transistor or SSR for a switch??

Discussion in 'The Projects Forum' started by LaZyLuke, May 10, 2009.

  1. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
    56
    0
    Hi.
    It's my 1st post so sorry if wrong place. I have tried to search for that topic but since I am not much familiar with electronics (yet) I might have missed it.

    The problem/dilemma is that I want to turn a little fan using a remote wire (wire that puts out current when the radio is ON, it gives about 200-300 mA @12V). I don't want to use this to power the fan as I am not sure if it will somehow affect the head unit. So what I am trying to do is to use a Transistor or a SSR to power up the fan using the main power wire from the battery (if its not clear yet I am talking about a car setup here). SSR are hard to get for me (I live in Canada- circuits components are available online only- or at my work, but they will have to order it up). The Transistor is little easier to get, and my question is - If transistor is also used to amplify current- can that damage the fan? (I know the V is constant but I don't know exactly how the transistor works so I want to be safe).

    Also If I wish to run more complex system then a fan (electric motors, lights, etc) could I use the same Idea (of course a higher Ic for the transistor)
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    You likely want to use a transistor, not an SCR. An SCR, when using DC, has the property of staying on until the current flows the other dirctions (as with AC) or until the current completly stops.

    A transistor, be if a BJT (Bipolar Junction Transistor, what I like to think of as conventional) or a FET will turn power on to the fan as long as the radio is on.

    This sounds more like what you want?

    BTW, you can make a homemade SCR using transistors, though it is easier to buy a finished device. SCRs are mainly used to control AC, as in dimmers and whatnot.
     
  3. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
    56
    0
    I was referring to Solid State Relay as SSR, I don't know if its same as SCR (I'm actually not familiar with SCR)..

    Yes that's exactly what I want- I was just worried if it can damage the fan (too much current- since on the definition of it says that TR is used to amplify current- but since its a constant Voltage I don't think it will matter...) - for future reference, if i want to run more stuff like that, I still use the TR only with higher Ic value??

    Thanks for a quick answer!
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    You can get a 2N3055 BJT (15A max) at Radio Shack. Do you know how to wire it or do you need a schematic?
     
  5. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
    56
    0
    I think I know. Base-remote wire, Collector- main power wire, Emitter positive terminal of the fan. If I am wrong let me know.

    Thanks for the responses!

    Isn't 15A too much? Also should I fused it?? I doubt the fan will pull more then 500mA (its one of those cooling fans for heat sinks from video cards)
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Fusing, yes.

    Heat sink, possibly, but probably not needed.

    Don't forget base resistors, they are necessary. Are you using a common emitter configuration?

    Don't forget to put a diode across the motor, it is also necessary to keep the transistor working.

    Something like this, ignoring the 555...

    [​IMG]
     
  7. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
    56
    0
    K, is there a simpler way to do it? I just thought its going to be fusing and Transistor. I don't understand any of that from the diagram. There is only one Vcc, where do I put the remote wire? Where main power wire goes? I thought it would be just like I said in the previous post - Base - remote wire carrying 300mA @12V, Main power wire- Collector (12V from battery), and Emitter goes to the fan....On this diagram I see something that looks like a Voltage Divider, or rather few fuses in some bizzare configuration (actually looks like only one fuse of 470 will be working when 555 assumed to be one node). Please explain if You can. Thanks, Luke.
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    I put that up there to show the transistor section, ignore the 555.
     
  9. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
    56
    0
    I understand, i stil dont know where i put the main power and remote wire, there is only one Vcc on the diagram.....
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Is this simpler for you?

    EDIT: If you're going to connect the motor directly to the battery you will need to fuse it. This is not shown in the schematic.
     
    Last edited: May 10, 2009
  11. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
    56
    0
    :] sorry for being slow, like i said i'm not very good at that stuff yet.... and YES it makes it all clear ;]... thanks again!
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    You're welcome. Also, if you're going directly to the battery put the fuse at the battery end of the motor wire. This way the wire is protected too! :cool:
     
  13. Suzkuz

    New Member

    May 9, 2009
    2
    0
    Can someone explain in greater depth the purpose of the diode across the motor?
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    We need a FAQ section to point people to when questions like this are asked. I clicked the FAQ button and entered "Back EMF" but no matches were returned. :eek:
     
Loading...