This is my first time asking a question here, so please be gentle?
I understand the theory of how mosfets work, but I'm having a lot of trouble deciphering all of the data sheet terms, to pick the right one.

I will be using an attiny85 to drive the mosfet. I have 3.3 volts with a maximum current of 4mA, to drive the gate. The circuit that I'm trying to switch on has a maximum of 24vdc, and a max current of 30mA.

My project will be positioned outdoors, so ambient temperatures will most likely range from freezing cold to 110 degrees Fahrenheit.

Any suggestions as far as Rds On (Max) @ Id, Vgs, or Vgs(th) (Max) @ Id, or Gate Charge (Qg) (Max) @ Vgs ranges?

  • \$\begingroup\$ What MOSFET are you currently looking at or currently trying to choose? \$\endgroup\$
    – user103380
    Nov 29, 2017 at 16:02
  • \$\begingroup\$ If you want the load to be connected to ground all the time (e.g. for safety reasons) then hi-side switching and thus P-ch MOSFETs can be used. Otherwise, low-side switching and thus N-ch MOSFETs can be used. The latter is easier, cheaper and less complex (I would go for this). VDS should be higher than supply (e.g. 60V); continuous drain current, ID should be higher than max load current (e.g. 200mA); gate-source threshold voltage, VGSth should be as low as possible (< 2V). The lower the Rds, the better. Input capacitance, Ciss ismportant if switching will be done frequently (e.g. 10kHz). \$\endgroup\$ Nov 29, 2017 at 16:18
  • \$\begingroup\$ See my recent answer to a different question. Same circuit will work. electronics.stackexchange.com/questions/341928/… \$\endgroup\$
    – user57037
    Nov 29, 2017 at 17:10

1 Answer 1


For a beginner all there parameters are overwhelming, I'm sure.

The best advice I can give is: do what others do.

So look on the internet (this site even) for something similar, to switch on/off some load working at a higher voltage using a micro controller.

Most of the parameters you mention are not relevant for switching something on/off, assuming you switch slowly, less than a few times per second.

You're looking for a MOSFET and that might not be the best choice as your microcontroller runs on 3.3 V and most MOSFETs really need a bit more like 5 V at least. Even the 2N7000 NMOS I often use has a maximum Vt of 3 V which would leave little margin to 3.3 V. If you find an NMOS with a max Vt of 2 V that'd do fine.

Since you're only switching 30 mA you can use a generic bipolar transistor as well. The voltage of the load is 24 V so Vce,max should be more than that. No worries, most transistors can handle 40 V or more.

A common BC547 can easily handle the job so I suggest to use the following circuit:


simulate this circuit – Schematic created using CircuitLab

I'm assuming you want to switch a relay and then the flyback diode (D1) is important as it prevents Q1 from damage.

If the load is not inductive (LEDs, resistor, lightbulb) you can skip D1.

The BC547 is a model that's popular in Europe, in the US you could use a 2n2222, it will do as well.


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