2N7000 Transistor: An In-Depth Overview

Thanks for sharing.

@marshalldunlop What is the purpose of your post? Most of my collection of MOSFETs can handle 10's of amps, a few maybe 5 or 6. Why should I limit myself to 200ma?

Do I ever need 10A, rarely, but if I can buy 2 MOSFETs for the same relative price, what is the harm of having a device that can handle more?

Keep in mind I am an 82 yo autistic Tube guy, my parts bins are only populated with stuff that works. I have no clue why they work, trust me many have tried to teach me, but it's hopeless.

I have just discovered ChatGPT can help, I just went through an exercise of asking for 3 examples of N and P 3.3V and 5V MOSFETs that could handle 10A at 60V. I went on Aliexpress and have a mix of 80 for $28.49 CDN shipping and taxes included.

NOW, if you want to teach me and many others what SPECIFICALLY to look for on a datasheet and HOW to interpret it, maybe some of us would learn something new. I mean in simple everyday street language, not full of meaningless acronyms. For me at least what I need to know is where on the datasheet does it say this device turns fully on at 4.5V or 3V. Be prepared to defend.

Thanks a ton for this - gives me a greater understanding of the capabilities of the part.

If I had to approach the selection of a MOSFET from the circuit design needs perspective, how would I go about selecting a MOSFET to switch on a 24V water pump?  Current draw unknown.  Would I have to run the pump and physically measure the current drawn?  And then select a MOSFET based on a corresponding continuous drain current > than pump draw?  What other variables would change?

@samc Larger voltage and current won't harm, lower will. The MOST important number is the fully on Gate voltage, either 5V or 3.3V. Also is it a low side N chan or Hi side P chan. N chan are more prolific, and 3.3V P chan are very rare.

Hi @samc,

re: If I had to approach the selection of a MOSFET from the circuit design needs perspective, how would I go about selecting a MOSFET to switch on a 24V water pump? Current draw unknown. Would I have to run the pump and physically measure the current drawn? ... etc ... etc.

  Designing a circuit, without knowing what it must achieve, is never a great approach. Of course, sometimes it is possible to take a rough guess, at least as a starting point, then do some experiments, possibly using a circuit based on the guesses. Depending on your ability to guess, luck, and so on, the result may be pleasing or disappointing.

Hopefully, it will not be dangerous, but clearly some sort of 'risk assessment', which, depending on the proposed course of action, may be just a few seconds thought or a much more involved process, should consider the 'worst case' scenario, before switching on.

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So do you need to determine the current demand of your pump? Yes... and more.

Perhaps, you can get a reasonable indication from a datasheet of the motor. You should consider all reasonable possibilities. e.g. What happens if the pump stalls? This might be due to a mechanical failure, such as bearing failure, or maybe because the mechanical load on the motor is excessive. Brushed motors, for example, tend to draw very large currents when stalled. If the power is not promptly removed, "magic smoke" and even a fire may be the result.

Similar, when power is first applied to a motor, it may take a much higher current, whilst it accelerates to normal running speed. The higher current during this phase may be enough to destroy an underrated MOSFET or other semiconductor switching device.

Also, switching voltages applied to any inductor, including the coils in an electric motor, usually results in high voltage spikes, which may also destroy an inadequately protected MOSFET or similar.

You should ensure that all of these (and more) considerations are taken into account.

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Of course, all of the above is taking a pessimistic view on life. By contrast, Bill (@dronebot-workshop) who kindly hosts this forum, has a number of excellent video + blog examples of safely driving electric motors with semiconductor switching circuits. But they only succeed because the characteristics of the motor have been considered, and appropriate switching devices have been chosen.

In addition, many of the examples will not use just a 'bare' MOSFET, but rather a circuit board or 'boxed' circuit, which has been designed for controlling motors of a certain type, rating and so on, which will usually include circuit elements to mitigate some of the issues I have just alluded to. Even if the board appears to have little more than a single chip, it is probable that chip will include appropriate functionality.

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So sorry, I can't provide a simple, positive answer to your question. It is like the proverbial "How long is a piece of string?" It maybe that the motor you have in mind is very small, robust, and can be driven by a very simple circuit. Or perhaps, it is a starter motor for huge, diesel engine for a lorry or a jet aircraft turbine.

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Please feel free to ask further questions. Please try to make each question as specific as possible, in the hope that you will get a matching, useful answer.

Best wishes, Dave

Hi Ron @zander,    (and @samc )

 RE: Larger voltage and current won't harm, lower will. The MOST important number is the fully on Gate voltage, either 5V or 3.3V. Also is it a low side N chan or Hi side P chan. N chan are more prolific, and 3.3V P chan are very rare.

  Ron, with due respect, please be careful when making very general statements like this, as it can be misleading to anyone who is less familiar with these components. In the 'right' circumstances, much of this statement is true, but the real world is complex, with many different situations to cope with.

e.g. It is obviously unwise to choose devices that inadequately rated, but higher rated devices tend to need more power to drive them, and exhibit higher leakage currents, etc. In some circumstances, these downsides may be of little consequence, but in other circumstances, they may be extremely disadvantageous. So chosing high voltage/current devices for situations that do not demand them, can introduce new difficulties.

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Similarly, N-channel MOSFETs are often used in high side switching of power circuits, such as in many buck regulators. N-Channel are often chosen for their lower drain-source resistance, compared to an equivalent P-channel device, which, in appropriate circumstances, more than compensates for the increased complexity of the driver circuit.

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P-channel devices are probably less common than their N-channel counterparts, presumably because of their higher drain-source resistance, meaning many designers will prefer N-Channel. This disadvantage will be accentuated if the gate drive voltage is limited to 3.3V. Thus, I am not surprised you found few high voltage/current P-channel devices with acceptable drain-source resistance characteristics, with a typical gate voltage drive of 3V or less. Such devices, if made, would make very few sales.

However, lower current/voltage P-Channel devices, with reasonable drain-source resistance characteristics, are available. e.g. Si2301BDS is -20V P-Channel, with Rds of <0.15 Ohm, with 2.5V gate drive, and passing 2A, in a small surface mount package.

Data sheet at https://www.vishay.com/docs/72066/si2301bds.pdf

Obviously, this is not a 'universal' answer to all situations, but is worthy of consideration in the right circumstances.

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Best wishes and take care my friend, Dave

@samc Try ChatGPT or equivalent, I normally have no use for that stuff, but compared to using the device selectors in DigiKey and similar, using ChatGPT was easy. I simply asked for 3 examples of a MOSFET that was fully on at 4.5V, and could handle 50V at 10A. Worked like a charm. Dave has tried to educate me in the art of reading Datasheets, but so far the organ has been rejected each time. I now have a collection of 80 MOSFETs for N chan, P chan, 3.3V, and 5V. All for under $30.

@davee Thanks a ton for the detailed response.  My problem is that the pump was an online purchase and its only "datasheet" is printed on a sticker on the side, and the only legible part of that is "24V"  😀 

But, you've given me enough pointers alongside @MarshallDunlop's initial post to at least get to some sort of experimental position.

Great stuff.

@samc Try using your phone's magnifier and illuminator. If it has macro mode use that. Take the best pictures you can and blow them up. Even a company name will help. If you have young eyes in the house or a neighbour/friend have them try, their eyes are MUCH better. In any case, once you have the best picture possible, submit it to google photo lookup, and every AI engine you have access to.

Any additional info beyond 24V is a clue. Do you know where you bought it? If so, let me know so I can have a look about. I will be gone most of the day riding the trails in my new machine but I will have a look tonight.

@zander Thanks Ron.  The label is much rubbed - and the script on it seems to have been Chinese (from what's left, that is).  I did try it through Google Translate - but no luck.  Thanks a ton for offering.

@samc I know what you mean, I have a few like that too. At this point you have a couple choices, guess and watch for smoke, replace it, OR look up similar spec'd devices with regard to pumping and see what their electrical specs are. Good luck.