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I'm trying to design a circuit to charge a capacitor bank with constant (limited) current and then discharge the capacitors through an LED. I've come up with the circuit in the schematic below, however looking at it now, I'm not sure if it's going to work the way I want it to, due to the 2 N-MOSFETS being in series (along with a small resistor also).

What I want is to slowly charge the capacitors at 128mA (blue path) up to 5V, and then use a digital 5V signal to switch on the MOSFET Q2, which will primarily discharge the capacitors through the diodes at high current (black path). Once the capacitors are discharged sufficiently, it should also keep the current coming from the power supply through the LEDs limited to 128mA. enter image description here

I have some concerns since I've never done or seen an arrangement of MOSFETS like this (even more so with one of them acting as a current limiter). Will this affect the operation of the MOSFET(s) and if so how?

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  • \$\begingroup\$ You see that all the time. It's called a half-bridge...sort of. I would be more concerned about how you overlooked the floating gate drive requirements of both Q1 and Q2. You clearly have a source follower For Q1/R9 and your Q2 is also a source follower of a sort. \$\endgroup\$ – DKNguyen Oct 3 '20 at 4:48
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    \$\begingroup\$ Your circuit can't work because Q2 has no Gate drive. Do you have a drive circuit for it? If so, show it to us. If not, is that why you are not sure if it's going to work the way you want? \$\endgroup\$ – Bruce Abbott Oct 3 '20 at 6:01

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