BJT Switch - Circuit Simulation

• dwn
In summary: LED - this reduces the voltage on the base of the transistor and should prevent it from going into conduction
dwn
I am reading The Art Of Electronics, and I realize this is more of an encyclopedia rather than a textbook. I have referenced other sources as this has mainly served as a compass for me to learn about each subject area.

My question is fairly simple and straightforward. I'm using multisim to replicate a circuit that was provided in the book. The author used a simple transistor switch to light an LED, to explain the function of a BJT. When I went to simulate the circuit, the LED lights up, even though it should not, at least until the SPST is closed.
What is causing the transistor to act like a short?

As far as I understand it, the collector is an open circuit, until the point in which the VB ≈ VE + VBE, at which point the collector would be saturated.

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dwn said:
My question is fairly simple and straightforward. I'm using multisim to replicate a circuit that was provided in the book. The author used a simple transistor switch to light an LED, to explain the function of a BJT. When I went to simulate the circuit, the LED lights up, even though it should not, at least until the SPST is closed.
What is causing the transistor to act like a short?

hi there
there is no LED in your circuit
also, because you have no resistance in either the collector or emitter circuit of the transistor, it's possible you have killed it with excess current
and it has failed in short circuit mode

so redo your circuit with the LED and it's current limiting series resistor, for a 10V supply it should be around 1k Ohm
cheers
Dave

dwn said:
As far as I understand it, the collector is an open circuit, until the point in which the VB ≈ VE + VBE, at which point the collector would be saturated.

As a general rule the bjt transistor will go into conduction with ~ 0.7 - 0.8V on the base
for any voltage higher than that, it will be in saturation
( caveat ... higher power bjt transistors require a bit more base voltage)Dave

dwn said:
I am reading The Art Of Electronics, and I realize this is more of an encyclopedia rather than a textbook. I have referenced other sources as this has mainly served as a compass for me to learn about each subject area.

My question is fairly simple and straightforward. I'm using multisim to replicate a circuit that was provided in the book. The author used a simple transistor switch to light an LED, to explain the function of a BJT. When I went to simulate the circuit, the LED lights up, even though it should not, at least until the SPST is closed.
What is causing the transistor to act like a short?

As far as I understand it, the collector is an open circuit, until the point in which the VB ≈ VE + VBE, at which point the collector would be saturated.
Here are a couple of BJT LED drive circuits that you can start simulating...

http://i.stack.imgur.com/OnVwa.png

And a Common-Emitter amplifier stage like above will have a V-I characteristic like this:

davenn
davenn said:
hi there
there is no LED in your circuit
also, because you have no resistance in either the collector or emitter circuit of the transistor, it's possible you have killed it with excess current
and it has failed in short circuit mode

so redo your circuit with the LED and it's current limiting series resistor, for a 10V supply it should be around 1k Ohm
cheers
Dave
Thank you Dave, silly mistake thinking that the light probe was multisims version of the LED. After changing that and adjusting the components to their original values everything worked as expected.

davenn
dwn said:
Thank you Dave, silly mistake thinking that the light probe was multisims version of the LED. After changing that and adjusting the components to their original values everything worked as expected.
cool

look at the variations that berkeman also posted

1. What is a BJT switch?

A BJT (Bipolar Junction Transistor) switch is a type of electronic switch that uses a transistor to control the flow of current between two terminals. It is commonly used in electronic circuits to switch on or off a load, such as a motor or a light bulb.

2. How does a BJT switch work?

A BJT switch works by using the base-emitter junction of a bipolar transistor as a control input. When a small voltage is applied to the base, it allows a larger current to flow from the collector to the emitter, effectively switching the transistor on. This allows the current to flow through the load connected to the collector terminal.

3. What is the difference between a BJT switch and a MOSFET switch?

The main difference between a BJT switch and a MOSFET switch is the way they are controlled. A BJT switch uses a small current to control a larger current, while a MOSFET switch uses a voltage signal. Additionally, MOSFET switches have a lower on-resistance and can handle higher currents, but they are more sensitive to static electricity and have a higher gate capacitance.

4. How do you simulate a BJT switch circuit?

To simulate a BJT switch circuit, you will need a circuit simulation software like LTspice or Multisim. Start by drawing the circuit diagram and selecting the appropriate BJT transistor from the software's library. Then, specify the parameters for the transistor, such as the collector current and base voltage. Finally, run the simulation and observe the behavior of the switch.

5. What are the common applications of BJT switches?

BJT switches are commonly used in electronic circuits for various applications such as motor control, audio amplifiers, and LED drivers. They are also used in logic gates and digital circuits to switch between two states (on and off). Additionally, BJT switches are used in power supplies to regulate voltage and current.

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