I want to design a circuit capable of controlling the voltage of either a inductive (electrovalve) or resistive (heating cord) load.

I want to have a variable voltage for these loads:

  • for the electrovalve (24V, 0.41A, 10W), to be able to power it at full voltage to switch it "on" and then decrease voltage just above the "switch-off" level, to reduce power consumption and heat generation.
  • for the heating cord (24V, 2.8A, 64W), to be able to control precisely their temperature.

What I did so far: Circuit draft

Few explanations about the components' choice:

  • The optocoupler is a ILQ2 because why not.
  • R1=1k. Vphotodiode=1.25V and VstatusLED=2.25V, so VR1=5-1.25-2.25=1.5V, so I=1.5/1k=1.5mA which is fine for the Arduino I/O pin
  • I chose a MOSFET IRF520nPbF for its ID=10A, which gives some margins for my application, without being too high. If I understand correctly VGS should be between 4V and 20V to switch it ON.
  • That is why I chose R1=R2: it gives a VGS=12V which is right in the middle of the 4-20V range.
  • However I am not sure about the absolute value of R1 and R2. I put 10k because this is what I found on other sites, but I can't explain why.
  • The Schottky SB5100-T flyback diode should be more than enough for my application, and I guess that for the heating cord it is useless but not harmful (?)

Considerations about component's switching time

  • Arduino PWM frequency is 490Hz (~2ms) for most pins, climbing to 980Hz (~1ms) for 2 of them.
  • ILQ2's rising/falling time is below 5µs --> OK
  • IRF520NPbF's rising/falling time is below 50ns --> OK

What do you think about the circuit's design and the component's choice? Did I miss something?

My electronic knowledges are old and weak, the circuit above is basically a copy-paste of small chunks of circuits found on various places on the web...

  • \$\begingroup\$ You should put the status led in parallels so that you are sure the total voltage drop is lower enough. \$\endgroup\$
    – Damien
    Jun 17, 2022 at 9:00
  • \$\begingroup\$ "IRF520NPbF's rising/falling time is below 50ns" With that gate drive? Probably not. \$\endgroup\$
    – Klas-Kenny
    Jun 17, 2022 at 9:02
  • \$\begingroup\$ A small elaboration on my comment above; the MOSFET has 330 pF input capacitance that needs to be charged to raise the gate voltage. Doing so through 10 kΩ takes some time. \$\endgroup\$
    – Klas-Kenny
    Jun 17, 2022 at 9:07
  • \$\begingroup\$ (1) I would suggest a much lower, or logical Vgs(thres) N-Ch Power MOSFET such as IRL540N. (2) Power MOSFET falling/rising time is not critical at all, because you PWM frequency is so low. \$\endgroup\$
    – tlfong01
    Jun 17, 2022 at 9:17
  • 1
    \$\begingroup\$ @tlfong01 If by "optoisolated relay" you mean something like the typical 8-relays board with SRD-05vdc-sl-c, their life expectancy is too low: the instrument I build should last several years and temperature control requires ON/OFF cycles every few seconds. \$\endgroup\$
    – Blacksad
    Jun 17, 2022 at 14:43


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