(OT) Polarity protection with MOSFET
Archiver
Posts: 46,084
Could someone point to me the circuit made with a single MOSFET that
provides polarity inversion protection without the typical diode voltage
dropout ?
Thanks
ACJacques
provides polarity inversion protection without the typical diode voltage
dropout ?
Thanks
ACJacques
Comments
Put the drain to the - battery terminal. The source connects to the load's
ground. The gate connects to the + battery terminal along with the power
input of the load.
The drop will be the FET's on resistance x the current. The on resistance is
often quite small (.1 ohm or less). This assumes that Vgs is less than the
power supply voltage (much less) so that the FET will turn on hard. If the
polarity is reversed, the FET does not turn on, and the device is isolated
by the FET's off resistance.
Regards,
Al Williams
AWC
*Floating point math for the Stamp, PIC, SX, or any microcontroller:
http://www.al-williams.com/awce/pak1.htm
>
Original Message
> From: A.C. Jacques [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=vm2FtWL8ZDYTg2-begQ-XPk4igK_bCf2wI6IQTWqfMTWpqdxcx81COgZtkkvF24vFB9QpLK-FXXIbVuwup5ieQ]acjacques@i...[/url
> Sent: Wednesday, August 23, 2000 4:03 PM
> To: basicstamps@egroups.com
> Subject: [noparse][[/noparse]basicstamps] (OT) Polarity protection with MOSFET
>
>
> Could someone point to me the circuit made with a single MOSFET that
> provides polarity inversion protection without the typical diode voltage
> dropout ?
>
> Thanks
> ACJacques
>
>
>
>provides polarity inversion protection without the typical diode voltage
>dropout ?
>
>Thanks
>ACJacques
>
Bob Pease does the following:
He puts an enhancement-type MOSFET in series with the load, and a 1Meg (not
critical) resistor from the (+) power terminal to the MOSFET gate. When you
apply power with the correct polarity, the MOSFET conducts, and you're in
business. With the polarity reversed, the MOSFET doesn't conduct, and
you're protected. With the low R-on of many MOSFETS, you drop very little
voltage across the MOSFET. Pease shows the approach in his book
"Troubleshooting Analog Circuits".
(+)-->>
o
>>
\
| |
| |
Power / |
In \ |
/ 1 Meg [noparse][[/noparse]Load]
\ |
| |
| |
(-)--<<
[noparse][[/noparse]S G D]
<<
/
MOSFET
(eg. IRF540)
Might be worth a try.
Steve
Steve Roberts: sroberts@s...
MOSFETs have an unavoidable internal diode pointing from source toward
drain. This diode would conduct for reverse polarity, so you have the load
in series with a diode when polarity is reversed, unless I am missing
something.
Ray McArthur
Original Message
From: Stephen Roberts <sroberts@s...>
To: <basicstamps@egroups.com>
Sent: Monday, August 28, 2000 2:16 PM
Subject: [noparse][[/noparse]basicstamps] Re: (OT) Polarity protection with MOSFET
> >Could someone point to me the circuit made with a single MOSFET that
> >provides polarity inversion protection without the typical diode voltage
> >dropout ?
> >
> >Thanks
> >ACJacques
> >
>
> Bob Pease does the following:
>
> He puts an enhancement-type MOSFET in series with the load, and a 1Meg
(not
> critical) resistor from the (+) power terminal to the MOSFET gate. When
you
> apply power with the correct polarity, the MOSFET conducts, and you're in
> business. With the polarity reversed, the MOSFET doesn't conduct, and
> you're protected. With the low R-on of many MOSFETS, you drop very little
> voltage across the MOSFET. Pease shows the approach in his book
> "Troubleshooting Analog Circuits".
>
>
> (+)-->>
o
>>
\
> | |
> | |
> Power / |
> In \ |
> / 1 Meg [noparse][[/noparse]Load]
> \ |
> | |
> | |
> (-)--<<
[noparse][[/noparse]S G D]
<<
/
> MOSFET
> (eg. IRF540)
>
> Might be worth a try.
>
> Steve
>
>
> Steve Roberts: sroberts@s...
>
>
>
>
Battery + and GATE and SOURCE connected together and
DRAIN will go to + LOAD.
there is NO GND connection in the MOSFET.
Just one component.
This is REVERSE from Al Williams pointed before.(conduct in BOTH
polarities)
ACJacques
rjmca wrote:
>
> Steve:
> MOSFETs have an unavoidable internal diode pointing from source toward
> drain. This diode would conduct for reverse polarity, so you have the load
> in series with a diode when polarity is reversed, unless I am missing
> something.
>
> Ray McArthur
>
>
Original Message
> From: Stephen Roberts <sroberts@s...>
> To: <basicstamps@egroups.com>
> Sent: Monday, August 28, 2000 2:16 PM
> Subject: [noparse][[/noparse]basicstamps] Re: (OT) Polarity protection with MOSFET
>
> > >Could someone point to me the circuit made with a single MOSFET that
> > >provides polarity inversion protection without the typical diode voltage
> > >dropout ?
> > >
> > >Thanks
> > >ACJacques
> > >
> >
> > Bob Pease does the following:
> >
> > He puts an enhancement-type MOSFET in series with the load, and a 1Meg
> (not
> > critical) resistor from the (+) power terminal to the MOSFET gate. When
> you
> > apply power with the correct polarity, the MOSFET conducts, and you're in
> > business. With the polarity reversed, the MOSFET doesn't conduct, and
> > you're protected. With the low R-on of many MOSFETS, you drop very little
> > voltage across the MOSFET. Pease shows the approach in his book
> > "Troubleshooting Analog Circuits".
> >
> >
> > (+)-->>
o
>>
\
> > | |
> > | |
> > Power / |
> > In \ |
> > / 1 Meg [noparse][[/noparse]Load]
> > \ |
> > | |
> > | |
> > (-)--<<
[noparse][[/noparse]S G D]
<<
/
> > MOSFET
> > (eg. IRF540)
> >
> > Might be worth a try.
> >
> > Steve
> >
> >
> > Steve Roberts: sroberts@s...
> >
> >
> >
> >