MOSFETs to stock for the parts bin? (and mosfet general questions)

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I have assortments of resistors and capacitors, and even bipolar transistors (https://www.aliexpress.com/item/Free-Shipping-18valuesX10pcs-180pcs-2N2222-S9012-S9013-S9014-A1015-C1815-S8050-S8550-TO-92-Transistor-Assorted/32323342275.html).

 

But what would be useful for MOSFETs?

 

I would foresee using them mostly for turning power on and off to circuitry and sensors (so fairly low current, either a couple/few mA up to a couple of dozen mA), enabling/disabling flow of charging power to batteries, motors for a volt or two up to a 12vdc, sprinkler valves, and possibly for actual appliances (such as hothouse heaters, but I assume that optically isolated relays are a better idea for that . . .)

 

While I managed to learn their theory ears ago in my physics degree,I remember very little, and am slowly picking up and reabsorbing bits and pieces.

 

As I understand it at the moment, p-channels would turn on/flow from a high signal from the avr, while n-channel would trigger with a low.

 

And until I  configure a pin as an output, the internal pulp resistor applies?  

 

In turn, does that suggest n-channel as the more frequent choice if I don't want any momentary activation before the sketch gets to configuring the pin?

 

And should I have an inline resistor between the pin and the mosfet gate?  if so, how much?

 

I'll be dealing almost entirely with 3.3v avrs, although they will frequently switch higher votages.

 

So which are worth stocking?  Which should I avoid?

 

So which would be useful to keep around?  

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The first thing is to find devices that will switch at 3.3V.  Some require much higher voltage, so be sure you check that spec first.  After that it's about price, size (current, power and/or on-resistance ratings) and case style.

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Here is an interesting thread on gate and pull-down resistors for a N-MOSFET: https://www.avrfreaks.net/forum/...

 

Basically, they recommended a 100 ohm gate resistor. The AVR pins usually also have 20-30 ohm internal resistance.

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For N-Channel Fet's:

2N7002 is one of the sort of traditonal type numbers, but it can only switch a few hundred mA.

BS170 is also a part for old farts :)

But both of these have a bit highish gate threshold voltage, which can be upto 3.0V but is still lower then your 3V3.

 

Are you looking for TO92 or chicken fodder like SOT23?

I recently bought a "sample book" on Ali for about USD 25, with 1800 transistors in it.

It's got 50 of the Si2300D which can switch 3Adc (From a SOT23!) and has a threshold voltage between 600mV and 1V5.

But there are thousands of FET's which fit your criterea. They should have a gate threshold voltage of 3V or lower and must be able to handle the current you put through them.

MOSfets are easy to destroy if misused, so get a bunch to experiment with.

Maybe a small MOSfet kit is a good idea, something like:

http://www.aliexpress.com/item/22values-10pcs-220pcs-SMD-MOSFET-Audion-Kit-SS8050-78L05-TL431-D882-AO3401-AO3400-SI2300-SI2306-SI2305/32885749784.html

 

But you can also switch much bigger loads directly.

If you have "logic level" compatible fets in TO220 you can switch 100A or even more, but this can easily go wrong in several ways. an when switching such high loads or wen PWM'ming the fet I recommend a "MOSfet gate driver" chip. such as IR2102 TC246, or even the output of a ne555 timer, which can also deliver quite a respectable 500mA on it's output and charge a MOSfet gate to 12V or so.

 

Experimenting is easier with TO92 and a breadboard, but I bought 20 adapter PCBs from SOT23 to 3-pin 0.1" header for EUR1.5 on Ali and those make it easy to experiment with SOT23 on a breadboard.

 

For P-Channel, Si2305 can also switch a few Amp and is logic compatible and pretty common and cheap.

But beware, some of these small high current fets can only tolerate a pretty low Source - Drain voltage. For Si2305 for example this is only 8V, but this is plenty if you only want to switch of some external logic IC's to save (battery) power.

 

But don't stare too much at those type numbers, there are thousands of FET's that fit your criterea.

 

Something else to consider are the "High-Side switches".  With these you can for example switch a 24Vdc power supply from your lowly 3V3 AVR and they often come with a lot of protection circuits built in (Under & over voltage, Over current, temperature).

 

In the reset state all AVR I/O pins are floating, and pullups disabled.

If you need to make sure that the fet stays of during power-up ( AVR RESET) then you will need some external circuitry.

Easiest is a resistor between Gate and Source and you have to be carefull in the order you initialize the port bits in you AVR.

 

Edit: 1n7002 -> 2n7002

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

Last Edited: Mon. Dec 17, 2018 - 12:27 PM
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1N7002 is one of the sort of traditional type numbers, but it can only switch a few hundred mA.

 

 

Or is that 2n7002?

 

aka someOldGuy

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Get yourself some BTS3035 & ZXMS004/5/6  "FETs".  Their inherent bullet-proof protection will make you glad you did.

 

I keep a few around to snack on.

 

https://www.diodes.com/assets/Datasheets/ds32247.pdf

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!