Hello, My name is Peter

@pajhum42

Welcome to the forum Peter. Post what you don't get about transistors to say the Components & Programming forum and you will get an answer or suggestions as to how to understand them.

John

@pajhum42 Welcome to the forum Peter. I think you may be the first member from Afghanistan. I also struggle with transistors, so you're not alone.

Posted by: @pajhum42

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Hi,  my name is Peter.  I'm a retired database programmer from north coast of California.  I'm trying to learn Ardunio programming (not hard so far) and electronics (harder, figuring out transistors is driving me nuts).

Welcome to the forum @pajhum42, (Peter)...

Yes, electronics is a tough subject matter to tackle, but the good news is that if your really interested, and are willing to read, study and research..there is a ton of information on the internet..indeed, google is your friend in this case..

As far as understanding how transistors work..The simple answer is using water analogy, the device is a valve to control the flow of electrons in the wires of a circuit, and in like manner that a valve controls water thru a pipe.. Transistors are mostly current driven devices..and there are many different types, some, like Mosfets, are voltage driven, but all still control the current, no matter what type they are.

here is a link to some basic transistors calculations that may help you to understand how they work, and how to use them in a circuit. https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/biasing-calculations/

Good luck, and keep us posted on your journey learning electronics..

Regards,

LouisR

@inst-tech Sorry to but in, but

I understand, but it's figuring out what size of resistor is needed and where to accomplish what purpose. MOSFETS are easier and I will always use a Logic Level MOSFET for a switch when I can (3.3V MOSFETS are more challenging to find, though)

And now back to your regularly scheduled program.

Posted by: @zander

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@inst-tech Sorry to but in, but

I understand, but it's figuring out what size of resistor is needed and where to accomplish what purpose. MOSFETS are easier and I will always use a Logic Level MOSFET for a switch when I can (3.3V MOSFETS are more challenging to find, though)

And now back to your regularly scheduled program.

@zander (RON), yes I understand that, and there's a little math involved in doing that..so here's a  link to how to figure that out.. https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/biasing-calculations/

Your right about using Mosfets for switching when higher currents are needed, but Jfets and some power transistors can be used when low voltage circuits are used as in your example of 3.3v...

When I when to tech school, only about 20% of the students that started out, made it through.. (we started with 42, graduated with 8 after 2 years!.. The math and physics killed most of them, and some just decided that this wasn't something they wanted to do after all...) Just as it is with hobbyist, many just want to play and build something..but run into trouble when things don't work as hoped for..But it's all learnable, it just takes patients, studying, and researching to find what your looking for..I've just been doing it for so long, I almost forgot how hard it was for me when I was first learning basics electronics in the Navy's 'A' where the success rate was about the same, 25%.. for 26 weeks. in that scenario, if you didn't make it, you were most probably would be chipping paint and scrubbing decks for the next 4 years...lol

At any rate, no one will ever tell you it's easy..but it is doable..!

regards,

LouisR

@inst-tech The reason I like MOSFETs is they are voltage-driven. I don't need to figure anything out. If it is a 5V MOSFET and I have a 5V logic pin, setting the pin HI turns on the MOSFET. I need to find a 3.3V version, and then I am a happy camper. I am at a point where I only care if the durn thing turns on. If I am only passing 100ma and it is rated at 10A, I don't care. The cost is peanuts, so it's no big deal.

However, I will file the link away and read it as I am not opposed to learning new tricks. I don't need more new tricks, just a 3.3V solid-state switch.

Posted by: @zander

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@inst-tech The reason I like MOSFETs is they are voltage-driven. I don't need to figure anything out. If it is a 5V MOSFET and I have a 5V logic pin, setting the pin HI turns on the MOSFET. I need to find a 3.3V version, and then I am a happy camper. I am at a point where I only care if the durn thing turns on. If I am only passing 100ma and it is rated at 10A, I don't care. The cost is peanuts, so it's no big deal.

However, I will file the link away and read it as I am not opposed to learning new tricks. I don't need more new tricks, just a 3.3V solid-state switch.

@ander (RON), I see your point.. so I'll recommend that you try using Logic level mosfets like RFP30N06LE with a Vgs (th) of 2V, and a 2N7000 with a Vgs (th) of 3V...

Keep in mind that there are many different types of mosfets, like Jfets, enhancement, and depletion, which all require specific biasing techniques.

Regards,

LouisR 

Hi @inst-tech,

 Thanks for your helpful suggestions.

  I hope you won't mind if add a little background information to your useful message.

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   I agree with what you have written, but a common misapprehension (which I am sure you realise, but I fear others might not) is to assume the Gate Threshold Voltage Vgs(th) is the 'switching on' voltage, implying that when that voltage is present the FET will be switched on.

However, it is more accurately a 'switching off' voltage. i.e. if the gate voltage is at that value, the transistor will not pass more than a small, specified current, typically 0.25 mA.

Switching the FET on, and achieving 'near' minimum (Rds) resistance, assuming that is required for the application, typically requires a considerably higher gate voltage, assuming the designer is not intending to  use a 50 A device whose leakage current maybe 0.25 mA current, to switch a load current of the same magnitude.

Some power FETs, including the numbers you mentioned, may manage a fairly low Rds value, when the gate voltage is in the range 4.5-5.0V, which just about makes them usable when driven by logic outputs that get very close to 5V, albeit their Rds will be considerably lower with a higher gate voltage, say 10V. Of course, most 5V logic gate outputs will often achieve this range, but their specification allows the 'high' output voltage to be considerably lower, say 4V, depending on temperature, process, etc., so this can be a bit of a design lottery, albeit for one-off amateur usage, this may not be a major issue.

However, with 3.3 V logic families, they cannot be reliably directly switched at all.

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So far, I have found and used small Fets with a lower switch-on rating, available at low cost from AliExpress, etc. e.g.  A2SHB (Si2302DS ?) (n-channel) and     A1SHB (SI2301DS ?)  (p-channel), but they only seem to be in the tiny SOT-23 package, meaning hot air soldering to an (adapter) PCB is the only advisable route, unless you have very good eyesight and steady hands. Some forum members are not comfortable with this limitation.

I hope this is useful to someone.

Best wishes, Dave

@davee , thanks for the information that those not as familiar, and  as usual, you are spot on.. and hence I defer to the data sheets for determining the usability of any component that I'm inclined to use for a particular circuit design.

Probably for those that are not as familiar with Mosfet technology as you and I might be, I would suggest that they take a look at this link: https://www.theengineeringknowledge.com/mosfet-bias-circuits/

I think this would simplify any explanation I could give..and is necessary information to the user on how the different types, ie: depletion,enhancement, Jfets, etc. , are biased for a particular circuit design. In the case of logic level switching, it may be crucial to understand this the difference between Vgs, and Vgs(th) or sometimes denotes as simply Vt.

regards,

LouisR