p-channel mosfet basic question

[Dave]

Hi all -

I've got a 2N5462 device that I'm trying to set up as a basic high-side
switch. I just want to turn on an LED to show the basic circuit connection
and have the MOSFET swtich it on and off with a toggle switch on the gate.
I connected the source to a +5VDC source, the gate through a 100-ohm
resistor to ground, the drain to a resistor connected to an LED to ground.
I put a toggle switch with a series 10K resistor from the +5VDC line to the
gate.

When I turn on the circuit power, the LED lights up without the toggle
switch pushed. I looked at the voltage on the gate and its sitting at
around 3.5VDC. Without the switch pushed, the gate should be pulled low
through the 100-ohm resistor. Why is it this high? Do I have this
connected incorrectly? Is the 2N5462 device actually an P-channel MOSFET
(as described in one datasheet) or a JFET amplifier (as described in another
datasheet)?

Any help would be appreciated.

Thanks.


Dave

 

[John Popelish]

Hi all -

I've got a 2N5462 device that I'm trying to set up as a basic high-side
switch. I just want to turn on an LED to show the basic circuit connection
and have the MOSFET swtich it on and off with a toggle switch on the gate.
I connected the source to a +5VDC source, the gate through a 100-ohm
resistor to ground, the drain to a resistor connected to an LED to ground.
I put a toggle switch with a series 10K resistor from the +5VDC line to the
gate.

When I turn on the circuit power, the LED lights up without the toggle
switch pushed. I looked at the voltage on the gate and its sitting at
around 3.5VDC. Without the switch pushed, the gate should be pulled low
through the 100-ohm resistor. Why is it this high? Do I have this
connected incorrectly? Is the 2N5462 device actually an P-channel MOSFET
(as described in one datasheet) or a JFET amplifier (as described in another
datasheet)?

According to this data sheet:
http://www.fairchildsemi.com/ds/MM/MMBF5460.pdf
It is a p-channel jfet that is on when the gate is at source
voltage and off when the gate is about 1.2 volts more
positive than the source. This is not the device you need
for a high side switch.

You need something more like:
http://www.zetex.com/3.0/pdf/BS250P.pdf
or a pnp transistor like 2N3906:
http://www.fairchildsemi.com/ds/2N/2N3906.pdf

 

[Eeyore]

Hi all -

I've got a 2N5462 device that I'm trying to set up as a basic high-side
switch.

Why a fet ?

Graham

 

[PeteS]

Hi all -

I've got a 2N5462 device that I'm trying to set up as a basic high-side
switch. I just want to turn on an LED to show the basic circuit connection
and have the MOSFET swtich it on and off with a toggle switch on the gate.
I connected the source to a +5VDC source, the gate through a 100-ohm
resistor to ground, the drain to a resistor connected to an LED to ground.
I put a toggle switch with a series 10K resistor from the +5VDC line to the
gate.

When I turn on the circuit power, the LED lights up without the toggle
switch pushed. I looked at the voltage on the gate and its sitting at
around 3.5VDC. Without the switch pushed, the gate should be pulled low
through the 100-ohm resistor. Why is it this high? Do I have this
connected incorrectly? Is the 2N5462 device actually an P-channel MOSFET
(as described in one datasheet) or a JFET amplifier (as described in another
datasheet)?

Any help would be appreciated.

Thanks.


Dave

According to my search, the 2N5462 is a P-channel JFET
http://www.onsemi.com/PowerSolutions/product.do?id=2N5462G

Not really suitable as a highside switch.

If you want to use a FET, find a P-Channel MOSFET.

Cheers

PeteS

 

[Dave]

Hi all -

I've got a 2N5462 device that I'm trying to set up as a basic high-side
switch. I just want to turn on an LED to show the basic circuit
connection and have the MOSFET swtich it on and off with a toggle switch
on the gate. I connected the source to a +5VDC source, the gate through a
100-ohm resistor to ground, the drain to a resistor connected to an LED to
ground. I put a toggle switch with a series 10K resistor from the +5VDC
line to the gate.

When I turn on the circuit power, the LED lights up without the toggle
switch pushed. I looked at the voltage on the gate and its sitting at
around 3.5VDC. Without the switch pushed, the gate should be pulled low
through the 100-ohm resistor. Why is it this high? Do I have this
connected incorrectly? Is the 2N5462 device actually an P-channel MOSFET
(as described in one datasheet) or a JFET amplifier (as described in
another datasheet)?

Any help would be appreciated.

Thanks.


Dave

My bad...

Many thanks to those that replied.

I got the device by looking for a P-Channel MOSFET but apparently I picked
the wrong part.

Dave