaker within the tower, and smoke (or in this case vapor) starts pouring out of the desktop

The smoke machine is a low resistance nichrome coil (~0.2ohm) immersed in propylene glycol. I want the capacitors (5.4V 250F) to provide the power necessary to run a high current through the nichrome which in turn heats and generates vapor. I need the capacitors to discharge when the RPi tells it to, which would only occur when the ATX is off.

Since the ATX will be off and no longer charging the capacitors when they are meant to be discharged, throttling the charging current won't be necessary for this application, but I also don't want the capacitors discharging unless the RPi is providing a signal or both the RPi and the ATX are off.

Answer 1

Perhaps something like this:

^{simulate this circuit – Schematic created using CircuitLab}

The IXDD609PI is a gate driver to switch the two MOSFETs smartly. The IPP120P04P4L03AKSA2 is a hefty P-channel MOSFET. R1 represents whatever is limiting the capacitor charge current. If you don't add external resistance it will be limited by the power supply (internal resistance and current limit), capacitor ESR, wiring resistance, M1 MOSFET on resistance and wiring resistance.

Common ground between the ATX supply and MCU is assumed. Heavy currents flow in some of the wires so some layout care is called for. Even the gate currents can be several amperes (briefly), hence ceramic MLCC capacitor C2 placed close to the gate driver chip power pins. A few ohms in series with each gate might be a good idea (5 to 10 ohms) to slightly slow the switching.

Keep in mind that if M1 and/or M2 fails, it/they will likely fail shorted, so your device should deal with that in a safe manner.

The driver is non-inverting, so a 0V input signal results in the capacitor charging through M1 and (if present) R1, and a 3.3V input results in the capacitor discharging through R2 and M2.