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Transistors: how to know which one is the right one? — Parallax Forums

Transistors: how to know which one is the right one?

IceFireProIceFirePro Posts: 86
edited 2010-06-20 14:21 in BASIC Stamp
Hello,

Getting into the mystery of transistors!

How can I know if a certain transistor can allow certain voltage/mA's to go thru it?

For example, with the BS2 start set I got a transistor that reads

2N
3904
-338

...which means nothing! How can I know whether I can apply, for example, 5 volts to it's input and connect a 50 mA diode or a light bulb to it's output?

How do I know when I need to combine the transistor with a relay switch, and when I can use only the transistor without relay for such application?

Thanks!

Comments

  • IceFireProIceFirePro Posts: 86
    edited 2010-06-17 04:05
    Here is the datasheet for the one I have:

    http://www.fairchildsemi.com/ds/2N/2N3904.pdf

    How do I read what I need from all this?
  • Mike GreenMike Green Posts: 23,101
    edited 2010-06-17 04:08
    1) Read the datasheet. This will tell you the maximum voltage, maximum current, and gain as well as all sorts of other information.

    2) Read the datasheet. If the transistor can handle the amount of current and voltage you need, then you probably don't need a relay. Relays have the advantage that they have isolation between the coil and the contacts. Sometimes you need that. Read the Wikipedia article on relays.

    Absolute maximum ratings are the most important. If you exceed them, you may break the device.

    Power dissipation tells you how much heat it can get rid of which depends on how much heat you generate. You need to learn about basic electronics otherwise this is just another fact you'll forget or won't make much sense of.

    DC current gain (hFE) gives you a rough idea of how much current the transistor will control given how much current is input to it. Read the Wikipedia article on transistors and the links from that. Again, this is basic electronics.

    Post Edited (Mike Green) : 6/17/2010 4:15:38 AM GMT
  • IceFireProIceFirePro Posts: 86
    edited 2010-06-17 04:18
    Thanks!

    In particular:

    Can I connect a 50 mA diode rated at 3V by connecting P0 to the transistor's "switch" pin; and connecting the Diode to Vdd and a resistor through the transistor?

    Is it correct to assume on my side that this data sheet tells me this transistor works on 6V for the "switch" pin; and can process 60 volts at 200 mA max over the two other pins?
  • IroneIrone Posts: 116
    edited 2010-06-17 09:00
    How Ya Doin'? A diode rated at 50 mA does not mean that it only puts out 50 mA! That is the most you may put thru it. More power will make it black and stinky. I looked thru the datasheet and could not find what you meant by the switch pin. The only three pins I found were the collector, base and emiter. These are·silicon pins that have been doped with impurities which makes them semiconductors. If you really want to know the truth these are just diodes carefully put together. None of them needs controlled by another diode. The center area of a transistor is where you find the depletion layer. The base pin lowers this depletion layer with current and since it is made very thin·it almost dissapears. The collector and the emitter sections have also a lot of care and planning done to them. Each has to be connected to specific places or it will not work very well at all. Please look up bipolar transistors on Wikipedia and read some of what they have written. There is also a section that tells you how to bias them too.

    Casey

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    I fish, therefore I lie!
  • Mike GreenMike Green Posts: 23,101
    edited 2010-06-17 14:30
    The connections to a transistor have specific names. Please learn them. Read the Wikipedia article on transistors for a start.
  • IceFireProIceFirePro Posts: 86
    edited 2010-06-20 04:16
    Question : In the "What's a microcontroller" manual they connect P8 to the Base of the transistor through 2 100kOhm resistors and no diode.

    Is this good, or is it always better to have a diode that will prevent electricity going back from the Base into the IO pin?
  • FranklinFranklin Posts: 4,747
    edited 2010-06-20 05:19
    It all depends on your circuit design how you implement protection.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    - Stephen
  • Mike GreenMike Green Posts: 23,101
    edited 2010-06-20 05:23
    Why do you want to have a diode? What are the resistors for? You need to learn the answers to those questions. They should be in the manual.

    If you mention something that's in a manual, it would be helpful to mention the page number or the figure number or the example number. What you're asking makes no sense without some context.

    You're asking a lot of questions and the questions themselves are not asked well. The way to get the best help in these forums is to:

    1) Make them as specific as possible.

    2) Give some context including a description of what you're trying to accomplish
  • IceFireProIceFirePro Posts: 86
    edited 2010-06-20 05:39
    Isn't it true that a transistor can return a dangerous amount of electricity back in the IO pin through it's BASE pin, and so I should use a diode in order to stop that from happening?
  • Mike GreenMike Green Posts: 23,101
    edited 2010-06-20 14:21
    No, not unless it's damaged or stressed beyond its absolute maximum ratings. Where would the "dangerous amount" of electricity come from?

    How is a transistor made? How does it work? What's the equivalent circuit (if it were made of simpler parts put together)? Read the Wikipedia article or maybe a chapter on transistors on a basic electronics text.
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