How Transistors Work -- The Learning Circuit 27

Hurray! Glad you found it helpful.

Yeah, it all started with diodes. Or you can think of LEDs (which are also diodes) many 5mm LEDs have a flat side at the cathode, so if you remember the diode symbol has a line on the cathode side, then you can remember that the flat side of the LED is the negative lead.

Also for the anions and cations, I love when other areas of science come into play and those mental connections really help solidify a concept. Glad you made that connection. I'm sure that's why the leads are named that way.

Thank you very much Karen.

I wish I had a teacher like you at high school. Learning physics at that time was really painful. No one but you on youtube mentions the fact that the arrow points toward the N-type region of the transistor.

I remember, a few weeks ago, I was trying to find out how to memorize the polarity of a diode’s anode. Now I know that the anode is where the P-type material is located and it is the positive terminal while the cathode is where the N-type material is located and it is the negative terminal.  It makes sense actually with electrochemistry as we have the same inverse relationship.  In an electrochemical cell, the anode (where the anions gather) is the positive terminal and the cathode (where the cations gather) is the negative terminal.

Also, in circuits diagrams,  the diode’s arrow points in the direction of the current flow. But here, this is the direction of the conventional current, that is the direction of positive charges, not electrons. So electrons move in the opposite direction. That means that the arrow points toward the electron donor, the N-type material, which is consistent. It is all clear now.

Good job Karen.  

You may find this animation on YouTube helpful to visualise what happens:

Transistors, How do they work ?

Remember that in reality it is electrons that flow but the convention (diagrams and mathematics) is to treat the positive charges as the carriers and not the electrons. Blame Benjamin Franklin, he guessed at whether it was the positive or negative charges and he got it wrong. Another confusing thing is that the arrows on an enhanced MOSFETs point in for the N Channel types and Out of the P Channel types which is the opposite of the standard BJT (transistors) devices.

John

I understand how it works, but something isn't right: The main current from the colector to the emiter, on an NPN transistor, is going against the diode that points in that direction.

How is this possible ? It's one of those different diodes ?

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.

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.

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 baldengineer 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.

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.

First, I would say double check the type of transistor you are using and make the you have the correct pinout. They are not all the same from transistor to transistor and it's easy to get them mixed up.

What voltage are you using? Can you share a picture of your circuit or a schematic of how you have everything hooked up? Something else in your circuit might be the answer.

Hello element 14, I am beginning to learn how to build simple circuits with leds and tried using a transistor and having problems. My leds are lighting up without a base current at all as soon as I connect collector and emitter. Any ideas on what im doing wrong?

A nice and simple tutorial for school guys.