When your project needs a transistor there are tons of choices. Which makes answering the question “Which transistor should I use or buy?” a daunting task. Fear not, before wading through spec sheet after spec sheet consider of these 4 general purpose transistors. Every electronics enginerd’s toolbox should have a few of each.
Transistors are one of the most versatile discrete components in electronics. In digital circuits they switch on and off while in analog circuits they are used to amplify signals. For most projects, they are used to turn on a load that would kill the I/O pin of a microcontroller or microprocessor. For most circuits either a BJT or MOSFET can be used, depending on the load current you need to switch.
In short the 4 I recommend are:
[Edit Note] Jan (comment below) points out that there are European Equivalents that may be more available if you are located in that part of the world. For NPN Check out the BC547, for PNP the BC557.
Here’s some more details on each of these.
BJTs
Bipolar transistors come in small packages, can be driven by I/O pins directly, and are VERY cheap. There are two variants, the NPN and PNP. These little guys are the workhorses of most control circuits, for small current applications. You’ll commonly find through-hole parts in the 3-pin TO-92 style package.
#1 NPN – 2N3904
NPN Transistors are used in low-side switch circuits. This means whatever you want to control is connected between the high voltage and the collector of the transistor.
A common transistor I use is the 2N3904. You can easily switch big 12 or 16V loads with this 40V transistor. The current is rated at 200mA which is enough for most relays.
#2 PNP – 2N3906
For high-side switch circuits, you need a PNP style BJT. A high-side circuit is where the load is placed between the emitter and ground. Since I recommended a 2N3904 for the NPN, I will suggest its complement: the 2n3906. It is also rated to 40V and 200mA.
#3 Power – TIP120
One of the advantages of BJTs is that they are easily driven from an Arduino or Raspberry Pi I/O pin. When configured as a “darlington pair”, they can provide significantly higher current capability than single transistors. The TIP120 is a darlington pair that can handle as much as 5A when in a relatively large TO-220 package. (You sometimes see the same package used for LM7805 linear regulators.) If you want to drive that much current, don’t forget the heat sink!
MOSFETs
When you have to drive a lot of amps of current, MOSFETs are awesome. However, most do not work at “logic levels” meaning they typically need 10-15V to switch them on. So most can’t be driven by an Arduino I/O pin’s 5V output. Which means, forgetting about a Beaglebone or Raspberry Pi.
#4 N-Channel (Logic Level) – FQP30N06L
These killer-transistors are rated for 60V and 30A. Not milliamps. Amps! (Though, you’ll need a heat sink!) They cost nearly 2X what a TIP120 costs, but they drive way way more current. The best part? They are “logic level” compatible and can be easily driven from 5V! This is why I keep a pile of FQP30N06s on hand.
Conclusion
These 4 general purpose transistors will cover a wide range of uses. Having a couple of each in your box will come in handy for nearly any project. Leave a comment below on which transistors you keep on hand.
I would like to build a Microprocessor from discrete transistors. Would look like the Monster6502. I decided to go for CMOS logic. Which transistor are best suited? My search resulted that many transistors are designed for high power, which is not the case with my processor. And thus they are usually more expensive. I found the FDV303N and FDV304P transistors by Fairchild. But they are SOT23 and therefore too tiny for soldering by hand, I fear.
PS: I didn’t examine the electrical charecteristics yet. Will it do soon after watching some of your videos 😉
I cannot suggest a transistor. I have never tried such a project. I would try contacting Eric and ask what he used on his 6502.
Thanks for you article. I am curious what you would recommend for a jfet. It seems that the j201 is the most popular choice, but it’s limitation of 50ma leaves me to wonder if there is a better choice ?
What I am using it for:
I believe I need to use a jfet for a robot project. The motordriver (Sabertooth) e-stop signal line connected high (+5v) will allow the motors to operate. Removing the +5v (line is pulled low) will stop the motor driver.
I have an e-stop button connected to to ground, and need to use that to break the 5v e-stop line (+5 to signal input). I am planning on using the j201, but am wondering if there is a better way to connect my “e-stop push button”. Thanks
First, JFETs are usually used for analog circuits. They are a good alternative to BJTs. (Are you confusing JFET with MOSFET? JFET is a type of MOSFET.) You typically wouldn’t use one in a switching application. An “enhancement mode” MOSFET is a better choice. Second, for an e-stop, I don’t understand why you would use a transistor at all. An e-stop switch is usually wired in at the main supply voltage. They are used to directly disconnect power in an emergency. I don’t know of any other way to wire one except at the supply.
If you’re using an e-switch to enable or disable a transistor, that is a very bad design choice. In the event the transistor fails shorted, your e-switch may do nothing. That creates a huge safety hazard.
Excellent, Thanks so much for you quick response. Agreed, the e-top would be best with a SPDT push-button type switch. The +5v line would be connected to the normally closed push-button connected to the signal line. Pushing the push-button would open the circuit – signalling the motor-driver to stop. I wanted to use a small SPST push-button wired to ground to trigger a transistor to open the circuit (Thus the JFET).
I thought the JFET would open a circuit when triggered LOW.?
I will have to read up on the “enhancement mode” MOSFET. I have not heard of that. Thanks 🙂
Watch this video on MOSFETs: https://www.youtube.com/watch?v=GrvvkYTW_0k.
Thanks, This is great stuff! I think the TO-220 will do the trick. Wish I could post schematics or pictures here – But you pretty much cover the answer in the above supplied video… Anyone want to buy a handful of useless JFET’s that I just bought – lol 🙂
For NPN I prefer 2N2222, for PNP 2N2907 , For power transistors, 2N3055 NPN or MJ2955 PNP, as mosfets IRF3205 55V 110A mosfet, IRFB4110 100V 180A mosfet and IRF2804 40V 280A mosfet.
Yep, I think the PN2222A and 2N2907 in to18 and to92 are the best, I use them all the time.
James, you hit right on the money with this article…incredibly useful information.
Those 4 transistors are super awesome to have in stock on the bench. I can’t tell you how many times I’ve used the 2N3904. For higher current, the TIP120 is hard to beat as well.
Another great general purpose NPN to have on hand is the PN2222A. They are super cheap and incredibly useful: from turning on fans, relays, or even higher power LEDs.
Keep up the great work and thanks for the article.
Mike
Hay all you guys out there wanting a list of parts to keep in your work bench parts bin this is a big list but you can pare it down. Maybe some one will do that and post it here (wink wink ).
http://forum.allaboutcircuits.com/threads/components-selection-guide.65137/
This was a brilliant article. Any chance you could do an article on the top 4 or 5 capacitors to have on hand?
really helpful … thank you very much !!
I am currently in the beginning stages of learning how to repair amplifiers and only problem I am having is identify the wrighting for direct part replacement . This is part number I am needing bad. But no luck. 80nf70 /czod T /Mar 247/.that’s all I see on side of 3 leg fet. If you could help i sure thank you
Would you know the equivalent of Ss8050 D331? Thanks
Sorry, I don’t know what that is.
LOL, obviously a transistor. Don’t worry about it.
http://www.mouser.com/ds/2/149/SS8050-117753.pdf
HELLO MY FRIEND! could you help me….
l would like to ask you for a type of transistor that can switch up a consumer of a 2A 3v by a digispark pin of a 3.3 v !
the supplier for the both of them is a3.7 lipo !
it is for model of airplane so I need a smd transistor and a “regular” too but a small or light one !
Thanks in advance
I would try a distributor’s website for that kind of search. For example, on Mouser.com you should be able to put in the parameters you want to get some options. (This is useful even if they do not ship to your country.)
thank you !
Great list… I also like to keep some trusty 2N7000 parts for general purpose switching at my bench.
I’m curious if you have a similar list for op amps…
Not yet, but good idea.
HELLO … ! you have a great site !!
i wanted to ask about BJT tutorial – you finished of a saying that the transistor is forcing to current from the co. to emi. to be 100 mA , and you get 6 v battery , then I’m not understand – what now this circuit got forced to be in v and I and what is the analyse that i do now for enter another consumer than a motor , lets say another motor (my motor don’t have any v properties , i just found that it draws between 60 to 200 mA on 5 v ?
Current is not forced at any time. Current is allowed to flow. The “beta” of the transistor is the ratio of current between Base/Emitter and Collector/Emitter. If the transistor has a beta of 100, and the current from the Base to Emitter is 1mA, then up to 100mA will flow between the Collector and Emitter. Beta varies from transistor to transistor and it varies depending on the amount of current flowing (which makes it sometimes difficult to predict.)
thank’s for the quick answer !
re: FQP30N06L “They are “logic level” compatible and can be easily driven from 5V! ”
But can they be driven from 3v such as from Raspberry Pi i/o?
Yes.
The max Vgs is around 2.5V. Even if you get one that’s pegged to the Max, it’ll at least turn on the FET even if Rds is a bit higher than usual.
For NMOS driven by 3.3V (1.8V also), smd package, I’d use Si2312 [email protected]
VGSth=0.8V, Ciss=740pF IDSmax=4.2A.
We should use these when the in/out capacitance is not a problem. In my current project, I need very small capacitance between drain/collector and source/emitter, SOT-23, so I’m limited to a NPN BJT instead.
MMBT2222, BCW66, BCX19 all look good.
Notice that the current gain of BJT depends on the target Ic. For my design, Ic would be 150mA, hence the above devices are suitable.
I also recomend SS8050 or S8050. Can handle upon 1.5A in a TO-92 Packaging. Here in Mexico are even cheaper than 2n3904.
Hi James,
very common in Germany is also the ULN2003, 6x 500mA/50V Darlington.
BUT it’s not 3.0A in total
http://www.farnell.com/datasheets/1690348.pdf
Thats a good list!
The european versions of the 3204 and 3906 transistors are like the BC 547 and BC 557. These are most common here (theres a huge list which compares BC and 2N types)
Anyway I´d like to add the BS170 low power mosfet to the list. I use it all the time instead of a low power NPN. Not logic level but have a look at the datasheet, DS-resistance is low enough @5V gate voltage…
The two high power logic level mosfets I always have at hand are:
IRLU 024N (55V, 17A; cheap and SMALL PACKAGE)
IRL 3803 (30V, 120A; quite expensive)
Your description of #1 is a Low side switch, not a high-side switch.
Look at Linear Tech data sheet for LT1089. It shows an NPN with collector tied to the positive rail, and the load is from emitter to ground.
Have a look at the block diagram for LTC4446. Control of the transistor that is connected to the positive rail is the high side, the transistor that is tied to ground is the low side.
Opposite mistake on #2. Your description is a high side switch, not low side.
Thanks for #4, good find.
Yes, you’re right. I got myself mixed up. This isn’t the first time I made this mistake… It’s been fixed.
Thanks.
I’m looking at my comment of april 2014, and wondering what is the ref to $4??? After thinking about it, it must be a misspront. I meant #4 🙂
Oops! Like facts on Wikipedia, that is easily changed. 😉