I need to switch a 400mA load with a PIC. Would it be better to use a logic level MOSFET or a Darlington? Are there alot of advantages one way or the other or is it more a matter of preferance in this case?

 

A darlington has a voltage loss of 0.8V to 1.5V. A half-decent Mosfet has an extremely low voltage loss.

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Uncle $crooge

 

Thanks Audio!

I don't know if i want to waste that much voltage on this project.

Sorry to be a pain, but when reading MOSFET datasheets, what value do I look at to know how many volts it takes on the gate to turn in ON? I have not used a MOSFET yet, so I don't know much about them.

 

Do a parametric search for logic level FETs. They are rated to saturate with 5 volts on the gate, vice 10 for the conventional ones. I came across one from IRC that was logic level, rated 20 volts at 180 amps, & only $2.18!

 

Okay, so for this one -
https://www.jameco.com/Jameco/Produc...dDS/178669.pdf
It shows Vgs(th) max 3V - Does this mean that more then 3v will damage it? What does the "conditions" of 1mA mean?

 

Vgs(th) is the voltage at which the MOSFET begins to turn on. The three numbers (min, typical, max) give some idea of the distribution of values. A typical would be the mean of the distribution while the min and max might rpresent the 3-sigma points. The condition of 1 mA represents the current that defines the threshold Vgs(th) since there is no standard definition.

One thing to note is, that if it doesn't begin to tun on until 3V, how hard will it be turned on at 5V? For that you have to refer to the graphs. 400 mA in a TO-92 package seems like a bit of a stretch to me. I'd look further for a part in a TO-220 package.

 

Never mind the graphs because they show typical parts. You can't order a typical part, you might get a weak but passing one.

The tiny 2N7000 Mosfet has a max continuous current of only 200mA.
With 10V on the gate and 200mA through it then its max saturation voltage loss is 1V when it is cold and 1.8V when it is hot. It will be pretty warm with a current of 200mA.

With only 4.5V on its gate, it turns on poorly and has a saturation voltage loss of a little more than with a gate voltage of 10V.

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Uncle $crooge

 

Of course that's what they show. It's more information then is contained in a line in a table. Why are you being so...ah prickly?

 

Parts are cheap.
I design circuits by using the minimum and maximum ratings so all my circuits work well. I don't have any circuits that don't work because the transistor passes the spec but doesn't have enough performance.

The yield from manufacturers varies as shown by the wide spread in the specs. Also, maybe somebody bought the entire "high specs" production before you got there.

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Uncle $crooge

 

And do you suppose that you are the only one who does that?

 

Absolutely use a MOSFET.

Look for Vgs(th) in the data sheet. That's the turn-on voltage. (2-4V typ)

Look for Gate-Source voltage in the ABSOLUTE MAX RATINGS section, or whatever it is called. This will be much higher than Vgs(th). (+/-20V or more typ).

A part we frequently use here at work is the IRF 7313. Dual N-channel enhancement mode mosfet in an SOIC8 package. Works fine with 5V out from a PIC. Just put a series R (100ohm) to isolate the PIC output from the gate capacitance of the fet, and a pull up or pull down resistor (10k) right at the gate so the FET is on/off at startup (before the pic takes control). We've got some other parts here too if you don't need the dual that I can recommend. But there should be lots of parts out there that will work.

ETA: Actually a much cheaper and better part to use would be this guy: NTD80N02-001

Do a search on that at digikey. They are 1.30ea. Very low on resistance, very high current. Work well with extremely low Vgs and Vds. Used them with 0.5V Vds and Id=4A with Vgs <3V here at work on a project. Worked fine.

 

I am with audioguru, I learned a long time ago when designing with bipolar transistors to use the minimum beta.

I started in electronics as a repair technician and worked my way up to engineering. Things I build for myself last like for ever. one comes to immediate mind, a digital clock with a MM5314N clock chip and flouresent display. Built about 24 years ago and still works fine.

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The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

More a matter of personal preference than anything else - and certainly commercially its far more common to find bipolars than FET's used for such a purpose.

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Thanks everyone.

Speakerguy - thanks for the recommendation!

 

For multiple loads, like segment displays or led arrays, I use the humble ULN2003 as a "single chip" solution with 7 channels, for single loads, a logic mosfet is definitely worth its coin.

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