Go Down

Topic: [sorted] Tesla member but never used a MOSFET: clarity please? (Read 4762 times) previous topic - next topic

JimboZA

Last visit to my preferred supplier I picked up a couple of IRL520N logic level mosfets. I've never used a mosfet before.

If I want to use one connected to an Arduino digital output does it need a current limiting resistor on the base oops I mean gate?
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

TomGeorge

Hi,
JimboZA, I usually use a 470R, no matter what the design along with 10K gate to source.

http://irf.custhelp.com/app/answers/detail/a_id/215

Tom...... :)
Everything runs on smoke, let the smoke out, it stops running....

Coding Badly


I have used them without.  But it's a not a good idea.  There is a brief surge of current when the MOSFET is switched which may damage the pin.  Anything to keep the current below 20mA will work.

Grumpy_Mike occasionally publishes a simple current-limiting circuit for LEDs (two transistors + resistor).  That circuit works great with a MOSFET (plus transistor and resistor).  I highly recommend it as a first circuit.


Paul__B

JimboZA, I usually use a 470R, no matter what the design along with 10K gate to source.
Actually, it should not be a 10k gate to source, but Arduino output to ground. You do not want a potential divider - however minor - between the Arduino and the FET.

JimboZA

The difference between reply 1 and reply 3 being on which side of the gate resistor the resistor to source / ground goes?

Would one of you guys mind doing a quick sketch when you have a moment?
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

TomGeorge

Hi,
At your service.

Tom..... :)
Everything runs on smoke, let the smoke out, it stops running....

JimboZA

Thanks Tom... so @Paul, you would prefer the vertical resistor (Tom's G-S one) to be on the left of the current limiter?

Tom's pic:



Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

MarkT

Last visit to my preferred supplier I picked up a couple of IRL520N logic level mosfets. I've never used a mosfet before.

If I want to use one connected to an Arduino digital output does it need a current limiting resistor on the base oops I mean gate?
Its not an easy question to answer definitively.  The Arduino outputs are rated at an
absoute max of 40mA, yet will be designed to handle a certain amount of capacitive load
(although how much isn't given in the datasheet).  Capacitive loads will mean brief
spikes of current as the outputs switch which could exceed 40mA, but last a few
nanoseconds.

So a small MOSFET with a few hundred pF of gate capacitance might be quite OK
to drive directly.  But what the maximum capacitive load is before a resistor should be
used is unclear.  Most modern MOSFETs have more gate capacitance that you would
expect to see on a logic signal (measured in nanofarads) so a resistor is generally
advised.  150 ohms is a good value, since the lower the better for switching speed.

If you have a beefy MOSFET and want to switch it fast, you should always consider
a MOSFET driver chip which boosts the current output to much higher levels safely,
reducing switching losses and appearing as a standard logic load to the microcontroller.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

JimboZA

I'm finding that the 10k pull down is mandatory: if I tickle the "G" the led (above the mosfet, between +ve and "D", "S" to ground) comes on but stays on when the 5V is removed, unless I have that pull down.

Is that normal for mosfets?
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

MarkT

Absolutely - the gate is one plate of a capacitor, that's why they are called "field effect
transistors" as the electric field between gate and channel is responsible for the thickness
of the induced channel which allows current to flow between source and drain.

The charge on the gate mirrors the charge forming the conducting channel (charge carriers
overwhelm the natural p or n-type of the channel to the opposite sign).

Since the gate is isolated on a thin layer of quartz (silicon dioxide) it will hold its charge for
quite a while.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

JimboZA

Ah ok thanks Mark... I was wondering about capacitance. I actually turned all the power off, and when I reapplied it (but with nothing tickling the gate) the LED was on again. I took a while for it to go off and I said to myself, self, this is a capacitor.

Right then, the mystery of MOSFETs is sorted, at least as much as I need to know.
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

MarkT

Another thing worth mentioning is that power MOSFETs are very unlike logic circuit
MOSFETs.  They use vertical current flow (the substrate is the drain), there is an
integral diode between source and drain that is part of the 3D structure of the device,
the gate voltage relative to the source is the important thing (gate to drain voltage
has little effect on behaviour).  On resistances of 0.001 ohm or lower are possible!

Logic MOSFETs are completely symmetrical, source and drain are interchangable,
and the substrate has to be kept at one of the supply rails to remain isolated.  On
resistances in MOSFETs in a typical processor are measured in k-ohms (but
gate capacitances in fF (femto farads)...)

Chips with integrated DMOS MOSFETs are a hybrid between the two - current flow
is horizontal since the drains have to be isolated from the substrate, and thus
the on-resistance is significantly higher than with discrete power MOSFETs (down to
about 0.2 ohms)
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

JimboZA

One other thing just struck me, and that's due to the word "gate" as distinct from "base" in bjt-speak.....

Are mosfets thus switches always?- whereas a bjt is only a switch when it's saturated and otherwise is an amplifier?

Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

MarkT

power MOSFETs originally were analog amplifiers, but the switching use
has come to dominate as all power electronics today is switchmode, more
or less.  Modern power MOSFETs are optimised for switching, not for low
distortion.

I think the gate/base difference was purely to have a different name and prevent
confusing circuits designed for FETs being used for BJTs and vice versa.  Although
SCRs also have gates, so its not particularly consistent.

The base was named after the early transistor design which had a sliver of silicon with
two dopant areas diffused in from opposites sides, nearly meeting in the middle, the
sliver was the base in a mechanical sense...

Incidentally IGBT's have gate/emitter/collector....
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Paul__B

Thanks Tom... so @Paul, you would prefer the vertical resistor (Tom's G-S one) to be on the left of the current limiter?
Yes, because as first stated, you do not want a potential divider reducing in any way the voltage applied to the gate.

It may be a small point, but - why not do it properly?

The term "gate" is more appropriate for FETs as that is what it is - it controls; opens and closes the path from source to drain, like a tap or valve.

The action of a transistor is quite different - it amplifies the current you feed into the base; this is the beta (ß) parameter.  The ß of a FET is almost infinite as virtually no current is drawn by the gate (except of course, to charge it up); it has instead a voltage/current transfer factor or "transconductance" like a (thermionic) valve (translation for Yanks: vacuum tube).

Go Up