Advise me some MOSFETs for learning purpose

Thread Starter

sairfan1

Joined May 24, 2012
77
I recently learned using transistors now i want to buy a kit for some common MOSFETs so that i can experiment and learn, can you please advise some N-Channel MOSFETs for switching and PWM purpose, (mostly using for turning on/off electronics modules, Relay & Motors)

100mA to 800mA 3.3v battery operated circuits SMD/DIP
1 to 3Amp 5v 12v 24v SMD/DIP (TO-92)
(I'm not looking for wide supporting range rather i would like to buy best fit, for example XYZ best fit for 700/800mA some ABC best fit for 150/100mA so that later i can use them in projects.)

I will get back to forum with some circuit setup to learn further.

Edit 1:
I have test PCBs where i can solder SMD MOSFET, breadboard compatible are also welcome but i need both as in one order i want to buy as much as i can (just to avoid shipping delays)
 
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dl324

Joined Mar 30, 2015
13,527
If you're breadboarding circuits, 2N7000 is a good starter. The threshold voltage is low enough for 5V circuits at several hundred mA. I don't know of any 10 cent P channel MOSFETs.

If you can handle SMT devices, I'd recommend AO3400 and AO3401. They're logic level devices that can handle several amps and cost around 2 cents each.

None of these devices have gate protection and I can tell you from experience that 2N700 are very easy to damage. NTA4151P does, but it has a maximum gate voltage of +/-6V.
 

Audioguru again

Joined Oct 21, 2019
3,846
Most Mosfets are used for fairly high currents then they are in a power transistor case with a metal tab to be bolted to a heatsink. Most need a gate voltage of 10V to fully turn on but there are some Mosfets that are called logic-level that need a gate voltage of only 4.5V to fully turn on. An old ones is IRF540. a logic level one is IRL540.
A low current Mosfet in a TO-92 case is a 2N7000 and 2N7002.
 

Yaakov

Joined Jan 27, 2019
3,755
Most Mosfets are used for fairly high currents then they are in a power transistor case with a metal tab to be bolted to a heatsink. Most need a gate voltage of 10V to fully turn on but there are some Mosfets that are called logic-level that need a gate voltage of only 4.5V to fully turn on. An old ones is IRF540. a logic level one is IRL540.
A low current Mosfet in a TO-92 case is a 2N7000 and 2N7002.
The 2N700x is a great MOSFET for experimentation and learning. They are cheap as dirt and so you can make mistakes, but they act like MOSFETs and that's what you want.
 

MaxHeadRoom

Joined Jul 18, 2013
23,923
I started using the 2n7000 from its early inception, late 20th century.
Most applications were interface from PC-based industrial control I/O interface.
Many still in operation today!
 

Papabravo

Joined Feb 24, 2006
17,291
What is gate protection, how can we achieve it, if its not built-in

I was looking at datasheet IRF540 (attached) Gate source threshold Min 2v does it mean at minimum 2v MOSFET will turn ON
No. Vgs(th) is the voltage where the MOSFET channel just starts to conduct a very little bit of drain current. to tun it on as a switch you will need to make Vgs considerably larger.
Rds(on) is specified as 77 mΩ (0.077 Ω) when Vgs = 10V
So five times the minimum threshold to turn the device on.
Gate protection is done with a series resistor and a pulldown to prevent a floating gate from destroying the device.
 

dl324

Joined Mar 30, 2015
13,527
What is gate protection, how can we achieve it, if its not built-in
This is how the device I mentioned does it.
1634855743219.png
When you're handling the devices, you need to make sure you don't apply a high enough voltage to damage the gate oxide or breakdown the device. If you're lucky, the device dies outright. If you're not, the device may appear to work when it isn't. The 2N7000 I damaged experienced high leakage.
 

Audioguru again

Joined Oct 21, 2019
3,846
The datasheet for the IRF540 says that the threshold voltage of 4V is needed for some of them to conduct a drain current only 250uA which is almost turned off. Some of them conduct 250uA when the gate-source voltage is 2V. They are all guaranteed to fully turn on when the gate-source voltage is 10V.
 

Thread Starter

sairfan1

Joined May 24, 2012
77
Rds(on) is specified as 77 mΩ (0.077 Ω) when Vgs = 10V
So five times the minimum threshold to turn the device on.
Sorry I was not able to understand what did you mean by "So five times the minimum threshold to turn the device on." can you please explain how are you calculating this, thanks.
 

Papabravo

Joined Feb 24, 2006
17,291
I'm learning so expect a goof question, I was able to find those minimum threshold values in datasheet like 2v-4v at 250uA, but i was not able to find guaranteed ON voltage as you said 10V, did you take it from Vgs = 10V in datasheet? or you find it through some calculation.

And also by full turn ON at gate-source voltage 10v means it can be used with 10v or above circuits? or am I going a wrong direction?
The information is there but maybe not in the explicit form that you might have expected. Let us take the IRF 540 datasheet.
1634915793156.png

The 2nd line shows Rds(on) at Vgs = 10 V of 0.077 Ω. Later in the datasheet you will see this value characterized as a MAXIMUM
This is telling you that the resistance of the channel is at a very low value when Vgs is equal to 10 Volts.
For some devices it might actually be a bit lower, but it would be rare to find one that was higher. It is effectively a maximum value at Vgs=10V. You could also make Vgs higher but Rds(on) would not change much.

1634916335493.png

With a .pdf datasheet (not an image) you can search for text items like VGS. Notice that the continuous drain current, Id, is specified at Vgs = 10V. Additional confirmation that the device is fully on in that particular configuration. Also note that +20V is the absolute maximum value of Vgs that you can use without risking damage to the device.

One more thing. Critical datasheet information is sometimes buried in the footnotes. DON'T ignore the footnotes.
 
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dl324

Joined Mar 30, 2015
13,527
I was looking at datasheet IRF540 (attached) Gate source threshold Min 2v does it mean at minimum 2v MOSFET will turn ON
1634919686691.png
1634919708273.png
2V is the minimum, but it could take as much as 4V to have a 250uA drain current.

This curve shows the gate to source voltage required for certain drain currents:
1634919799638.png
Note that the lowest Vgs indicated is 4V and they're showing a drain current of about 200mA.

If you wanted the MOSFET to have a drain current of 1A, I'd provide a Vgs of at least 5V.
 

Audioguru again

Joined Oct 21, 2019
3,846
The threshold voltage of a Mosfet is when it begins to turn on. It is actually almost turned off.
As I said before, an IRF540 needs a Vgs of 10V to fully turn on. The datasheet shows it as its on resistance.
An IRL540 (see the L?) is a logic level Mosfet that is fully turned on with a Vgs of 5V but all of them are turned on very well with a Vgs of 4V.

The graphs on a datasheet are for a "typical" device that you cannot order and buy. You get whatever device passes the range of specs. The above graph of a "typical" IRF540 device shows a on-current of 200mA when Vgs is 4V but some conduct only 250uA.
 

Papabravo

Joined Feb 24, 2006
17,291
The threshold voltage of a Mosfet is when it begins to turn on. It is actually almost turned off.
As I said before, an IRF540 needs a Vgs of 10V to fully turn on. The datasheet shows it as its on resistance.
An IRL540 (see the L?) is a logic level Mosfet that is fully turned on with a Vgs of 5V but all of them are turned on very well with a Vgs of 4V.

The graphs on a datasheet are for a "typical" device that you cannot order and buy. You get whatever device passes the range of specs. The above graph of a "typical" IRF540 device shows a on-current of 200mA when Vgs is 4V but some conduct only 250uA.
This is an important point about datasheets representing the mean (average) device for a set of random variables (the performance specifications) and then specifying that 99%+ of the devices will be within 3 standard deviations (±3σ) from the mean. That is why competent designers come up with designs that don't depend on specific values, or even a narrow range of values, for those parameters.
 

dl324

Joined Mar 30, 2015
13,527
Or at least within one standard deviation. (≈68%)
Exactly. Rants about not being able to order typical parts is just that - a rant. You have a 68% chance of getting something considered typical and about a 95% chance of getting something within 2 sigma.
 

Papabravo

Joined Feb 24, 2006
17,291
Exactly. Rants about not being able to order typical parts is just that - a rant. You have a 68% chance of getting something considered typical and about a 95% chance of getting something within 2 sigma.
Exactly. PISSENANMOANENALLTHELIVELONGDAY
 
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