I need a transistor whose base will be driven by a controller (3mA max base current). the Vce(sat) should be less than 0.5 or max 0.7. And collector current required by solenoid is 400mA. I am unable to find a transistor with such specification. can you suggest any? single or darlington is also ok

  • \$\begingroup\$ what is the time of operation? is it short periods of for long periods of time with 400ma constantly being pumped through the transistor? \$\endgroup\$
    – KyranF
    Commented Apr 3, 2014 at 6:07
  • \$\begingroup\$ en.wikipedia.org/wiki/Sziklai_pair \$\endgroup\$ Commented Apr 3, 2014 at 6:08
  • \$\begingroup\$ are you doing parts selection? you looking for surface mount components? or through hole? what is your budget? are you good at soldering? could a very low power NPN transistor turn on a 12V biased MOSFET (a really good option by the way) ? \$\endgroup\$
    – KyranF
    Commented Apr 3, 2014 at 6:10
  • \$\begingroup\$ hi, i want SMD component..and ya , i want to select only transistor with the given specification \$\endgroup\$
    – Priyanka
    Commented Apr 3, 2014 at 6:14
  • \$\begingroup\$ its a door lock application in which the solenoid will be used to open n close the door... \$\endgroup\$
    – Priyanka
    Commented Apr 3, 2014 at 6:15

1 Answer 1


With a distinct lack of detail, I shall prescribe this component:


This is a surface mount NPN transistor. It has a beta value of 140-150, at 400ma load. Your 3ma max will drive 400ma through it. It will dissipate ~100mW through it during this time, but it is rated to 300mW.

do NOT put >5V into it's base pin. The max Collect-Emitter voltage is 18V, so respect that too.

dont forget a simple barrier/rectifier diode (1N4001 or whatever) parallel to your solonoid

  • \$\begingroup\$ yes , the controller pin will output 3.3v as high..so no problem with it... \$\endgroup\$
    – Priyanka
    Commented Apr 3, 2014 at 6:56
  • \$\begingroup\$ okay, so 1k Ohm resistor will give you the 3mA from 3.3V. nice work! good luck. if you agree, please vote as answer whenever you like. \$\endgroup\$
    – KyranF
    Commented Apr 3, 2014 at 6:57
  • \$\begingroup\$ hi, in the datasheet of mmbt123S the gain of the transistor in saturation mode is 10 (i.e Ic=300mA, Ib=30mA). so for 3mA of Ib how it wil give 400mA? gain should be higher, right? correct me if i am wrong. \$\endgroup\$
    – Priyanka
    Commented Apr 3, 2014 at 8:07
  • \$\begingroup\$ @Priyanka not sure what you mean, where on the datasheet does it say that? Please see Fig. 2, "Typical DC Current Gain vs Collector Current" And follow it to 0.4A and up to the gain, its between 100 and 150. Remember, Ic = Beta * Ib \$\endgroup\$
    – KyranF
    Commented Apr 3, 2014 at 8:21
  • \$\begingroup\$ see, i want to use this transistor as a switch , that is , it will be used in saturation mode , so my question what will be the gain when it will be in saturation? \$\endgroup\$
    – Priyanka
    Commented Apr 3, 2014 at 8:30

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