FET (TOSHIBA 2SK941) SWITCHING

[Tuurbo46]

Hi
Im currently having a switching problem. Im applying 5V to the gate of this
fet and the driven circuit works correctly, but when the gate is switched
off, the micro which is driving the gate does not go completely to zero and
therefore the switching circuit stays on slighty (0.007v).

I have tried putting a 1N4001 diode in series with the gate and the mirco
and this doesnt work. Also i have tried putting a 10k resistor from the gate
to the source(0V) and this also does not work.

At this point im stuck. Would my gate voltage read 0V if i ran the output
voltage from micro through a precision voltage supply (e.g 3 pin chip)?

Please could anybody foward there idea?



Cheers Turbo

 

[Walter Harley]

"Tuurbo46" <tuurbo46@yahoo.co.uk> wrote in message
news:d2uhqj$9id$1@newsg3.svr.pol.co.uk...

Hi
Im currently having a switching problem. Im applying 5V to the gate of
this fet and the driven circuit works correctly, but when the gate is
switched off, the micro which is driving the gate does not go completely
to zero and therefore the switching circuit stays on slighty (0.007v).

I have tried putting a 1N4001 diode in series with the gate and the mirco
and this doesnt work. Also i have tried putting a 10k resistor from the
gate to the source(0V) and this also does not work.

At this point im stuck. Would my gate voltage read 0V if i ran the output
voltage from micro through a precision voltage supply (e.g 3 pin chip)?

Please could anybody foward there idea?

What are you trying to switch, and how are you measuring this voltage? The
difference between 7mV and 0V is often unimportant in switching
applications. I can't tell from your message whether the 7mV is on the gate
or at the load, but I doubt it matters either way. If it does matter,
you're probably using the wrong switching technique altogether.

If you're just measuring the voltage at the output of the switch, especially
if there's no load on it at the time, don't sweat it. Other than the fact
you're measuring a little voltage there, is everything working properly (I
*think* I understand the answer to be yes)?

Zero only exists inside computers. There's no such thing as zero in the
real world.

 

[Tuurbo46]

Hi

Im trying to switch a radio on and off at certain times by a micro to save
power because its in a remote location. The FET circuit is switching the
radio on and off.

The gate voltage has to be 0V, else there is still a circuit between the
drain and the source. Currently there is 0.007V on the gate and therefore
the drain and source are still in use (therefore radio is wasting power).

When the gate to the FET is 5V the ground 0V to my radio ( through the drain
and source) is connected and therefore my FET driven circuit is working.
The problem in having is my radio is in a remote location and there is no
mains power supply and the FET circuit is switching the radio on and off.
At the moment this is not happening and when the radio is supposed to be off
it isnt because the gate voltage is 0.007V above 0V. I am therefore trying
to make this small error to 0V.

Sorry for the complicaiton.

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:J9idnXfRz9JSTM_fRVn-tQ@speakeasy.net...

"Tuurbo46" <tuurbo46@yahoo.co.uk> wrote in message
news:d2uhqj$9id$1@newsg3.svr.pol.co.uk...
Hi
Im currently having a switching problem. Im applying 5V to the gate of
this fet and the driven circuit works correctly, but when the gate is
switched off, the micro which is driving the gate does not go completely
to zero and therefore the switching circuit stays on slighty (0.007v).

I have tried putting a 1N4001 diode in series with the gate and the mirco
and this doesnt work. Also i have tried putting a 10k resistor from the
gate to the source(0V) and this also does not work.

At this point im stuck. Would my gate voltage read 0V if i ran the
output voltage from micro through a precision voltage supply (e.g 3 pin
chip)?

Please could anybody foward there idea?


What are you trying to switch, and how are you measuring this voltage?
The difference between 7mV and 0V is often unimportant in switching
applications. I can't tell from your message whether the 7mV is on the
gate or at the load, but I doubt it matters either way. If it does
matter, you're probably using the wrong switching technique altogether.

If you're just measuring the voltage at the output of the switch,
especially if there's no load on it at the time, don't sweat it. Other
than the fact you're measuring a little voltage there, is everything
working properly (I *think* I understand the answer to be yes)?

Zero only exists inside computers. There's no such thing as zero in the
real world.

 

[Larry Brasfield]

"Tuurbo46" <tuurbo46@yahoo.co.uk> wrote in
message news:d2ukkc$eso$1@news8.svr.pol.co.uk...

Hi
Hi.
Im trying to switch a radio on and off at certain times by a micro to save power because its in a remote location. The FET
circuit is switching the radio on and off.

I believe it is doing just that and agree with Mr. Harley's
take on the importance of what you see. A MOSFET
with 7 mV gate-to-source is effectively off if it is the
usual (enhancement mode) kind. (If it was a depletion
mode device, it would be on.) I suggest you put either
a current meter (if you have one) or a 100K resistor
(with your mill-voltmeter across) in series with your
nominally off circuit and measure the actual current
when the MOSFET is supposedly off. That should
show whether there is a real problem or or not.

The gate voltage has to be 0V, else there is still a circuit between the drain and the source. Currently there is 0.007V on the
gate and therefore the drain and source are still in use (therefore radio is wasting power).

Your premise is wrong. Lookup the meaning of
"threshold voltage" in relation to MOSFETs.

When the gate to the FET is 5V the ground 0V to my radio ( through the drain and source) is connected and therefore my FET driven
circuit is working. The problem in having is my radio is in a remote location and there is no mains power supply and the FET
circuit is switching the radio on and off. At the moment this is not happening and when the radio is supposed to be off it isnt
because the gate voltage is 0.007V above 0V. I am therefore trying to make this small error to 0V.

Is the radio observably not off? If it continues
to draw power, the cause is something beyond
the 7 mV you mention. In that case, it is time to
show some kind of schematic.

Sorry for the complicaiton.

No worry, all par for the course.

[quotes and sig only follow]

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message news:J9idnXfRz9JSTM_fRVn-tQ@speakeasy.net...
"Tuurbo46" <tuurbo46@yahoo.co.uk> wrote in message news:d2uhqj$9id$1@newsg3.svr.pol.co.uk...
Hi
Im currently having a switching problem. Im applying 5V to the gate of this fet and the driven circuit works correctly, but when
the gate is switched off, the micro which is driving the gate does not go completely to zero and therefore the switching circuit
stays on slighty (0.007v).

I have tried putting a 1N4001 diode in series with the gate and the mirco and this doesnt work. Also i have tried putting a 10k
resistor from the gate to the source(0V) and this also does not work.

At this point im stuck. Would my gate voltage read 0V if i ran the output voltage from micro through a precision voltage supply
(e.g 3 pin chip)?

Please could anybody foward there idea?


What are you trying to switch, and how are you measuring this voltage? The difference between 7mV and 0V is often unimportant in
switching applications. I can't tell from your message whether the 7mV is on the gate or at the load, but I doubt it matters
either way. If it does matter, you're probably using the wrong switching technique altogether.

If you're just measuring the voltage at the output of the switch, especially if there's no load on it at the time, don't sweat
it. Other than the fact you're measuring a little voltage there, is everything working properly (I *think* I understand the
answer to be yes)?

Zero only exists inside computers. There's no such thing as zero in the real world.
--

--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Pooh Bear]

Tuurbo46 wrote:

Hi

Im trying to switch a radio on and off at certain times by a micro to save
power because its in a remote location. The FET circuit is switching the
radio on and off.

The gate voltage has to be 0V, else there is still a circuit between the
drain and the source. Currently there is 0.007V on the gate and therefore
the drain and source are still in use (therefore radio is wasting power).

All mosfets have a threshold voltage below which they don't conduct. 7 mV on the
gate is never going to turn on an enhancement mode mosfet., so that certainly
isn't your problem.

When the gate to the FET is 5V the ground 0V to my radio ( through the drain
and source) is connected and therefore my FET driven circuit is working.
The problem in having is my radio is in a remote location and there is no
mains power supply and the FET circuit is switching the radio on and off.
At the moment this is not happening and when the radio is supposed to be off
it isnt because the gate voltage is 0.007V above 0V. I am therefore trying
to make this small error to 0V.

It's not the 7 mV.

What's the supply voltage for the radio that you're switching ?
What's the load current for the radio ?
What is the mosfet's drain voltage in on and off conditions ?


Graham

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote (in
<4252F360.35EA9B2@hotmail.com&gt about 'FET (TOSHIBA 2SK941) SWITCHING',
on Tue, 5 Apr 2005:

When the gate to the FET is 5V the ground 0V to my radio ( through the drain
and source) is connected and therefore my FET driven circuit is working.

It's difficult to understand what you are doing. Have you put the FET in
the negative supply to your radio? Is the radio grounded? Is the power
supply grounded?
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Pooh Bear]

Tuurbo46 wrote:

Hi
Im currently having a switching problem. Im applying 5V to the gate of this
fet and the driven circuit works correctly, but when the gate is switched
off, the micro which is driving the gate does not go completely to zero and
therefore the switching circuit stays on slighty (0.007v).

Why are you using what appears to be an obsolete fet btw ? Can't even find data
in the usual places.


Graham

 

[John Popelish]

Tuurbo46 wrote:

Hi

Im trying to switch a radio on and off at certain times by a micro to save
power because its in a remote location. The FET circuit is switching the
radio on and off.

The gate voltage has to be 0V, else there is still a circuit between the
drain and the source. Currently there is 0.007V on the gate and therefore
the drain and source are still in use (therefore radio is wasting power).

When the gate to the FET is 5V the ground 0V to my radio ( through the drain
and source) is connected and therefore my FET driven circuit is working.
The problem in having is my radio is in a remote location and there is no
mains power supply and the FET circuit is switching the radio on and off.
At the moment this is not happening and when the radio is supposed to be off
it isnt because the gate voltage is 0.007V above 0V. I am therefore trying
to make this small error to 0V.

Sorry for the complicaiton.

If this small gate voltage is accompanied by measurable drain current,
I suspect you have blown the gate insulation with static and the
mosfet is no longer functional.

 

[Walter Harley]

"Tuurbo46" <tuurbo46@yahoo.co.uk> wrote in message
news:d2ukkc$eso$1@news8.svr.pol.co.uk...

When the gate to the FET is 5V the ground 0V to my radio ( through the
drain and source) is connected and therefore my FET driven circuit is
working. The problem in having is my radio is in a remote location and
there is no mains power supply and the FET circuit is switching the radio
on and off. At the moment this is not happening and when the radio is
supposed to be off it isnt because the gate voltage is 0.007V above 0V. I
am therefore trying to make this small error to 0V.

Like Graham said: it's not the 7mV that's your problem. The problem is
either that you've got the circuit configured wrong, or that you've blown
the FET, or both.

Can you draw a simple schematic here to show what you're doing? (Google
"aacircuit" to find a simple, easy, free ASCII schematic editor.) Also, is
the 2SK941 an N-channel or a P-channel FET? (And as Graham asks, can you
point us to a datasheet somewhere?)

FETs are analog devices, not digital. They don't have a well-defined "on"
and "off", even though we sometimes talk about them that way. Rather, the
max current from drain to source is determined by the voltage from gate to
source (or drain, but that's another story) in a fairly smooth way. Below a
certain Vgs, very little current flows. As Vgs approaches a "threshold"
voltage of a couple of volts (depending on the FET), more and more current
is allowed to flow; and then, as Vgs continues to increase, at a certain
point the max current hits its limit. If you look at some FET datasheets
you can see graphs of Id versus Vgs that will show you what I mean.

The point being, 7mV is way below threshold, so you should expect a small
amount of current to flow but (a) no different than if Vgs was zero and (b)
not enough to power the radio.

However, there are certain very common mistakes that newbies make in
designing FET circuits, and I suspect (and so does John, I think) that
you've made one of them. Thus the request for you to show us a schematic.

 

[Fred Bloggs]

Tuurbo46 wrote:

Hi
Im currently having a switching problem. Im applying 5V to the gate of this
fet and the driven circuit works correctly, but when the gate is switched
off, the micro which is driving the gate does not go completely to zero and
therefore the switching circuit stays on slighty (0.007v).

I have tried putting a 1N4001 diode in series with the gate and the mirco
and this doesnt work. Also i have tried putting a 10k resistor from the gate
to the source(0V) and this also does not work.

At this point im stuck. Would my gate voltage read 0V if i ran the output
voltage from micro through a precision voltage supply (e.g 3 pin chip)?

Please could anybody foward there idea?

Sounds like you have Drain/Source leads reversed on the MOSFET...have
you verified with an ohmmeter- there is an internal body diode with
anode at Source and cathode at Drain...

 

[Tuurbo46]

Yes you are all correct and i made a silly error(school boy error). 7mV on
the gate is erelavent! I disconnected the wire from the FET to the radio
and put an amp meter in series with it and when the gate was 5V the radio
was using 150mA, which is correct. When the gate was 7mV the wire to the
radio was 0mA. The thing which confused me yesterday was that the (on led)
on the radio was dimly visable when off (7mV on gate). This was attributed
to a big capacitor in the radio. This can also be seen when all leads are
disconnected from radio and the led is still dim for a short period( 1
min ).

The lesson i have taken from this is dont try and short cut jobs and use a
multimeter more.

Thanks for your help guys and girls.


"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:4252F360.35EA9B2@hotmail.com...

Tuurbo46 wrote:

Hi

Im trying to switch a radio on and off at certain times by a micro to
save
power because its in a remote location. The FET circuit is switching the
radio on and off.

The gate voltage has to be 0V, else there is still a circuit between the
drain and the source. Currently there is 0.007V on the gate and
therefore
the drain and source are still in use (therefore radio is wasting power).

All mosfets have a threshold voltage below which they don't conduct. 7 mV
on the
gate is never going to turn on an enhancement mode mosfet., so that
certainly
isn't your problem.


When the gate to the FET is 5V the ground 0V to my radio ( through the
drain
and source) is connected and therefore my FET driven circuit is working.
The problem in having is my radio is in a remote location and there is no
mains power supply and the FET circuit is switching the radio on and off.
At the moment this is not happening and when the radio is supposed to be
off
it isnt because the gate voltage is 0.007V above 0V. I am therefore
trying
to make this small error to 0V.

It's not the 7 mV.

What's the supply voltage for the radio that you're switching ?
What's the load current for the radio ?
What is the mosfet's drain voltage in on and off conditions ?


Graham

 

[Rich Grise]

On Wed, 06 Apr 2005 17:30:16 +0100, Tuurbo46 wrote:

Yes you are all correct and i made a silly error(school boy error). 7mV
on the gate is erelavent! I disconnected the wire from the FET to the
radio and put an amp meter in series with it and when the gate was 5V the
radio was using 150mA, which is correct. When the gate was 7mV the wire
to the radio was 0mA. The thing which confused me yesterday was that the
(on led) on the radio was dimly visable when off (7mV on gate). This was
attributed to a big capacitor in the radio. This can also be seen when
all leads are disconnected from radio and the led is still dim for a short
period( 1 min ).

The lesson i have taken from this is dont try and short cut jobs and use a
multimeter more.

Thanks for your help guys and girls.

Good Job!

And thanks for reporting back!

Now, all that's left is to learn to bottom-post. ;-)

Cheers!
Rich

 

[Pooh Bear]

Rich Grise wrote:

Now, all that's left is to learn to bottom-post. ;-)

Chuckle.


Graham