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I tried to decrease value of Rg resistor on the gate of the MOSFET (the one on the right) but then the MOSFET does not open enough and i get low currents trough it, i have this setup on breadboard and results are very close to simulation, when i have voltage at 10V and current fully closed i read 15-20mA on the DMM.

I use IRF630 for MOSFETS and the circuit is powered off laptop charger 19V.


Edit: There is also error in regulation, preventing the mosfet to close fully as there is 3.2V at gate when the voltage is closed, i guess that is the reason it does not close.

Link to simulation: Falstad Simulation

  • \$\begingroup\$ I think you should start with a vary basic MOSFET circuit, like a controlled resistor and on/off switch. I think you lack of a basic MOSFET working principle. \$\endgroup\$ Apr 20, 2021 at 20:15
  • \$\begingroup\$ @MarkoBuršič i did, i started with just one MOSFET, the voltage controlling one on left and i'we got what i wanted it opens from 0 (not fully, about 100mV) to 15V, but got stuck on second one cannot close it fully. \$\endgroup\$
    – josip
    Apr 20, 2021 at 20:34
  • 1
    \$\begingroup\$ You are trying to drive a high side N-MOSFET, but it doesn't work like that. You can split the circuit and leave only the left side, and you'll see that it won't work either. \$\endgroup\$ Apr 20, 2021 at 21:04
  • \$\begingroup\$ What is the goal in this circuit? In your picture you are achieving regulation with both op-amps, so from that perspective it's kinda working. Your actual Falstad link has different resistor values on the right gate that will keep it from working. \$\endgroup\$
    – W5VO
    Apr 20, 2021 at 21:59
  • \$\begingroup\$ @W5VO i need basic adjustable power supply for my projects like arduino or i will use it as battery charger etc, yes sorry that resistor would be 100k i left it low when i was playing with the circuit before posting. Yes regulation is so the current/voltage stays the same as load changes. \$\endgroup\$
    – josip
    Apr 20, 2021 at 22:05

2 Answers 2


In Falstad’s sim, you must define the Beta (RdsOn) and not just Vt . You had the default beta=0.02 which is a high Ron like 4000 series CMOS. The properties allow you compute RdsOn for the Vg, Vgs(th) and Beta value.

Also all IC’s are ideal voltage sources, so to simulate current limit add 220 to output or if CMOS 1k and inside if using a closed loop.

Logic would be 300 ohms in series for 4xxx 50 Ohms for 5V logic and 25 Ohms for 3.3. logic family. if you cared about load capacitance and ESR etc, you add those if that is important.

Normally I might choose Vout for 15V max and not divide 19 with 10k series to 2k shunt. The FET has a body diode option which don’t need to add since you defined

So you can model any RdsOn by selecting Vgs, Vgs(th) and use Beta somewhere between 1 and 50 for starters.

  • \$\begingroup\$ Yeah thanks i totally forgot on that option, was a while since i last used Falstad :) \$\endgroup\$
    – josip
    Apr 21, 2021 at 0:22

Actually you are wasting your time on that circuit. A high side N-MOSFET shall have an additional isolated power source (like in my schematics 0-15V) to be able to control the load current/voltage. There are charge pumps for that, if you don't want to use a separate power.

So, forget it or change everything.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Maybe, to avoid changing everything, the OP could use a depletion MOSFET. I don't know if this exists in Falstad simulator. \$\endgroup\$ Apr 21, 2021 at 13:04
  • \$\begingroup\$ @CharlesJOUBERT unfortunately no depletion mosfets at hand, i have maybe 1 PMOS, i use mostly salvaged stuff for my projects i try to buy as little possible, i think i have also latching problem on both OP-amp regulators, but not sure, its hard to find good information on web about these things. \$\endgroup\$
    – josip
    Apr 21, 2021 at 15:03
  • \$\begingroup\$ Ok but how to control current? \$\endgroup\$
    – josip
    Apr 21, 2021 at 15:07

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