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I am planning to buy some MOSFETs for testing and playing around, to decide if I would use one instead of a BC338 transistor (see my intended application below).

My "filtering algorithm" would be:

  1. Small package (TO-92 preferrably);
  2. Small power dissipation (less heat = more light!);
  3. Saturated below 5V at the gate;
  4. Small price!;
  5. P channel (allows for common ground between Arduino and Led module);
  6. Not overengineered (my max current is not expected to go above 1A);
  7. Containing the proverbial "L suffix", meaning Logic-friendly;

I have been looking for a good candidate using "arduino led mosfet" search words, and tried some big-supplier sites (Mouser, Futurlec, DigiKey), but couldn't find a comparative table of Mosfet families, or even a "one place" to search for candidates to a given application, based on "layman readable" numeric parameters.

Also, if you could give me two or three part numbers that would be suitable for my application (see below), I would be glad!

Intended Application:

Use an Arduino GPIO (5V) to switch a module of 18 red leds running at 6V. The leds are wired in nine parallel groups, where each group has two leds and one 330 ohm resistor in series. The led brightness is quite sensitive to voltage drop, so a low "ON" resistance is welcome (preferrably below 10 ohm).

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  • \$\begingroup\$ Most manufacturers have selection tables on their websites to choose between their products. I don't know of any really good table to compare across manufacturers. \$\endgroup\$ – The Photon Jun 19 '15 at 23:37
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Don't be shy about spec'ing your own part from a distributor's website. It's not as hard as you'd think, once you learn which parameters are important in which situations. Based on the information you've provided, I'll guide you through selecting a usable MOSFET on Digikey.

Note that I am not validating your schematic design in any way, since you didn't provide one. One thing that stood out to me in your description was the LED supply voltage (6V) and the FET's gate voltage (5V). Make sure you fully understand how to interface a P-Channel MOSFET before you make this circuit. You will have to do more than just connect an Arduino pin directly to the gate.

Anyway, on to Digikey:
1. Search for "MOSFET" and click on the "In Stock" checkbox.
2. Choose "FETs - Single" under Discrete Semiconductor Products.
3. We want to whittle-down the 16,000+ options as much as possible, but without limiting ourselves. First, select the two "P-Channel" options under the FET Type filter, since we want a P-Channel FET.
4. Select all of the "Logic Level Gate" variations under the FET Feature filter.
5. Digi-Reel, Tape & Box, and Tape & Reel are codenames for "minimum order is, like, a million". So select everything in the Packaging filter except those three.
6. You said the power supply is 6V, so you shouldn't need to filter under the Drain to Source Voltage (Vdss).

There should be a large amount of FETs left. At this point I'd sort by price and start looking at what the least expensive components are like. The main parameter that's left is current. Forget about what Digikey is reporting under "Current - Continuous Drain". Those numbers are usually unrealistic values advertised by the manufacturer. You shouldn't expect to push that much current through the FET unless you've specifically designed it for that purpose (i.e., thermal considerations).

Instead, let's approach it another way by picking a FET based on its Rds(on). Let's assume something in a small package will have a thermal resistance of about 100 degrees C/W. That means for every Watt of power, it will increase 100 degrees C. Actually, 100 degrees rise in temperature is probably a good design point. That leaves a little bit of room before the typical maximum silicon temperature of 150C. So we want to pick a FET that will dissipate no more than 1W at the 1A you specified: $$P=I^{2}R$$ Rearranging and solving for R: $$R=\frac{P}{I^{2}}=\frac{1W}{1^{2}A}=1\Omega$$ Now we can start from the top of the price-sorted list and look for FETs that have 1\$\Omega\$ or less Rds(on). At this point, any one you choose will do fine. Just read the datasheet first to avoid surprises later!

By no means is this a comprehensive method for choosing a FET for all circumstances. But for the simple application you're doing, this method is good enough.

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  • \$\begingroup\$ Thanks for your detailed step-by-step! Also, the P channel indeed wasn't the right choice, for the reason you mentioned, so I will look for N channel. \$\endgroup\$ – heltonbiker Jun 20 '15 at 3:34
  • \$\begingroup\$ You can get just one part on a Digi-Reel. I mean you probably wouldn't want to because of the extra $7 reel cost (US site) on top of the parts cost, but they will make it for you. I accidentally got 10 of something on such a reel and was mildly amused. \$\endgroup\$ – Spehro Pefhany Nov 14 at 3:48
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I think your problem is that you don't know what you don't know - you don't understand the many various parameters of MOSFETs & how they might apply to your needs, and so you've dismissed several of the best & easiest ways to find a MOSFET for one's needs: Digikey, Mouser, Parts.io, Element14/Farnell/Newark, Radio Spares, TME, etc. Those first few listed are great for drilling down through the parameters that are important to you, to arrive at a small subset to choose from, which you can then order by cost, for example.

So I would encourage you to go reading up on them. There are no 'layman's MOSFET selection' tools I'm aware of - that's what electronics engineering is all about. I'm sure you'll find a wealth of info on how to use & specify MOSFETs, on this website in other questions, on the hobbyist sites like Sparkfun, Adafruit, et. al., and on other forums like EEVblog (& many others).

To address a few other of your questions:

(a) searching for MOSFETs with 'arduino' as a keyword is like searching for jeans with 'Nike' as a keyword - the two have no inherent link, and the MOSFETs that one published project based on Arduino might use will have been chosen specifically with knowledge of that projects needs for a MOSFET (regardless of what microcontroller family or IDE was used to program it), and an understanding of MOSFETs.

(b) With a LED supply voltage (+6V) which is above that of your Arduino GPIO (+5V), a p-channel 'high-side drive' arrangement will leave you with a hard time ensuring the array actually switches off, if driven from a GPIO that can't do open-drain (as is the case for Arduino's GPIOs). For your initial experimentation I would highly recommend an n-channel 'low-side drive' arrangement.

(c) 'L' is not a universal suffix for MOSFET part number to indicate that they're 'logic level gate drive voltage' types - for that you need a search criteria (Vgs-threshold), or a search field dedicated ("Standard" and "Logic Level") to it, and checking datasheet.

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  • \$\begingroup\$ I studied a bit more today, and have to agree that the "filterable table" I was looking for is exactely what DigiKey et al. provide. It doesn't help that the part codes doesn't seem to follow any logic. For example, if you take the BC family of transistors, there are some numerical "series" on it, and I didn't find a similar sequential naming on Mosfets yet. Finally, your tip about P channel being a bad idea is correct, figuring it out beforehand saved me from a big frustration :D. Thanks! \$\endgroup\$ – heltonbiker Jun 20 '15 at 3:24
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    \$\begingroup\$ "part number families" were a lovely idea in the 70s & 80s when transistors & discrete logic chips were coming out in ever newer families with different overall operational parameters (74xx, 74SLxx, 74ALSxx, 74HCxx, etc etc) where the xx was usually the same function, especially when it came to finding second sources for components (many manufacturing policies demanded at least 1 2nd-source before you could design with it). But those days are all but gone in modern components, part numbers are a dog's breakfast & nothing you can systematically go by without an encyclopedic memory. \$\endgroup\$ – Techydude Jun 20 '15 at 5:29

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