Does anyone know what this part is?

 

That's a MOSFET. You should check out some schematic symbol guides. Here's an example:

 

If a resistor on the schematic says +-5%, is that essential? If all I have is one with +-1%, can I substitute?

 

If a resistor on the schematic says +-5%, is that essential? If all I have is one with +-1%, can I substitute?

Yes of course. That just refers to the actual resistance versus the stated value and closer is better. Unless the design was poorly done and relied on a resistor being out of tolerance front the stated value.

Note that resistors can also have a temperature tolerance specification. So, ±5% is ambiguous. Lacking any other information, I would assume it refers to the ohms value.

 

Also, while it appears that it is an N-channel mosfet, how can I determine exactly which one I need since it isn't listed on the schematic?

Thank you,
J

 

Also, while it appears that it is an N-channel mosfet, how can I determine exactly which one I need since it isn't listed on the schematic?

Thank you,
J

You need to know:
1) the voltage you plan to switch (Vds)
2) the voltage you want to use for control (Vgs)
3) the current you expect to run through it
4) the power the MOSFET will have dissipate, which is a function of current and MOSFET resistance (Rds on)

Make sure your MOSFET can handle the Vds you intend to switch.

Make sure your control voltage meets or exceeds to Vgs used in the datasheet for testing (usually 5V for logic level MOSFETs and 10V for standard units.) Also be sure your control voltage doesn't exceed Vgs max rating.

Look in the datasheet to determine the on state resistance (Rds on) for the particular control voltage you'll be using. Then calculate power dissipation from current and Rds on:
power (W) = I^2 * Rds-on
Make sure that the MOSFET can dissipate that amount of power as is, or determine what size heat sink is required in order to keep it cool.

These are general guidelines as a starting point. For some applications, you'd need to consider many other factors in detail. For other applications you'll find that with a little experience you have to components for easy jobs, and you rarely have to check the datasheet anymore. How specific you have to get depends on the application.

I'm guessing that this project is small and simple enough to get away with using almost any logic level unit, but I'm not seeing enough specs to know for sure.

 

Oh, just to clarify on the gate voltage question, you don't necessarily have to have a control voltage that matches what they used for testing. If your microcontroller is a 3.3V unit, you may still be able to drive a logic level MOSFET directly from it. You'd have to look at the graphs in the datasheet to find the Rds-on at that particular control voltage. In many cases, that resistance is already low enough for the intended application at 3.3V Vgs, even if most of the specs are based on 5V Vgs.

 

Hello,

An other parameter for selecting a mosfet is the Vth (threshold voltage).
A lower threshold voltage will most times lead to harder switching.

Bertus

 

Thank you very much!