What type diode for battery reversal protection?

[royalmp2001]

Do I need a signal diode (like IN4148) or a rectifier diode (like
IN4001) to add to the +ve line to protect against polarity reversal?
Would the available supply after the diode be 0.6V less? Is there any
way of avoiding this?
Thanks, everyone

 

[Rich Grise]

On Sat, 26 Feb 2005 10:01:53 -0800, royalmp2001 wrote:

Do I need a signal diode (like IN4148) or a rectifier diode (like
IN4001) to add to the +ve line to protect against polarity reversal?
Would the available supply after the diode be 0.6V less? Is there any
way of avoiding this?

Yes - never discharge your pack to the point that any cell reaches
zero volts. By the time the reverse voltage across the cell reaches
a diode drop, the cell has already been destroyed.

In other words, don't do it.

Sorry,
Rich

 

[royalmp2001]

9V Battery supply and about 7mA current

 

[royalmp2001]

Er...Rich...its a rectangular 9V Battery actually

 

[John Popelish]

royalmp2001 wrote:

9V Battery supply and about 7mA current

I would probably use a 1N5817 schottky in series. At this current,
the drop is only about 0.2 volts.
--
John Popelish

 

[John Popelish]

royalmp2001 wrote:

Do I need a signal diode (like IN4148) or a rectifier diode (like
IN4001) to add to the +ve line to protect against polarity reversal?
Would the available supply after the diode be 0.6V less? Is there any
way of avoiding this?
Thanks, everyone

At rated current, it may be more like 1 volt less. If you use a
schottky diode it will be about half as much loss. If you use a
mosfet hooked up backwards (turned on channel in parallel with the
body diode when correctly connected), it can be considerably less than
that, but you may have to complicate the circuit a bit to provide
spike protection for the gate. What supply voltage and load current
are we talking about?
--
John Popelish

 

[Robert Monsen]

John Popelish wrote:
[snip...]

If you use a
mosfet hooked up backwards (turned on channel in parallel with the
body diode when correctly connected), it can be considerably less than
that, but you may have to complicate the circuit a bit to provide
spike protection for the gate.
[snip]

How does one go about protecting the gate? I heard about this trick in
passing from one of those "Bob Pease" online lectures, and was
intrigued, but he just said 'fet', and didn't go into detail, so I never
too the time to figure out what it was about.

It seems like a useful technique. I guess just ensuring that the gate
doesn't go outside it's max rating limits is enough.

It also seems like somebody would offer one of these as a discrete
protection device, with built in limiting for the gate, and datasheet
characterization for max voltages, Ron, currents, etc. Or, do the normal
specs work for this backward connection?

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[Don Bruder]

In article <pan.2005.02.26.21.07.53.554632@example.net>,
Rich Grise <richgrise@example.net> wrote:

On Sat, 26 Feb 2005 10:01:53 -0800, royalmp2001 wrote:

Do I need a signal diode (like IN4148) or a rectifier diode (like
IN4001) to add to the +ve line to protect against polarity reversal?
Would the available supply after the diode be 0.6V less? Is there any
way of avoiding this?

Yes - never discharge your pack to the point that any cell reaches
zero volts. By the time the reverse voltage across the cell reaches
a diode drop, the cell has already been destroyed.

In other words, don't do it.

Sorry,
Rich

Sorry, Rich, but I don't think your answer is very useful...

Unless I'm misunderstanding him horribly, he's looking for info on what
to use as what I've always heard called "steering diodes", so that no
matter which way a battery/battery pack gets hooked to the circuit, the
polarity is correct, not info on keeping a dead cell from reversing a
pack.

Royal:
What I've always done (in non-weight-critical applications) is "over
design" the power supply (Add an extra cell, ferinstance) appropriately
to compensate for the diode drop that will be involved, and then plug
the power (Either AC or DC...) into a bridge rectifier's inputs. Put DC
on the inputs of a bridge, and it's going to put out DC of the right
polarity on the output pins regardless of which way the supply voltage
is hooked up. Free bonus: If your circuit doesn't have a problem
handling AC ripple, you're also set to just hook up the correct voltage
AC (from an ultra-cheap wall-wart, for instance) to the same pins you'd
hook the battery to. If it does have a problem with ripple, a couple of
caps and coils "downstream" from the power input can easily clean things
up to tolerable when used with AC, yet sit there quietly doing nothing
if the device is being fed DC.

--
Don Bruder - dakidd@sonic.net - New Email policy in effect as of Feb. 21, 2004.
Short form: I'm trashing EVERY E-mail that doesn't contain a password in the
subject unless it comes from a "whitelisted" (pre-approved by me) address.
See <http://www.sonic.net/~dakidd/main/contact.html> for full details.

 

[Rich Grise]

On Sun, 27 Feb 2005 03:25:52 +0000, Don Bruder wrote:

Rich Grise <richgrise@example.net> wrote:

On Sat, 26 Feb 2005 10:01:53 -0800, royalmp2001 wrote:

Do I need a signal diode (like IN4148) or a rectifier diode (like
IN4001) to add to the +ve line to protect against polarity reversal?
Would the available supply after the diode be 0.6V less? Is there any
way of avoiding this?

[wrong answer]

Sorry, Rich, but I don't think your answer is very useful...

Unless I'm misunderstanding him horribly, he's looking for info on what
to use as what I've always heard called "steering diodes", so that no
matter which way a battery/battery pack gets hooked to the circuit, the
polarity is correct, not info on keeping a dead cell from reversing a
pack.

OOpps! Yes, I've reread the original post again, and you're absolutely
right. Please disregard what I said.

Thanks,
Rich

 

[John Popelish]

Robert Monsen wrote:

John Popelish wrote:
[snip...]
If you use a
mosfet hooked up backwards (turned on channel in parallel with the
body diode when correctly connected), it can be considerably less than
that, but you may have to complicate the circuit a bit to provide
spike protection for the gate.
[snip]

How does one go about protecting the gate? I heard about this trick in
passing from one of those "Bob Pease" online lectures, and was
intrigued, but he just said 'fet', and didn't go into detail, so I never
too the time to figure out what it was about.

It seems like a useful technique. I guess just ensuring that the gate
doesn't go outside it's max rating limits is enough.

It also seems like somebody would offer one of these as a discrete
protection device, with built in limiting for the gate, and datasheet
characterization for max voltages, Ron, currents, etc. Or, do the normal
specs work for this backward connection?

I just had to design one of these for a device that runs on 4 AA
cells. I added 100k in series with the gate to the normally positive
battery clip, drain normally negative clip and source to circuit
negative rail and a 6V8 zener, anode to source, cathode to to gate.
Since people may be touching the cells as they touch the battery clip,
I had to allow for body capacitance discharge into the clips without
zapping the anti reverse fet switch. Put the cells in backward and
you have only 100k in series with a forward biased zener across the
battery.

--
John Popelish