Mosfet as a 12V Switch (and opto)

[[email protected]]

I've been stuck on this for awhile. I've got a MOSFET that is not
behaving the way I expect.

I was hoping it would act similar to the following animation where
source to drain is controlled by the gate:
http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html

But when applying 12V between the source and the drain, my load (a
light) slowly illuminates as if a capacitor is powering up. I did not
expect anything to happen since nothing was applied to the gate.

Is there any way to get it to work like an on/off switch like the link
above applied I can do?

Is there any way someone can draw or point me to schematics that
specifically describe the proper way, maybe even with an optoisolator
involved? Sorry for asking for that but I've really frustrated. I
got this to work with normal transistors, but I'm hoping the FETs will
perform better.

I'm using this kind of MOSFET:
http://www.fairchildsemi.com/ds/FQ/FQP7N10.pdf

And (eventually when I get the basic part figured out) this kind of
optocoupler:
http://www.ortodoxism.ro/datasheets/vishay/83717.pdf

I appreciate anyone's help and patience as this is my first attempt at
electronics work.

 

[John Fields]

I've been stuck on this for awhile. I've got a MOSFET that is not
behaving the way I expect.

I was hoping it would act similar to the following animation where
source to drain is controlled by the gate:
http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html

But when applying 12V between the source and the drain, my load (a
light) slowly illuminates as if a capacitor is powering up. I did not
expect anything to happen since nothing was applied to the gate.

---
When the gate is floating it can/will charge up since there's
nothing to drain the charge away and, eventually, slowly turn the
device ON.

Is there any way to get it to work like an on/off switch like the link
above applied I can do?

---
Yes. Try this: (View in Courier)


..+12V>------+-------+
.. | |
.. | O [LAMP]
.. | O |
.. | D
.. +-----G NCH
.. | S
.. [1K] |
.. | |
..GND>-------+-------+

Is there any way someone can draw or point me to schematics that
specifically describe the proper way, maybe even with an optoisolator
involved? Sorry for asking for that but I've really frustrated.

---
Don't worry about it; this is seb.

If you want to use an opto, try this:

..+12V>-----+----------+-------+
.. | | |
.. [1K2] [1K2] [LAMP]
.. | | |
.. | O | D
.. S1| +-----G FQP7N10
.. | O | S
.. | +----+ | |
.. +--|A C|--+ |
.. | | |
.. +--|K E|--+ |
.. | +----+ | |
.. | 4N35 | |
..GND>------+----------+-------+

Notice, however, that in this instance, when S1 is made the lamp
will go off.

 

[Jamie]

I've been stuck on this for awhile. I've got a MOSFET that is not
behaving the way I expect.

I was hoping it would act similar to the following animation where
source to drain is controlled by the gate:
http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html

But when applying 12V between the source and the drain, my load (a
light) slowly illuminates as if a capacitor is powering up. I did not
expect anything to happen since nothing was applied to the gate.

Is there any way to get it to work like an on/off switch like the link
above applied I can do?

Is there any way someone can draw or point me to schematics that
specifically describe the proper way, maybe even with an optoisolator
involved? Sorry for asking for that but I've really frustrated. I
got this to work with normal transistors, but I'm hoping the FETs will
perform better.

I'm using this kind of MOSFET:
http://www.fairchildsemi.com/ds/FQ/FQP7N10.pdf

And (eventually when I get the basic part figured out) this kind of
optocoupler:
http://www.ortodoxism.ro/datasheets/vishay/83717.pdf

I appreciate anyone's help and patience as this is my first attempt at
electronics work.

The Gate is very high impedance, It's acting on surrounding static
fields. You need to pull the gate down to the SOURCE side of the fet.
A resistor to lower the input impedance.. then you'll get what you
1 meg sounds like a round number.

If you're doing a sink switch (that's one that pulls the load to
common), you use a N-channel with the source on the common..
If you're trying to do a source type of switch (+ volts on) to a
load. you use P-channel on the + side output with the SOURCE on
the + rail. then you simply pull the Gate to common which will turn it
on..
Beware of the fet that is being used. Make sure you read the spec's on
what the GATE (Vgs) can handle and should have for voltage.
that's about it in a nut shell I guess.

 

[[email protected]]

The Gate is very high impedance, It's acting on surrounding static
fields. You need to pull the gate down to the SOURCE side of the fet.
A resistor to lower the input impedance.. then you'll get what you
1 meg sounds like a round number.

If you're doing a sink switch (that's one that pulls the load to
common), you use a N-channel with the source on the common..
If you're trying to do a source type of switch (+ volts on) to a
load. you use P-channel on the + side output with the SOURCE on
the + rail. then you simply pull the Gate to common which will turn it
on..
Beware of the fet that is being used. Make sure you read the spec's on
what the GATE (Vgs) can handle and should have for voltage.
that's about it in a nut shell I guess.

Thanks, I'll try your suggestions and follow up with the results asap.

 

[[email protected]]

I've been stuck on this for awhile. I've got a MOSFET that is not
behaving the way I expect.

I was hoping it would act similar to the following animation where
source to drain is controlled by the gate:
http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html

But when applying 12V between the source and the drain, my load (a
light) slowly illuminates as if a capacitor is powering up. I did not
expect anything to happen since nothing was applied to the gate.

---
When the gate is floating it can/will charge up since there's
nothing to drain the charge away and, eventually, slowly turn the
device ON.

Is there any way to get it to work like an on/off switch like the link
above applied I can do?

---
Yes. Try this: (View in Courier)

.+12V>------+-------+
. | |
. | O [LAMP]
. | O |
. | D
. +-----G NCH
. | S
. [1K] |
. | |
.GND>-------+-------+

Is there any way someone can draw or point me to schematics that
specifically describe the proper way, maybe even with an optoisolator
involved? Sorry for asking for that but I've really frustrated.

---
Don't worry about it; this is seb.

If you want to use an opto, try this:

.+12V>-----+----------+-------+
. | | |
. [1K2] [1K2] [LAMP]
. | | |
. | O | D
. S1| +-----G FQP7N10
. | O | S
. | +----+ | |
. +--|A C|--+ |
. | | |
. +--|K E|--+ |
. | +----+ | |
. | 4N35 | |
.GND>------+----------+-------+

Notice, however, that in this instance, when S1 is made the lamp
will go off.
---

I got this to work with normal transistors, but I'm hoping the FETs will
perform better.

I'm using this kind of MOSFET:
http://www.fairchildsemi.com/ds/FQ/FQP7N10.pdf

And (eventually when I get the basic part figured out) this kind of
optocoupler:
http://www.ortodoxism.ro/datasheets/vishay/83717.pdf

I appreciate anyone's help and patience as this is my first attempt at
electronics work.

Thanks JF, and Jamie. It works perfectly now... best of all, the FET
is generating virtually no heat unlike all the regular transistors I
tried which would have needed heat sinks.

 

[Jamie]

I've been stuck on this for awhile. I've got a MOSFET that is not
behaving the way I expect.

I was hoping it would act similar to the following animation where
source to drain is controlled by the gate:
http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html

But when applying 12V between the source and the drain, my load (a
light) slowly illuminates as if a capacitor is powering up. I did not
expect anything to happen since nothing was applied to the gate.

---
When the gate is floating it can/will charge up since there's
nothing to drain the charge away and, eventually, slowly turn the
device ON.

Is there any way to get it to work like an on/off switch like the link
above applied I can do?

---
Yes. Try this: (View in Courier)

.+12V>------+-------+
. | |
. | O [LAMP]
. | O |
. | D
. +-----G NCH
. | S
. [1K] |
. | |
.GND>-------+-------+

Is there any way someone can draw or point me to schematics that
specifically describe the proper way, maybe even with an optoisolator
involved? Sorry for asking for that but I've really frustrated.

---
Don't worry about it; this is seb.

If you want to use an opto, try this:

.+12V>-----+----------+-------+
. | | |
. [1K2] [1K2] [LAMP]
. | | |
. | O | D
. S1| +-----G FQP7N10
. | O | S
. | +----+ | |
. +--|A C|--+ |
. | | |
. +--|K E|--+ |
. | +----+ | |
. | 4N35 | |
.GND>------+----------+-------+

Notice, however, that in this instance, when S1 is made the lamp
will go off.
---

I got this to work with normal transistors, but I'm hoping the FETs will
perform better.

I'm using this kind of MOSFET:
http://www.fairchildsemi.com/ds/FQ/FQP7N10.pdf

And (eventually when I get the basic part figured out) this kind of
optocoupler:
http://www.ortodoxism.ro/datasheets/vishay/83717.pdf

I appreciate anyone's help and patience as this is my first attempt at
electronics work.

Thanks JF, and Jamie. It works perfectly now... best of all, the FET
is generating virtually no heat unlike all the regular transistors I
tried which would have needed heat sinks.

That's because of the turn on state resistance between the drain and
source.
You can saturate a bipolar transistor and get good results.

The key is saturation which simulates low resistance paths that
generates the lowest heat possible for that component.

 

[[email protected]]

On Sun, 07 Oct 2007 09:46:07 -0700, "[email protected]"
I've been stuck on this for awhile. I've got a MOSFET that is not
behaving the way I expect.
I was hoping it would act similar to the following animation where
source to drain is controlled by the gate:
http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html
But when applying 12V between the source and the drain, my load (a
light) slowly illuminates as if a capacitor is powering up. I did not
expect anything to happen since nothing was applied to the gate.
---
When the gate is floating it can/will charge up since there's
nothing to drain the charge away and, eventually, slowly turn the
device ON.
Is there any way to get it to work like an on/off switch like the link
above applied I can do?
---
Yes. Try this: (View in Courier)
.+12V>------+-------+
. | |
. | O [LAMP]
. | O |
. | D
. +-----G NCH
. | S
. [1K] |
. | |
.GND>-------+-------+
Is there any way someone can draw or point me to schematics that
specifically describe the proper way, maybe even with an optoisolator
involved? Sorry for asking for that but I've really frustrated.
---
Don't worry about it; this is seb.
If you want to use an opto, try this:
.+12V>-----+----------+-------+
. | | |
. [1K2] [1K2] [LAMP]
. | | |
. | O | D
. S1| +-----G FQP7N10
. | O | S
. | +----+ | |
. +--|A C|--+ |
. | | |
. +--|K E|--+ |
. | +----+ | |
. | 4N35 | |
.GND>------+----------+-------+
Notice, however, that in this instance, when S1 is made the lamp
will go off.
---
I got this to work with normal transistors, but I'm hoping the FETs will
perform better.
I'm using this kind of MOSFET:
http://www.fairchildsemi.com/ds/FQ/FQP7N10.pdf
And (eventually when I get the basic part figured out) this kind of
optocoupler:
http://www.ortodoxism.ro/datasheets/vishay/83717.pdf
I appreciate anyone's help and patience as this is my first attempt at
electronics work.

Thanks JF, and Jamie. It works perfectly now... best of all, the FET
is generating virtually no heat unlike all the regular transistors I
tried which would have needed heat sinks.

That's because of the turn on state resistance between the drain and
source.
You can saturate a bipolar transistor and get good results.

The key is saturation which simulates low resistance paths that
generates the lowest heat possible for that component.

That's good to know in case I want to make this device AC capable.
For now I've set my mind on FETs. Hopefully they'll have some
advantage down the road. Thanks again.

 

[John Fields]

I've been stuck on this for awhile. I've got a MOSFET that is not
behaving the way I expect.

I was hoping it would act similar to the following animation where
source to drain is controlled by the gate:
http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html

But when applying 12V between the source and the drain, my load (a
light) slowly illuminates as if a capacitor is powering up. I did not
expect anything to happen since nothing was applied to the gate.

---
When the gate is floating it can/will charge up since there's
nothing to drain the charge away and, eventually, slowly turn the
device ON.

Is there any way to get it to work like an on/off switch like the link
above applied I can do?

---
Yes. Try this: (View in Courier)

.+12V>------+-------+
. | |
. | O [LAMP]
. | O |
. | D
. +-----G NCH
. | S
. [1K] |
. | |
.GND>-------+-------+

Is there any way someone can draw or point me to schematics that
specifically describe the proper way, maybe even with an optoisolator
involved? Sorry for asking for that but I've really frustrated.

---
Don't worry about it; this is seb.

If you want to use an opto, try this:

.+12V>-----+----------+-------+
. | | |
. [1K2] [1K2] [LAMP]
. | | |
. | O | D
. S1| +-----G FQP7N10
. | O | S
. | +----+ | |
. +--|A C|--+ |
. | | |
. +--|K E|--+ |
. | +----+ | |
. | 4N35 | |
.GND>------+----------+-------+

Notice, however, that in this instance, when S1 is made the lamp
will go off.
---

I got this to work with normal transistors, but I'm hoping the FETs will
perform better.

I'm using this kind of MOSFET:
http://www.fairchildsemi.com/ds/FQ/FQP7N10.pdf

And (eventually when I get the basic part figured out) this kind of
optocoupler:
http://www.ortodoxism.ro/datasheets/vishay/83717.pdf

I appreciate anyone's help and patience as this is my first attempt at
electronics work.

Thanks JF, and Jamie. It works perfectly now... best of all, the FET
is generating virtually no heat unlike all the regular transistors I
tried which would have needed heat sinks.

 

[John Fields]

That's because of the turn on state resistance between the drain and
source.
You can saturate a bipolar transistor and get good results.

---
But not as good as you can with a MOSFET.

Bipolars can't generally get to Vce(sat) below 0.3V, while MOSFETS
can get to milliohms so, local dissipation for a given current load
will generally be less for a MOSFET.