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Can a transistor be triggered by a higher voltage than it is controling

timswift

Can a transistor be triggered by a higher voltage than it is controlling.

 

 

Tim

  • Hi Tim!

     

    The easiest way to see what is going on is to try a simulator, for example, click here. (It requires Java, and may ask for your permission to unblock it - it is safe, this simulator has been around for many years).

     

    Once you do that, you'll see this appear:

    image

    Then, it is possible to adjust the sliders on the right, and see the effect on the circuit. Hover over any wire or component, and information will appear about the current and voltage. For example, I hovered over the wire circled in blue, to see the current through the wire at that point, and the voltage there with reference to the ground.

     

    You'll notice that as you increase the control voltage slider from zero, the voltage at the base approaches around 0.7V, current starts to flow through collector and emitter (i.e. yellow blobs will start to move). As you increase the voltage to anything, the voltage on the base does not change much, it stays at around 0.7. This means that current is flowing through the 1k (1000 ohm) resistor equal to I=V/R or (Vcontrol-0.7)/1000. Also notice that the current through the 10 ohm resistor at the top is about 100 times the current through the 1k resistor, when the voltage at the base of the transistor is around 0.7V. This remains the case up to about 0.5A (500mA) (which happens to be I=V/R or 5/10 ).

  • Hi Tim,

     

    A transistor is like a fancy diode.

    When the base voltage exceeds a trigger level, the current flow from the base to the emitter also initiates a current flow from the collector to the emitter.

     

    So while you can place a high voltage on the base, you must ensure that you do not exceed its current flow.

     

    You can usually do this with a proper resister arrangement to limit the voltage and thus the current flow.

     

    Plus the short answer is always yes, once!  image

     

    So check with someone about how to properly bias the transistor you plan to use.  They can help you set up the circuit.

     

    DAB

  • N-channel MOS FETs are often have their gates driven higher than the voltage they're controlling.  This is needed to get the full voltage across the FET from drain to source.  Also, a higher gate voltage makes the N channel wider which reduces its on-resistance.

     

    There is a nifty component for doing this called a "high-side driver", e.g., the Linear Technology LTC1154.  An NFET with high-side driver is more efficient than using a PFET because an NFET has lower resistance than a PFET of the same physical size.

  • Recognize the difference between a MOSFET and a bipolar transistor in this case.  A bipolar transistor functions on current into the base (and looks like a diode as previously posted) so you use a current limiting resistor to determine base current from the voltage you are sensing.  A MOSFET has an insulated gate and senses voltage across the gate and looks more like a capacitor in it's model so a series resistor from the high voltage to the gate of a MOSFET only limits the current when the gate breaks down when the voltage goes too high (well, it also limits the rise/fall time due to the capacitance of the gate as well).  To deal with high voltages with a MOSFET, you need to use a voltage divider of some sort and even better a clamp diode limiting how high the gate can go.

     

    mikey