I know HOW things work, I just dont understand all the little intricacies involved. Such as this example, I have no clue what type of transistors I should get for my project.
What I'm doing is driving a solenoid from my arduino (same schematic as a relay). At my local electronics supply store, these are the only things on the shelves.
Also, a side question, does anyone know of some good helpfull electronics forums? I feel bad for putting these questions on the Arduino forums when they are not always arduino-specific.
According to the datasheet for the solenoid I bought (page here: http://tinyurl.com/ybg5gyc), heres the stats:
Voltage: 24vdc
Resistance: 60.6
Power: 10.1 W
Current: 399 mA
Ill be the first to admit that I never memorized electrical theory or how to compute the numbers (Im just a software guy )... but does that mean I need to find a transistor that has around 399 mA? Should I find one with MORE than 399, or LESS?
Also, does that mean I should find a 24v transistor? Even though I'll be driving it from the 5v arduino?
If you were to decide that you could stock up on about 6 or 7 different common transistors... having the right 6 or 7 devices could handle most of your needs. And I think it's probably better to stick with Bipolar transistors initially and the move to Field Effect Transistors, like MOSFETS, as you get more comfortable.
My recommendation is below. Granted I'm in the USA, so I am not so keen on EU Philips style numbers, just the JEDEC's numbers.
2N2222 - The ubiquitous NPN Gen Purpose low current high speed switch (600ma) (Think... small relays)
2N2219 - NPN Gen Purpose 800ma switch (better for some bigger relays)
2N3904/2N3906 - Small Signal NPN/PNP pair. (200ma) I imagine these are pretty close to what's inside standard TTL logic chips but they are just as much at home in audio circuits.
TIP120 - NPN Power Darlington switch (Handles a few watts)
2N3055 - A high power transistor workhorse... (But outclassed by Mosfets)
In my opinion, with these in your collection... you could build pretty much any of the basic projects you would want to with the Arduino.
Just pick one that has headroom above what you will be doing to it.
IE; you will draw 400ma. A transistor capable of 600 would be OK, 800 might be better... but it will cost more. So then it comes down to "which one you do have?".
The "2n2222" you'd get is actually most likely a "PN2222" because it's in TO-92 casing, which is much more common than the original "can" style case (whose technical name escapes me at the moment).
It's maximum current rating is 800 mA. I just blew one a few days ago by accidentally drawing 1A+ (Magic smoke from transistors is not pleasant ;D).
Just for reference, the below Radio Shack shows the "common" transistor types for simple projects. The MOSFET has the least internal resistance when appropriately set up.
The MOSFET has the least internal resistance when appropriately set up.
This is wrong.
The MOSFET has internal resistance. A transistor does not have any internal resistance, it has a saturation voltage but these are not the same thing. So you can't say it has the "least"
The saturation voltage does not vary linearly with current and so the heat dissipation will not vary linearly with current on a transistor. Where as a FET on resistance is fixed (given a gate voltage) so the heat dissipation will vary linearly with current.
True in general you will get lower heat generated by an FET doing switching but it is not always the case.
The MOSFET has internal resistance. A transistor does not have any internal resistance, it has a saturation voltage but these are not the same thing.
I agree with the "...not the same thing" I always understood internal resistance is a function of the saturation level. A transistor has no internal resistance? What causes them to become hot when a high current is applied across them? Cold fusion perhaps?
Yeah, I guess I'm a geek. I actually get a kick out downloading and collecting PDF datasheets. If I ever download one, even just for a quick reference... I SAVE it and archive it for later. I've assembled a number of 3 ring binders with datasheets I refer to often.
There is nothing more frustrating than have an old part in your collection and not having a datasheet anymore.
I am seriously into Electronics Part recycling.
Example: I still have some old HDSP-2416 displays and it took me ages to find a datasheet again after they "dried up" years back.
The crazy thing is, I have some old books from Motorola with transistors and diodes. Hundreds upon hundreds are listed all the way from 2N297-2N6097. Most have barely been seen by anyone around now. It always makes me wonder... if we still have the 2N2222... why not the 2N404. (hmmm bad choice, I think 404 was Germanium) Anyway... what makes wonder why one device is more popular and long lasting over the thousands of others?
pwillard, I do the same thing - collect datasheets and recycle electronics; I have done this since I was a little kid, bugging the neighbors for broken pieces of junk for me to "take apart" (I still have some of those old parts today).
My greatest recycling "finds" (all were purchased, for very little):
Core memory unit with planes constructed of bakelite and large ferite beads (each one is about 3-4 mm in diameter) - no drive electronics, and very beat up.
Core memory unit PCB (core is in the center of the board with a metal plate covering it), with drive circuitry surrounding it.
Altair 8800 with flat toggles, missing its top cover, but I managed to get a ton of cards (lots of RAM, serial/parallel card, disk drive card, etc). It doesn't have the original Intel 8080 card, instead has the Z-80 upgrade (this was a common upgrade). It needs a major restoration done to it before I can apply power (there would probably be a fire).
The thing is... if you can find some 10 Years or older electronics junk... you will probably still find lots of discrete transistors you could easily re-use inside. Old Stereos, TV's, etc.
You just de-solder one... look up the datasheet... see if it matches close what you need and give it a try. I have a home-made Go-NO GO Transistor tester/PNP/NPN identifier that still comes in handy for this.
For Arduino use, the major transistor characteristics to keep in mind are: Type (NPN/PNP), Max Collector-Emitter Voltage, Max Collector Current. Somewhat Less important is switching speed and "gain" value and temperature/thermal ratings.
)... but does that mean I need to find a transistor that has around 399 mA? Should I find one with MORE than 399, or LESS?