This sounds like a good solution

Thanks
Richard

Dave Platt wrote:
In article ,
Dr. A.T. Squeegee wrote:


I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Power Schottky diodes are available that have a forward voltage
drop of 0.3 or less. More expensive, yes, but they really do work.


Another trick I've seen is to use a hefty power MOSFET (one with a low
Rds[on]), with the source and drain leads reversed, in series with the
power. Depending on the voltages involved, you can either connect the
gate directly to the opposite side of the power supply, or (if the
supply voltage may exceed the Vgs limit) to a high-Z resistive divider.

When a power supply of the correct polarity is hooked up, the MOSFET's
intrinsic substrate diode is biased forward (conducting normally) and
the MOSFET is turned on "hard" (giving a low Rds, and thus a very low
forward voltage drop). If the supply is hooked up backwards, the
MOSFET is turned off (the gate is pulled down towards hard depletion)
and the substrate diode is reverse-biased, switching it off.

On Sun, 11 Jan 2004 21:17:43 -0000, (Dave Platt)
wrote:

Another trick I've seen is to use a hefty power MOSFET (one with a low
Rds[on]), with the source and drain leads reversed, in series with the
power. Depending on the voltages involved, you can either connect the
gate directly to the opposite side of the power supply, or (if the
supply voltage may exceed the Vgs limit) to a high-Z resistive divider.

Nice one Dave

This sounds like a good solution

Thanks
Richard

Dave Platt wrote:
In article ,
Dr. A.T. Squeegee wrote:


I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Power Schottky diodes are available that have a forward voltage
drop of 0.3 or less. More expensive, yes, but they really do work.


Another trick I've seen is to use a hefty power MOSFET (one with a low
Rds[on]), with the source and drain leads reversed, in series with the
power. Depending on the voltages involved, you can either connect the
gate directly to the opposite side of the power supply, or (if the
supply voltage may exceed the Vgs limit) to a high-Z resistive divider.

When a power supply of the correct polarity is hooked up, the MOSFET's
intrinsic substrate diode is biased forward (conducting normally) and
the MOSFET is turned on "hard" (giving a low Rds, and thus a very low
forward voltage drop). If the supply is hooked up backwards, the
MOSFET is turned off (the gate is pulled down towards hard depletion)
and the substrate diode is reverse-biased, switching it off.

In article ,
Dr. A.T. Squeegee wrote:

I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Power Schottky diodes are available that have a forward voltage
drop of 0.3 or less. More expensive, yes, but they really do work.

Another trick I've seen is to use a hefty power MOSFET (one with a low
Rds[on]), with the source and drain leads reversed, in series with the
power. Depending on the voltages involved, you can either connect the
gate directly to the opposite side of the power supply, or (if the
supply voltage may exceed the Vgs limit) to a high-Z resistive divider.

When a power supply of the correct polarity is hooked up, the MOSFET's
intrinsic substrate diode is biased forward (conducting normally) and
the MOSFET is turned on "hard" (giving a low Rds, and thus a very low
forward voltage drop). If the supply is hooked up backwards, the
MOSFET is turned off (the gate is pulled down towards hard depletion)
and the substrate diode is reverse-biased, switching it off.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Doing a bit of reading - this device might do the trick

http://www.linear.com/pdf/4411f.pdf

Richard


Richard Hosking wrote:
Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Richard

On Sun, 11 Jan 2004 19:44:19 +0800, Richard Hosking
wrote:

Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Richard

Rick,
one way for reverse protection is to include a bridge recifier in the
rigs power line ( internally fitted ), but this causes a drop in
available volts.

I prefer a diode into a relay coil, with the relay contacts enabling
the power when the relay coil is correctly polarised and powered (
also internally fitted ).

This enables full volts to the rig and also gives reverse protection,
however this is at the cost of further ( and some may say excessive )
drain of the power source.
Over voltage should be by fuse and a crowbar circuit. Do a search on
crowbar for details.

The diode + relay circuit should be obvious to you, I use a 9v relay
coil with a 1N4001 in the activation side and a DPDT contact,in
parallel, to switch the power. For the power you are talking about a
14pin DIL relay should be adequate.

HTH, Mike W, G8NXD qthr

I'm not understanding something here. You want to design a power supply that
protects against reverse voltage and overvoltage. OK. Is this power supply a
battery or are you actually building a power supply that runs from the wall
outlet?

The requirements are quite different, depending on what you are trying to
achieve.

Jim



Richard Hosking
shared these priceless pearls of wisdom:

-Dear all
-I want to design a power supply for a low power rig with protection for
-reverse and overvoltage.


Jim Weir, VP Eng. RST Eng. WX6RST
A&P, CFI, and other good alphabet soup

In article ,
says...

Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Power Schottky diodes are available that have a forward voltage
drop of 0.3 or less. More expensive, yes, but they really do work.


--
Dr. Anton Squeegee, Director, Dutch Surrealist Plumbing Institute
(Known to some as Bruce Lane, KC7GR)
kyrrin a/t bluefeathertech d-o=t c&o&m
Motorola Radio Programming & Service Available -
http://www.bluefeathertech.com/rf.html
"Quando Omni Flunkus Moritati" (Red Green)

Doing a bit of reading - this device might do the trick

http://www.linear.com/pdf/4411f.pdf

Richard


Richard Hosking wrote:
Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Richard

On Sun, 11 Jan 2004 19:44:19 +0800, Richard Hosking wrote:

Dear all
I want to design a power supply for a low power rig with protection for
reverse and overvoltage. The requirement would be about 1A at 12-14V.
What is the best way of achieving this? I guess a diode would give
reverse voltage protection but the 0.6V drop is a problem.

Richard

Hi Richard.

If you do decide to use a switching supply, the diode
or diodes in the output of the switching circuit may
me be all the reverse voltage protection you need.
Make sure that the output smoothing capacitor can
handle any voltage it is likely to encounter.

A crowbar and fuse will protect against overvoltage.
A hefty Thyristor (SCR or whatever they call them now)
or triac makes a good crowbar.

73, Ed. EI9GQ.

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