chosing the right transistors
Britannicus
Posts: 98
Skylight has been helping me with a bunch of queries about switching.
Can somone help me with a further query on transistors as switches.
How can I establish if a transistor is an appropriate one for my STAMP - clearly there must be "rules" - where could I find them ?
initially I'm hoping to switch 6V at 1 amp - what criteria should i search on to make sure that it can be switched by the Pins and not get fried by the current being switched. I look at the data sheets and I'm baffled by the huge volumes of data. Is there a handy guide to picking the right transistor, that a newbie can understand ?? - I can muddle through NPN and PNP, but that's about it.
over to you gurus !
Can somone help me with a further query on transistors as switches.
How can I establish if a transistor is an appropriate one for my STAMP - clearly there must be "rules" - where could I find them ?
initially I'm hoping to switch 6V at 1 amp - what criteria should i search on to make sure that it can be switched by the Pins and not get fried by the current being switched. I look at the data sheets and I'm baffled by the huge volumes of data. Is there a handy guide to picking the right transistor, that a newbie can understand ?? - I can muddle through NPN and PNP, but that's about it.
over to you gurus !
Comments
All of those should be available at Radio Shack. You are right, there are way too many transistors or transistor-like components to choose from. You will get a few favorites in your parts drawer and select from those as needed. The trouble / blessing with electronics components is normally no ONE component must be used. As a general rule, you can select one of a hundred. Transistors typically fall in that category.
A great tool is Jameco or Digikey's website. You can select by parameters, for example: You need to switch 5V at 100mA at a frequency of 20kHz. That should give you a huge number of results. Your selection starts narrowing to a handful of components when you need extreme parameters, 90V or 5A or 2.4GHz for example.
Aside from that if you want to pick the BEST component for a job, you're in for a lot of reading
Happy harvesting!
Tomato
*the nice thing about LCD TV's is that everyone is throwing out their old CRT's. THat means treasure for us!
There are a few things you want to look for. here are the most important things:
1) I_c, collector current. This is the current that will be switched to your load, so you want to make sure the transistor's collector can handle it. If you're load pulls 1A, a little transistor like a 2n2222 will pop (it can handle no more than like 200mA)
2)V_ceo, the maximum allowed voltage across the collector and emitter. Exceed this voltage and your transistor will arc over.
3)H_fe, gain. For small signal transistors like the 2n2222, this is usually like 200. Medium power transistors (the square shaped ones with the heatsink tab) usually have gains around 50-100, and high power transistors (old fat daddies like 2n3055) have annoyingly low gains around 10-50.
4) V_ce(sat), saturation voltage. This is the voltage between the collector and emitter at saturation. Ideally, this would be 0, meaning that when your switch is on, it's a short circuit. Usually, this is 0.2-0.4V
5) P_c, collector power dissipation. THis is the maxiumum power dissipation that the transistor can handle before you get the magic smoke lol. If you're switching cleanly, this won't be an issue since the transistor doesnt consume much power (either full current 0 voltage, or full voltage 0 current). But if your application requires greater power dissipation than the spec sheet says is allowed, you can bolt the transistor to a heatsink. Probably not necessary unless you're switching at kHz or more.
Those are the most important things for switching that I can think of at the moment. Hope it helps1
-Tomato
Once again Nuggets of wisdom - Radio Shack desn't have much of a UK presence, so I tend to rely on a variety of sources - our nearest equivalent is Maplins, which I resort to as a last resort, as i find components to be typically double what I can get most of them elswhere. Good point about the CRT trashing - I work for an IT company and we get quite a few. might just have a chat with the stores guys and see what happens to them.
The problem I get is not the availability of data, it's the fact that I can't understand it when I find it ! terms like "medium power" only mean something if you know what low and high look like - so i really appreciated your explanations, tomato.
As transistors go, there are so many kinds out there, it is hard to point to the "best" one. There are so many factors one needs to look at. Like, the forward voltage drop, max power dissipation, max base voltage, base threshold voltage..etc. Mouser is a very good international source of components, you should give them a go.