NPN Transistor as switch

Discussion in 'General Electronics Chat' started by cevval, Jul 9, 2012.

  1. cevval

    Thread Starter Active Member

    Dec 6, 2011
    The literature I got from internet says that, if a NPN transistor is to be used as a switch, the load shall be at the power feed side of the transistor and the emitter shall be connected to GND. But, I prefer that, the load will be at ground side of the transistor. My try worked well on the bread board. Please, see the attached drawing for more clarification.
    The drawing shows the load as a lamp. But in my project, the load is other electronic circuit with different function, so that my switching circuit with NPN transistor will power on or off the other circuit. Will it be reliable for long term, or damage the other circuit or itself.
    Thanks in advance.
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  2. luvv


    May 26, 2011
    The document in correct, npn devices are best when used as a "low side" (after the load)switch.
    And pnp devices are commonly used as "high side" (before the load) switches.
    Your circuit is fine to switch this way so long as you are not exceeding the specs of the transistor.
    Check out the e-book section for more info on transistors and switching.
    cevval likes this.
  3. Markd77

    Senior Member

    Sep 7, 2009
    Measure the voltage difference across the collector emitter of the transistor, this is higher than if it was connected the other way so some power is wasted and this voltage will change slightly depending on what the load current is. The transistor will get warmer than connected the other way. A higher voltage is required at the base and also the base can go lower than the emitter (check the datasheet to see if this is bad). If none of this is a problem then you are OK.
    cevval likes this.
  4. Engr


    Mar 17, 2010
    You may want to add a resistor on the emitter terminal, either before or after the lamp. The resistor will act as a current limiter of the circuit. The resistor that you are going to add depends on how bright you want your lamp to be. Without the resistor, maximum current could flow from collector - to - emitter and could cause the transistor to get very hot and eventually get damaged.
    cevval likes this.
  5. crutschow


    Mar 14, 2008
    You are using the circuit as a emitter follower, thus the load voltage is always about 1.5V below the base voltage (for the Darlington transistor you show). So strictly speaking, the transistor is not acting as a switch. But if the base control voltage is sufficient to generate the desired load voltage, then you are OK.
    cevval likes this.