Specification Clarification for FETs

Question

I use a MOSFET in my project, and I don't know what model of MOSFET I need. I do have a list of specs, but I don't know what they would be called on the datasheet.

I need some help translating my list of required specs into something I can put into an interactive search on an electronics store website.

• Switching voltage (The voltage at which the MOSFET lets current flow)
• Maximum current it can pass from the source to the drain. (How many Amps can I run through it)
• Maximum voltage it can hold back when the gate has no current.(How much voltage can it hold back when not switched 'on')
• Gate capacitance (No idea)

Or, would I be able to just use a switching transistor? I feel like it would simplify things if I could. I am using this circuit:

Edit: Would this MOSFET(datasheet) work? I would be supplying the circuit with 11.4V, the current after R2 would be about 10mA, and L1 gets supplied with about 1A.

Answer 1

From your other question you need to be able to switch 3A and you need to switch 3A so that when the MOSFET is on it isn't going to get too warm so maybe say "it shouldn't dissipate more than 100 mW at 3A. This means its on resistance (\$R_{DS(on)}\$) is a maximum of:

\$\dfrac{power}{current^2}\$ = 11 milli ohms.

Next you need to be able to turn that fet on down to 11 miili ohms with the gate voltage you have available - maybe this is 5 volts or maybe it's 10 volts - you have to work that out then look at a the graphs on the data sheet for several FETs and decide if the gate voltage you can supply is ok for switching it. Here's an example of the type of graph to look for: -

I'm not saying this FET will do the job, I'm just pointing out the graph you need to look for. On the graph above, with a gate voltage of 3.3 volts and a drain current of 3 amps, the drain source voltage will be about 0.45 volts meaning, the on-resistance is 0.45/3 = 150 milli ohms.

Yes it's a little tricky for a novice but you will be doing your self a favour finding the FET yourself and gaining that knowledge you can use countless times over (well maybe a few!).

Also make sure your voltage rating is well above what the circuit might generate and don't forget a reverse biased diode across the coil rated at least at 3 amps.