"Peabody" <waybackNO584SPAM44@yahoo.com> wrote in message
news:20170215-175018.222.0@Peabody.ssl.astraweb.com...
> I don't know if it's a problem. But among other things, I have a
> microcontroller that's used to +3.3V, and I don't know how much -9V
> current
> it takes to do harm. I just don't know.
I would be shocked if any logic device is unable to survive at least as much
reverse current as the ESD protection diodes (i.e., the input current limit
on a CMOS input logic pin), if not the Vdd/Vss pin current limit (which is
carried by transistors not diodes, but the diodes are at least as big as the
transistors, see--). So, 10 to 100mA, not ~uA.
I can't conceive of a reason how or why a CMOS IC should fail from that.
It's just substrate forward bias!
And, for that matter, I would be very concerned if I discovered a chip that
does die so easily! Assuming I have any choice in its selection, of
course...
In any case, you can prevent such exposure by shunting Vss to Vdd with
another small schottky, which will keep it well below a PN junction Vf.
Tim
--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com
Reply by George Herold●February 15, 20172017-02-15
On Wednesday, February 15, 2017 at 2:52:41 PM UTC-5, mako...@yahoo.com wrote:
> On Wednesday, February 15, 2017 at 2:30:23 PM UTC-5, George Herold wrote:
> > On Wednesday, February 15, 2017 at 12:30:48 PM UTC-5, mako...@yahoo.com wrote:
> > > one more question....
> > >
> > > for the reverse polarity protection function, why is 100uA leakage a problem?
> > >
> > > Reverse polarity protection is just to keep the magic smoke in if someone momentary connects the battery backwards... ...no?
> > >
> > >
> > > 100uA is not going to blow anything...
> > > Mark
> >
> > Do Schottky diodes leak that much? That seems like a lot.
> > (un-measurable on my DMM uA scale... leakage less than 50 nA.)
> > (Well that's a small diode.. 1N5711)
> >
> > George h.
>
> it's not diode leakage I'm taking about
>
> the 100 ua leakage is from the PNP circuit when the battery is connected backwards.
>
> The OP is concerned this 100 uA leakage will damage his load.
>
> A SHUNT diode will forward conduct the 100uA leakage to ground and almost no reverse voltage will appear across the load.
>
> m
Oh sorry my mistake.
GH.
Reply by ●February 15, 20172017-02-15
On Wednesday, February 15, 2017 at 2:30:23 PM UTC-5, George Herold wrote:
> On Wednesday, February 15, 2017 at 12:30:48 PM UTC-5, mako...@yahoo.com wrote:
> > one more question....
> >
> > for the reverse polarity protection function, why is 100uA leakage a problem?
> >
> > Reverse polarity protection is just to keep the magic smoke in if someone momentary connects the battery backwards... ...no?
> >
> >
> > 100uA is not going to blow anything...
> > Mark
>
> Do Schottky diodes leak that much? That seems like a lot.
> (un-measurable on my DMM uA scale... leakage less than 50 nA.)
> (Well that's a small diode.. 1N5711)
>
> George h.
it's not diode leakage I'm taking about
the 100 ua leakage is from the PNP circuit when the battery is connected backwards.
The OP is concerned this 100 uA leakage will damage his load.
A SHUNT diode will forward conduct the 100uA leakage to ground and almost no reverse voltage will appear across the load.
m
Reply by George Herold●February 15, 20172017-02-15
On Wednesday, February 15, 2017 at 12:30:48 PM UTC-5, mako...@yahoo.com wrote:
> one more question....
>
> for the reverse polarity protection function, why is 100uA leakage a problem?
>
> Reverse polarity protection is just to keep the magic smoke in if someone momentary connects the battery backwards... ...no?
>
>
> 100uA is not going to blow anything...
> Mark
Do Schottky diodes leak that much? That seems like a lot.
(un-measurable on my DMM uA scale... leakage less than 50 nA.)
(Well that's a small diode.. 1N5711)
George h.
Reply by John Larkin●February 15, 20172017-02-15
On Wed, 15 Feb 2017 00:21:09 -0600, Peabody
<waybackNO584SPAM44@yahoo.com> wrote:
>John Larkin says...
>
> > Why not use a series schottky diode, instead of
> > complicated circuits?
>
>I just wanted to avoid even that voltage drop. Any drop
>just means the battery has to be replaced sooner.
Batteries usually discharge at nearly constant voltage, and drop off
rapidly at end of life. So a series diode won't affect lifetime much.
You might give up 300 mv to a schottky diode.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
Reply by ●February 15, 20172017-02-15
On Wednesday, February 15, 2017 at 12:50:23 PM UTC-5, Peabody wrote:
> makolber@yahoo.com says...
> >one more question....
> >
> >for the reverse polarity protection function, why is 100uA leakage a
> >problem?
> >
> >Reverse polarity protection is just to keep the magic smoke in if someone
> >momentary connects the battery backwards... ...no?
> >
> >
> >100uA is not going to blow anything...
> >Mark
>
> I don't know if it's a problem. But among other things, I have a
> microcontroller that's used to +3.3V, and I don't know how much -9V current
> it takes to do harm. I just don't know.
put a reverse diode on the inside of the protection to short any small leakage
so if the battery is connected backwards, 100uA will flow through the diode to ground
Mark
Reply by Peabody●February 15, 20172017-02-15
makolber@yahoo.com says...
>one more question....
>
>for the reverse polarity protection function, why is 100uA leakage a
>problem?
>
>Reverse polarity protection is just to keep the magic smoke in if someone
>momentary connects the battery backwards... ...no?
>
>
>100uA is not going to blow anything...
>Mark
I don't know if it's a problem. But among other things, I have a
microcontroller that's used to +3.3V, and I don't know how much -9V current
it takes to do harm. I just don't know.
Reply by ●February 15, 20172017-02-15
one more question....
for the reverse polarity protection function, why is 100uA leakage a problem?
Reverse polarity protection is just to keep the magic smoke in if someone momentary connects the battery backwards... ...no?
100uA is not going to blow anything...
Mark
Reply by Peabody●February 15, 20172017-02-15
Jasen Betts says...
> if preserving the battery is important, why are you
> using "9V", which is one of the most expensive forms? or
> is that 6 AA cells in series?
Part of the circuit needs unregulated 9-ish volts, and the
9V battery is just easier to deal with for size and weight
than multiple AAs.
Reply by Jasen Betts●February 15, 20172017-02-15
On 2017-02-15, Peabody <waybackNO584SPAM44@yahoo.com> wrote:
> John Larkin says...
>
> > Why not use a series schottky diode, instead of
> > complicated circuits?
>
> I just wanted to avoid even that voltage drop. Any drop
> just means the battery has to be replaced sooner. Also, a
> P-channel mosfet requires no additional parts, has
> essentially no voltage drop, and in case it even matters,
> only costs a little more that a Schottky diode.
if preserving the battery is important, why are you using "9V",
which is one of the most expensive forms? or is that 6 AA cells
in series?
--
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