How a 2N2222A NPN turns on a P channel Mosfet

Other Hardware General Electronics
1

How A 2N2222A NPN transistor turns on a P-channel MOSFET

By acting as a low-side switch that pulls the MOSFET’s gate voltage down to ground. Since P-channel MOSFETs turn on when their gate-source voltage () is negative (i.e., gate is lower than source), the 2N2222A enables this by pulling the gate below the source voltage.

Mechanism and Circuit Setup:

P-Channel MOSFET Setup:

The Source is connected to the positive supply (e.g.,). A pull-up resistor (e.g.,) 10 kΩ being a very common and effective starting point, is connected between the Gate and the Source to keep the MOSFET off by default.

2N2222A (NPN) Operation:

The Emitter is connected to ground, and the Collector is connected to the MOSFET gate.

Turning On:

When a high signal (e.g.,from a microcontroller) is applied to the 2N2222A base, the transistor turns on.

Gate Voltage Action:

The 2N2222A collector drags the gate voltage down to nearly ground (). Because the source is at, the gate becomes negative relative to the source (), causing the MOSFET to turn on.

Turning Off:

When the 2N2222A base signal goes low, it turns off, allowing the pull-up resistor to bring the gate back up to source voltage, turning the MOSFET off.

This setup allows a low-voltage, low-current signal to control a high-voltage, high-current load.

To turn on a P-channel MOSFET using a 2N2222A (an NPN transistor), you use the 2N2222A as a "low-side driver" or level-shifter. This is necessary because a P-channel MOSFET turns ON when its gate voltage is significantly lower than its source voltage.

How the Circuit Works In a typical high-side switching configuration, the P-channel MOSFET's source is connected to the positive supply (e.g., +12V).

The Off State (Default):

A pull-up resistor is connected between the MOSFET's gate and its source (+12V).

When the 2N2222A is "OFF" (no voltage at its base), the pull-up resistor, 10 kΩ being a very common and effective starting point, holds the MOSFET gate at the same voltage as the source (+12V). Since the gate-to-source voltage () is 0V, the MOSFET remains OFF.

Turning it ON (The 2N2222A's Role):

When you apply a small positive voltage (approx. 0.7V) to the base of the 2N2222A, it turns ON and conducts current from its collector to its emitter (ground).

The 2N2222A's collector is connected to the MOSFET's gate. When the 2N2222A conducts, it drags the gate voltage down toward 0V (ground).

This creates a negative (e.g., the gate is 0V while the source is +12V, so). This negative potential allows current to flow through the MOSFET, turning it ON.

Typical Component Values:

Base Resistor (2N2222A):

Typically 1kΩ to 10kΩ to limit current from your control signal (like an Arduino).

Pull-up Resistor (MOSFET Gate):

Typically 1kΩ to 10kΩ. A lower value (like 1kΩ) allows the MOSFET to turn off faster by charging the gate capacitance more quickly.

2N2222A Specs:

This transistor is ideal for this role because it can handle the small currents needed to pull the gate low and supports voltages up to 40V.

Why use a 2N2222A instead of connecting directly?

If your control signal (e.g., a 3.3V or 5V microcontroller) is lower than the supply voltage (e.g., 12V), a direct connection cannot turn the MOSFET off.

The microcontroller's "high" signal (5V) would still be 7V lower than the 12V source, keeping the P-channel MOSFET partially or fully ON.

The 2N2222A acts as a level shifter, allowing a low-voltage signal to control a high-voltage load safely.

2

The word salad is a waste of paper when a simple schematic would work. When working with MOSFETs the gate voltage is ALWAYS REFERENCED to the source. If the source is at +5 and the gate is at ground the Voltage Gate to Source is -5V.

image
image392×306 7.99 KB

R3 is not necessary but helps when testing without a load.

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3

Looks like a chatbot produced that summary, which is difficult to read with useless boldface font, unnecessary commentary and formatting.

3 Likes
4

This configuration is known as a "high side switch" and a Google image search shows countless examples many, incidentally, with errors such as the mosfet upside down or a N channel mosfet where a P channel should be.

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5

Ok, now that you gave us the ChatGPT result, was there a question?

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6

It was AI genereated, I thought is was readable and informative, I’ll bet you’re the type that if you won the 6 billion dollar lottery, you would cry that you had to pay tax on it. LOL

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For a beginner your schematic means nothng

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Without a schematic, the AI word salad would be very confusing to a beginner.

2 Likes
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No Sir, I posted this to help people who are trying to learn the finer details, like I always try to do. This is AI genereated, and you can learn a ton from AI if you read what it states, and if you don’t understand what it states, you can ask it to explain it in more layman type terms, and it will do just that. AI is a big well liked tool, in my tool box. It can be wrong, nothing on planet Earth is 100%, but AI is closer that I’ll ever get. :slight_smile:

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Those people can spend 10 seconds asking a chatbot, just as you did.

No need to clutter up the Arduino forum with random circuit trivia.

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True, but some people forget they have these tools, in my experience people ask me questions all the time, about any subject, that they could easily google themselves, but they forget to do so.
Why have forums now if you have AI? Because people like feedback too.

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So how do I know what you have posted is wrong or right?
Have you built and tested this circuit with real components?

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That’s true too, but hey, word salad first, since most people, know how to read, how many people would want a schematic first, especially a newbie? I still struggle with automotive schematics, LOL. Once they read it, if they are serious, they can google for a schematic.

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If automotive schematics were displayed like the schematic in post #2 you wouldn't struggle so much but in many manuals they are not.

A newbie wouldn't get anything from reading the AI.

1 Like
15

Yes Sir,
I am a relative newbie to electronics, and I have built a PCB, that blinks my Harley Davidson fog lights, that I installed, and the PCB has an Atmega 328 microcontroller, with the Arduino script that does the blinking, and also has a police light blink sequence.

It uses two 2N2222A NPN transistors and two P channel Mosfets, it works beautifully.

I did this all with google, forums, U

My Harley Blinking Fog Ligts Schematic_2
My Harley Blinking Fog Ligts Schematic_21850×1113 169 KB

pwork, and especially AI for help.

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Yes, but like you say, they are not. I remeber when I first got into electronics as an enthusiast, I am not a pro, I was really confused by schematics, I still am on deep ones like vehicles, but I keep at it, and AI has helped me more than anything. People say you won’t learn if AI tells you the answer, but that is not true because if you don’t understand what AI has explained to you , you can ask it to explain what you are stuck on, in a more simplified style and it will do that. Learning Arduino scripts is a perfect example.

17

If you had mentioned that in your first post and showed your schemaic, your explanation would have carried a little more weight.
Many people do use AI to format and organize their thoughts but it's always a good idea to back it up with real data.

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18

Yes Sir,
I posted that to help newbies, I didn’t want to go as deep as what I showed you, but I understand your thoughts.

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Your post has the characteristics of a tutorial and these are best created by people with a thorough grasp of the subject matter otherwise it is the blind leading the blind.

1 Like
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If post 15 is a project, that should be posted in the Show Case category. If that is accompanied with a post similar to post 1 but better organized and less wordy it might serve the purpose of educating new folks. Just posting the OP is not helpfull as many have told you.
My personal thougths re post 15 is it might benefit from some reorganization but that is just 50+ yrs of gut feeling.