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How to use MOSFETs ?

J

Jan Nielsen

Jan 1, 1970
0
Hi,

I was told on another forum, that the way to switch "large" currents is
a MOSFET since a regular transistor cant, atleast not from a mcu output.

Now I have a few of these IRF530 MOSFETs, and are trying to get them
working.

I connect ground/common to one of the outer legs, a led (cathode) to the
other outer leg.
The anode of the led to VDC+ (4V)
Then I connect the base to a 1K resistor and then to common/ground.

So in my understanding, when I apply power the led should be off, well
its not, no matter where I connect the base with 1K resistor, the led is
always on.

I tried searching google for some guides, and found some, but I cant get
their exsamples to work, the led is either always on or off, no matter
what the gate/middle led is.

/Jan
 
L

Lord Garth

Jan 1, 1970
0
Jan Nielsen said:
Hi,

I was told on another forum, that the way to switch "large" currents is
a MOSFET since a regular transistor cant, atleast not from a mcu output.

Now I have a few of these IRF530 MOSFETs, and are trying to get them
working.

I connect ground/common to one of the outer legs, a led (cathode) to the
other outer leg.
The anode of the led to VDC+ (4V)
Then I connect the base to a 1K resistor and then to common/ground.

So in my understanding, when I apply power the led should be off, well
its not, no matter where I connect the base with 1K resistor, the led is
always on.

I tried searching google for some guides, and found some, but I cant get
their exsamples to work, the led is either always on or off, no matter
what the gate/middle led is.

/Jan

You don't connect ground to "one of the outer legs". Insure you connect
the Vss leg ground. That is the 'S' lead or source.

Connect the LED cathode to the Vdd leg. That is the 'D' or drain pin. You
then need a current limiting resistor in series with the LED anode. The
value of the resistor depends upon the power supply voltage and the current
specified for the LED. 10mA is a good round number. Connect the other
end of the resistor to the positive power source.

The 'G' or gate of the IRF-530 doesn't require a limiting resistor as the
device
is a MOSFET. It is an N channel MOSFET so applying a positive voltage to
this pin will activate the LED. Grounding the pin will deactivate the LED.

The device is sensitive to static electricity so you can damage it by
touching
the pins or by not observing ESD precautions.
 
J

John Popelish

Jan 1, 1970
0
Jan said:
Hi,

I was told on another forum, that the way to switch "large" currents is
a MOSFET since a regular transistor cant, atleast not from a mcu output.

Now I have a few of these IRF530 MOSFETs, and are trying to get them
working.

I connect ground/common to one of the outer legs, a led (cathode) to the
other outer leg.
The anode of the led to VDC+ (4V)
Then I connect the base to a 1K resistor and then to common/ground.

So in my understanding, when I apply power the led should be off, well
its not, no matter where I connect the base with 1K resistor, the led is
always on.

I tried searching google for some guides, and found some, but I cant get
their exsamples to work, the led is either always on or off, no matter
what the gate/middle led is.

The first hit is the data sheet:
http://kitsrus.com/pdf/irf530.pdf

Terminal 3 is the source, which goes to ground.

Jumper that to terminal 1 to turn the gate off, and connect
your led and current limiting resistor between the positive
supply and terminal 2 (also the tab), which is the drain.

This should result in a very dim or off led (just the
leakage current).

Turning the mosfet on is accomplished by bringing the gate
up to a positive voltage with respect to the source.
Unfortunately, from the Electrical Characteristics section,
you will find that 10 volts is needed to get the thing
essentially all the way on. The gate threshold voltage
(where the channel just begins to turn on) may be anywhere
between 2 and 4 volts.

If you want to switch a large current from a logic level
signal, you need a logic level (low gate threshold) device.
For instance:
http://www.fairchildsemi.com/ds/RF/RFP3055LE.pdf
 
E

ehsjr

Jan 1, 1970
0
Jan said:
Hi,

I was told on another forum, that the way to switch "large" currents is
a MOSFET since a regular transistor cant, atleast not from a mcu output.

Now I have a few of these IRF530 MOSFETs, and are trying to get them
working.

I connect ground/common to one of the outer legs, a led (cathode) to the
other outer leg.
The anode of the led to VDC+ (4V)
Then I connect the base to a 1K resistor and then to common/ground.

So in my understanding, when I apply power the led should be off, well
its not, no matter where I connect the base with 1K resistor, the led is
always on.

I tried searching google for some guides, and found some, but I cant get
their exsamples to work, the led is either always on or off, no matter
what the gate/middle led is.

/Jan

_____
| o |
|_____|
/_____/|
| IRF ||
| 530 ||
|_____|/
| | |
| | +----Source
| +------Drain
Gate

Source to ground, drain to LED, LED to resistor,
resistor to +

Ed
 
J

Jan Nielsen

Jan 1, 1970
0
ehsjr skrev:
_____
| o |
|_____|
/_____/|
| IRF ||
| 530 ||
|_____|/
| | |
| | +----Source
| +------Drain
Gate

Source to ground, drain to LED, LED to resistor,
resistor to +

Ed
Oh, I thought gate was the middle, since the datasheet didnt show a
clean pinout, atleast not the one I found.

Thanks, it works.

/Jan
 
R

Robert Wilson

Jan 1, 1970
0
Jan said:
Hi,

I was told on another forum, that the way to switch "large" currents is
a MOSFET since a regular transistor cant, atleast not from a mcu output.

Now I have a few of these IRF530 MOSFETs, and are trying to get them
working.

I connect ground/common to one of the outer legs, a led (cathode) to the
other outer leg.
The anode of the led to VDC+ (4V)
Then I connect the base to a 1K resistor and then to common/ground.

So in my understanding, when I apply power the led should be off, well
its not, no matter where I connect the base with 1K resistor, the led is
always on.

I tried searching google for some guides, and found some, but I cant get
their exsamples to work, the led is either always on or off, no matter
what the gate/middle led is.

/Jan
Hello,
A MOSFET gate looks superficially like a Capacitor. It gets more
interesting when you begin to drive them at higher frquencies. At the
same time if is a large device then you will begin to see an appreciable
charging current pulses from the output of your Micro. In this regard,
I would install a current limiting resistor, in order not to overload
the output pin spec.

In the case of a large MOSFET, the drive power is equal to P = f*C*V^2,
where f is drive frequency, C is the gate capacitance, and the V is the
drive voltage. Some people like to drive them with a negative voltage
(me included) when off, such that any dv/dt seen on the drain-source
route, does not take the gate positive due to the drain-source
capacitance, and so turning it on mistakenly.

There is usually a resistor stated as suitable for the gate drive of the
FET, you see it in the spec data sheets. This should be taken as the
minimum resistance to drive the fet with. Also remember if you are
switching the FET *really* smartly you will see some RF emission due to
the harmonics of the very fast transtition of voltage due to the
switching speeds.

Is that of use?

Cheers,

Rob Wilson,
Robstech Consulting Ltd, UK.
 
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