HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Apr 27, 2011, 03:14 PM
Registered User
Joined Apr 2007
146 Posts
Discussion
Mosfet P-channel Linear Region control

Does anyone have a circuit for controlling Mosfets in the Linear Reagion.

I am locked into using a p-channel mosfet and cant use PWM.

I need to be able to controller the Mosfet with a pot and i have a heat since that will control the heat.

Thanks for all the help.

Mike
mazur50 is offline Find More Posts by mazur50
Reply With Quote
Sign up now
to remove ads between posts
Old Apr 27, 2011, 06:41 PM
ltc
AMA 97737
ltc's Avatar
United States, MA, Mendon
Joined Mar 2010
2,388 Posts
Voltage or Current control?
Simple single op amp would work, would need additional details (input voltage, frequency response, voltage rail, etc.)

BTW, why are you locked into a P channel?
ltc is offline Find More Posts by ltc
Reply With Quote
Old Apr 27, 2011, 06:58 PM
Registered User
Ohio
Joined Nov 2003
1,655 Posts
just invert this for a P-fet
jglenn is offline Find More Posts by jglenn
Reply With Quote
Old Apr 27, 2011, 07:00 PM
Registered User
Ohio
Joined Nov 2003
1,655 Posts
Or try a linear amp!
jglenn is offline Find More Posts by jglenn
Reply With Quote
Old Apr 27, 2011, 07:05 PM
Registered User
Ohio
Joined Nov 2003
1,655 Posts
P-ch the arrow points out. Just modulate the gate voltage. They are just like

grids in tubes!
jglenn is offline Find More Posts by jglenn
Reply With Quote
Old Apr 28, 2011, 09:27 AM
Registered User
Joined Apr 2007
146 Posts
I am looking for voltage control. at the moment.

I have to use a P-channel becasue that is the side i have access to.
mazur50 is offline Find More Posts by mazur50
Reply With Quote
Old Apr 28, 2011, 09:32 AM
Registered User
Joined Apr 2007
146 Posts
Quote:
Originally Posted by jglenn View Post
just invert this for a P-fet
How do you determin R1 and R3
mazur50 is offline Find More Posts by mazur50
Reply With Quote
Old Apr 28, 2011, 10:44 AM
"Simplify, then add lightness"
Raleigh,NC
Joined Nov 2000
2,701 Posts
Quote:
I am looking for voltage control. at the moment.

I have to use a P-channel becasue that is the side i have access to.
Why a mosfet instead of a BJT? How close to the high side rail does the output need to come?

If you want to control the load voltage and can stand the output not coming completely to the high side voltage then you would be much better off using a NPN in common collector or N-Channel mosfet in common drain configuration. That way the output voltage is equal to the input voltage minus the junction or threshold voltage.

Using a P-Channel in common source will actually control the current through the load and you would need feedback(ie an op amp) to directly control the voltage of the load.

PS Added the quote because that is all the info we have. The quality of the output depends on the quality of the input.
jeffs555 is offline Find More Posts by jeffs555
Last edited by jeffs555; Apr 28, 2011 at 10:53 AM.
Reply With Quote
Old Apr 28, 2011, 10:51 AM
Registered User
Joined Apr 2007
146 Posts
I need the output to be as close to the rail as possiable, that is why i dont want to use BJT. I am using a BJT now.

Can you give me a little more insight to your second line.

Mike
mazur50 is offline Find More Posts by mazur50
Reply With Quote
Old Apr 28, 2011, 05:57 PM
Registered User
Ohio
Joined Nov 2003
1,655 Posts
How do you determin R1 and R3?

The fet gate voltage when running linear is sort of like the grid of a tube, a proportional
change affects conduction. The spec sheet will have the range, set up the pot divider
to cover that. There may be differences from device to device. And non-linearities.
jglenn is offline Find More Posts by jglenn
Reply With Quote
Old Apr 28, 2011, 06:08 PM
ltc
AMA 97737
ltc's Avatar
United States, MA, Mendon
Joined Mar 2010
2,388 Posts
A rail to rail op amp, a FET and a few resistors. Linear, easy to scale and limit.
Simple
ltc is offline Find More Posts by ltc
Reply With Quote
Old Apr 28, 2011, 07:00 PM
Registered User
Ohio
Joined Nov 2003
1,655 Posts
This circuit works well, driven by an opamp.

There are also power opamps like the EL2008 that can drive 1A at RF freq.
jglenn is offline Find More Posts by jglenn
Reply With Quote
Old Apr 28, 2011, 07:12 PM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,182 Posts
When a MOSFET is operating in its 'linear' region, Gate voltage controls Drain current. The action is similar to how Base current controls Collector current in a bipolar transistor, but at low currents the Gate contol is close to 'pinch-off' and highly non-linear. It is also quite temperature dependent.

A pot is OK if you don't mind constantly adjusting it, but if you want a stable output voltage then you will need an amplifier and feedback loop.
Bruce Abbott is offline Find More Posts by Bruce Abbott
Reply With Quote
Old Apr 28, 2011, 08:43 PM
Registered User
Ohio
Joined Nov 2003
1,655 Posts
Thanks for the curves. Some find that in digital switching, the rise and fall

are actually linear. Slow ramp or slew causes heating. Too fast causes spikes.
jglenn is offline Find More Posts by jglenn
Reply With Quote
Old Apr 28, 2011, 11:14 PM
Registered User
Joined Apr 2007
146 Posts
I was looking at using the irf4905 because I have a few. How would I start.
mazur50 is offline Find More Posts by mazur50
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
For Sale Cox P-40 Control Line badbill Aircraft - Fuel - Airplanes (FS/W) 2 Mar 16, 2011 12:12 PM
Discussion control line flying in the Ottawa, Ontario, Canada region GrahamC Control Line 0 Apr 24, 2007 09:41 AM
Looking for P-Channel High-Side MOSFET Mr.RC-CAM DIY Electronics 20 Dec 13, 2004 04:39 PM
Question How To Control The Six Mosfet In Bldc CLEMENT DIY Electronics 1 Nov 07, 2004 05:14 PM
Speed Control Not Linear BQuick Electric Heli Talk 2 Jun 06, 2002 09:21 AM