stuck on transistor symbols?

Discussion in 'General Electronics Chat' started by jonisonvespa, Apr 7, 2012.

  1. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
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    0
    hi
    i want to use this surface mount npn transistor to switch a relay for a arduino project im building, i wast to use a so23 surface mount transistor could someone please tell be the symbols for the max current and max voltage for the base pin to work out what value resistor to use?
    thank you

    http://www.rapidonline.com/pdf/47-3372.pdf
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The normal rule of thumb for switching a relay with a Bipolar transistor is that the base current should be one-tenth of the highest likely collector current.

    The data sheet linked to in your post shows that the transistor is rated at 1A maximum continuous collector current, at which level a 100mA base current would be needed. I would be wary of using such a small transistor at these levels, as heating may be the limiting factor.

    First start by finding out how much current your relay actually needs, work out a tenth as much for the base drive, and finally check how much current drive the Arduino can deliver (they are not something I'm familiar with.). If these figures do not tie up, you may need to use some other driver device for the relay, such as a Darlington transistor, or probably better a logic-level MOSFET.

    Finally, whatever driver device you use, do not forget to fit an anti back-emf device (typically, a silicon diode having voltage and current ratings at least equal to those of the relay coil) across the relay coil to minimise the switch-off voltage surge. If you do not do so, the voltage spike generated when the relay is turned off may destroy the driver.
     
    Last edited: Apr 7, 2012
  3. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    hi
    thanks for your help, yes i have a diode across my relay coil for protection, (a 5v relay) the output from arduino is 5v 40ma max i have found a surface mount mosfit FDN359AN transistor, capable of 2.7a 30v so plenty what is the rule of thumb for gate current to operate it ie what symbol am i looking for in the speck sheet?
    http://www.rapidonline.com/pdf/47-0208.pdf
    thank you
     
    Last edited: Apr 7, 2012
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    FETs steady state gate current is zero (there is a brief pulse at turn on and turn off you can forget here).

    So it is safe and desirable to connect the gate direct to the I/O pin, no resistor required or desired.
     
  5. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    ok that sounds perfect for my application, the only thing that im a bit concerned is looking at the speck sheet am i right in saying vgs is the symbol to turn on voltage for the device?

    VGS(th) Gate Threshold Voltage VDS = VGS, ID = 250 μA 1v min 1.6 typical 3 V max
    thank you
     
  6. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    Vgs is indeed the voltage that turns on the FET but the important parameter for switching is not the Threshold Voltage but the Vgs ON resistance at the voltage you will be applying to the gate (shown in the row labeled "RDSON -- Static Drain Source On-Resistance". That parameter show how well it turns on (On Resistance) for the given gate voltages.
     
  7. jonisonvespa

    Thread Starter Member

    Mar 25, 2012
    49
    0
    thanks very much for the help appreciate it.
     
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