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I made this schema and I want to ask, how transistor control MOSFET. What that s8050 transistor do with the IRFZ44N MOSFET. Please, help.enter image description here

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  • \$\begingroup\$ The S8050 is an NPN complement to the PNP S8550. (I just pulled apart and repaired some circuits using both for a pair of h-bridges for a motor, in fact.) It's got a fairly high collector current specification despite being packaged in a TO-92. You could get by with other small-signal BJTs with smaller collector current specs -- so you don't need to stick with that particular one. You probably should add a base resistor (in series between the I/O pin and the base) to limit the base current, though. With that added resistor: when D3 is LO, the NFET is ON and when D3 is HI, the NFET is OFF. \$\endgroup\$
    – jonk
    Apr 24, 2019 at 22:37
  • \$\begingroup\$ I would avoid spending much time on this primitive design . It is flawed with omissions, details and poor choices. Even the transistor would have been damn hot. \$\endgroup\$ Apr 25, 2019 at 0:45

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It does 2 things:

  1. It inverts the signal. A logic high now turns the MOSFET off, while a logic-low turns it on.
  2. More importantly it allows a low voltage logic signal to drive the MOSFET. This way the MOSFET can have 12V on its gate and turn on fully. Without the NPN you would only be driving the gate with 5V which is not enough.

I would add a small base resistor so you don't burn up your transistor.

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  • \$\begingroup\$ Hi, thanks! Do I understand it right? So, 5V won't be enough because I have heater which need 12V, this 12V can be provided via MOSFET transistor. MOSFET can be controller with 5V from NPN. This 5V is minimum or maximum value to control MOSFET? \$\endgroup\$ May 21, 2019 at 22:15
  • \$\begingroup\$ @TimcoVanco The MOSFET needs MORE than 5V to operate. The NPN can operate on 5V, it then provides switching of 12V on the MOSFET gate. \$\endgroup\$ May 22, 2019 at 14:35
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The s8050 allows the 5V Arduino output to control the MOSFET gate which requires at least 10V. It also inverts, so high= off. There should be a base resistor, 1K or 470 ohms would be fine. Any general purpose NPN silicon transistor will work here.

The turn-on is rather slow because of the relatively high 10K pull-up, so this should be used with a slow PWM frequency to minimize heating of the MOSFET.

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  • \$\begingroup\$ Hi, thanks! Please, from where you know about minimum 10V for MOSFET gate? Why I need base resistor ? (sorry maybe for amateur questions) \$\endgroup\$ May 21, 2019 at 22:17
  • \$\begingroup\$ See the datasheet page 2, 3rd item down, Rds(on) is only specified with 10V gate voltage. Base resistor is needed to limit the current to the transistor base. Otherwise it will be unnecessarily high and could eventually damage something (either the Arduino output or the transistor). \$\endgroup\$ May 21, 2019 at 22:30
  • \$\begingroup\$ Thanks! And one more question, why 1k or 470 ohms base resistor is needed? Why not more or less? \$\endgroup\$ May 21, 2019 at 22:40
  • \$\begingroup\$ Base current when high is about (Vout - 0.7)/R so with a bit less than 5V out you'll get about 3.8 to 8mA. That's well within the capability of the Arduino output. Higher values would also acceptable since you only need perhaps 0.1mA-0.2mA to fully saturate the transistor. \$\endgroup\$ May 21, 2019 at 22:44

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