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 21:23:21 GMT, (Mike W)
wrote:

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

indeed, if you could tell me about a MOSFET without protexion diode

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On Sun, 11 Jan 2004 21:23:21 GMT, (Mike W)
wrote:

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

indeed, if you could tell me about a MOSFET without protexion diode

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In article ,
J M Noeding 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

indeed, if you could tell me about a MOSFET without protexion diode

That's why I mentioned hooking it up with the source and drain leads
in the reverse of the usual orientation.

In the normal hookup situation, the intrinsic/substrate/protection
diode would prevent the circuit from working as one would want (as you
clearly understand). However, by hooking up the MOSFET "backwards"
(e.g. an NMOSFET with the drain on the negative side), the orientation
of the intrinsic diode is reversed. It'll be forward-biased when the
power supply is hooked up the way you want it, and would be
conductive. It probably won't have a chance to conduct, though, since
you'll be biasing the gate up several volts above either the source or
drain pins, turning on the MOSFET, and the relatively low Rds[on] of
the MOSFET will keep the voltage drop across the MOSFET down below the
substrate diode's voltage drop.

If the supply connections are reversed, the MOSFET itself will be
biased off, and the intrinsic protection diode will be reverse-biased
and will not conduct.

It's an odd trick. Many (most?) JFETs have a highly symmetrical
internal construction - the distinction between source and drain is a
matter of convention rather than construction. A typical MOSFET's
construction is not entirely symmetrical... but, according to the
description I've read of this trick in Pease's book, it _is_
symmetrical enough to allow it to be used with source and drain
reversed, in this particular application. By flipping 'em around, you
make the intrinsic diode work for you rather than against you.

--
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!

In article ,
J M Noeding 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

indeed, if you could tell me about a MOSFET without protexion diode

That's why I mentioned hooking it up with the source and drain leads
in the reverse of the usual orientation.

In the normal hookup situation, the intrinsic/substrate/protection
diode would prevent the circuit from working as one would want (as you
clearly understand). However, by hooking up the MOSFET "backwards"
(e.g. an NMOSFET with the drain on the negative side), the orientation
of the intrinsic diode is reversed. It'll be forward-biased when the
power supply is hooked up the way you want it, and would be
conductive. It probably won't have a chance to conduct, though, since
you'll be biasing the gate up several volts above either the source or
drain pins, turning on the MOSFET, and the relatively low Rds[on] of
the MOSFET will keep the voltage drop across the MOSFET down below the
substrate diode's voltage drop.

If the supply connections are reversed, the MOSFET itself will be
biased off, and the intrinsic protection diode will be reverse-biased
and will not conduct.

It's an odd trick. Many (most?) JFETs have a highly symmetrical
internal construction - the distinction between source and drain is a
matter of convention rather than construction. A typical MOSFET's
construction is not entirely symmetrical... but, according to the
description I've read of this trick in Pease's book, it _is_
symmetrical enough to allow it to be used with source and drain
reversed, in this particular application. By flipping 'em around, you
make the intrinsic diode work for you rather than against you.

--
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

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

Fort hose who want to see schematics, here's a web page with both the MOS
and the relay protection circuits in various versions:

http://www.g4wif.fsnet.co.uk/q_tech12.htm

It comes from the pages of G-QRP club.


--
Sverre Holm, LA3ZA
---------------------------------
www.qsl.net/la3za

Fort hose who want to see schematics, here's a web page with both the MOS
and the relay protection circuits in various versions:

http://www.g4wif.fsnet.co.uk/q_tech12.htm

It comes from the pages of G-QRP club.


--
Sverre Holm, LA3ZA
---------------------------------
www.qsl.net/la3za

This may need re-stating. The reverse polarity protection goes IN THE
RADIO, not the power supply. Right fellas?
--
Steve N, K,9;d, c. i My email has no u's.


??? Fort Hose... ??? Cute also:

When the clock gets near 12:00 I stick my head in teh co-workers office &
say
Djeet Jet? If the response is no, then;"Tsqueet!"



"Jim Weir" wrote in message
...

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