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How Transistors Work -- The Learning Circuit 27

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The Learning Circuit
sudo Sergeant
The Ben Heck Show

 

In today's Episode Karen discusses transistors. Rather than using a physical, mechanical switch, a transistor can act as an electronic switch, using signals to turn it on or off.  She'll go over what they are, how they work, and some types of transistors.  She'll cover Bipololar junction transistors (BJT), NPN transistors, PNP transistors, and darlington transistors.

 

The Learning Circuit 37: Community Feedback: Revisiting Transistors

Transistors eBook Part 1

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  • Ok I figured out what my issue was I got some parts mixed up. The transistor I was using wasnt a transistor it was actually a lm35 temp sensor. They both look exactly the same lol embarrassing. I dont have one other question though, I dont have a variable bench power supply yet so whats the best way to achieve the .7 volts from constant 5v power supply? its actually an old cell phone charger I cut lol.

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  • Ok I figured out what my issue was I got some parts mixed up. The transistor I was using wasnt a transistor it was actually a lm35 temp sensor. They both look exactly the same lol embarrassing. I dont have one other question though, I dont have a variable bench power supply yet so whats the best way to achieve the .7 volts from constant 5v power supply? its actually an old cell phone charger I cut lol.

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  • in reply to jasonshader

    Aha! You are far from the first person to assume a TMP was a transistor. So 0.7V at the base is the threshold. Meaning if you're below 0.7V, say 0V or an open circuit, your transistor will be "off". If the base voltage is above 0.7V, the transistor will turn "on" allowing current to flow between the emitter and collector.

    Let's look at a 2N4401. I pulled these from the data sheet.

     

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    According to this, any connection on the transistor can withstand the 5V from your power supply. So technically you could supply the 5V to the base and it would be safe, and being above the 0.7V threshold, it's enough voltage to turn your transistor on.

     

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    But this transistor has a minimum saturation voltage of 0.75V. So you'll need at least that. Tbh, I'm not sure about the maximum Vbe(sat). I think it is the top end of the range that it could be, meaning you might need more than 0.75V to achieve saturation, as much as 0.95 or 1.2V depending on current. I don't think it's the maximum allowable voltage that the transistor can handle between the base and emitter.

     

    But again, as and I discussed in the follow-up video, transistors are current-controlled devices, so you need to mind your currents. It's best to put a resistor in line with the base of your transistor.

     

    Here's a graphic I found that may help you out. Here there's a 0.7V drop across the base and emitter and a 0.15V drop across the emitter and collector. They use a 1kohm resistor to limit current across the base and emitter.

    Transistor Circuit