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'[EE]: Protecting Against Hooking Up a Battery in R'
2002\10\05@162245 by Dale Botkin

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From what I have seen, typically it's not done at all, or at most there's
a reverse biased diode across +V and ground to blow the power suply fuse
if it's connected bass-ackwards.  At least that's how most consumer type
stuff is, and a disappointingly large amount of non-consumer stuff as well
(like ham radio gear).

I like a bridge rectifier (so AC or either polarity DC will work fine),
followed by a regulator if needed.  I s'pose one could use a Zener and
MOSFET or something like that to cut off the input power if it's too high,
but a switching regulator that will accept higher input voltages seems
like a better idea.  Of course I'm assuming this is the input to a power
supply.

Dale
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On Sat, 5 Oct 2002, Donovan Parks wrote:

> How is this typically done?  How would you do it?

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2002\10\05@164859 by Russell McMahon

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An elegant, effective  and not especially expensive method is to use a
single MOSFET.
Having written this I see that while the circuit is as simple as they come
(one FET in many cases) this is quite a good tutorial to some less usual
aspects of FET use - I rcommend that beginners interested in some finer
points of low speed FET usage read and understand the following.

If an N channel device is used connect Drain to battery negative, Source to
load ground and Gate to battery positive. The FET must have a Vth (turn on
voltage) comfortable less than the battery voltage and a Vgsmax (max gate
turn on voltage) less than the battery voltage.

The reverse from usual polarity of Drain & Source connection with "normal"
Gate to Source polarity is due to the FET body diode which now must be in
the "always on" direction. This works for a MOSFET but does NOT work for a
bipolar transistor in the same arrangement as the FET is a "two quadrant
switch" which will turn on for either polarity of Drain to Source voltage as
long as the Gate is forward biased relative to the Source. (For an N
Cchannel FET this is with Gate positive relative to Source).

When the battery is connected correctly the body diode is on BUT the FET is
also on and over-rides the body diode to give a low forward on resistance.
An approrpriately chosen MOSFET can give a very low Rdson (on resistance)
and a ver low voltage drop - MUCH better than if a diode was used. When the
battery is reversed both body diode and FET are turned off. With low voltage
battery systems this arrangement can give a vastly extended battery life
compared to a series diode. Note that the FET Vthreshold MUST be low enough
to allow thr FET to be well turned on. Most FETs need around 8 volts plus,
"logic" FETs need 5 volts or less and there are some logic FETs needing only
1 to 2 volts. Choose appropriately. Be aware that FETs with low Vth usually
also have reduced Vgsmax. You MAY need a resistor and zener to protect the
gate depending on FET specs. The gate current required is zero (apart from a
tiny amount of current needed to charge the gate capacitance initially) and
the gate resistor needs be sized only to carry zener current. (Zener voltage
will be lower than nominal for large resistor values and will be thermally
less stable than normal - not a problem here). Be aware that if a large
valued gate resistor is used the stored charge MAY hold the FET on for some
while after correct polarity is removed.

If you want the FET in the positive lead for some reason, use a P Channel
MOSFET with Drain to battery positive, Source to load positive and gate to
ground.



           Russell McMahon



{Original Message removed}

2002\10\06@171511 by Roman Black

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Donovan Parks wrote:
>
> Hello,
>
> I'd like to build a circuit that will ensure the input voltage is between 0V and 30V.  What I was planning to do was use a fuse in series with a 30V zener diode:
>
>     +V ----fuse------- Vout
>                       |
>                 zener (30V)
>                       |
>                     gnd
>
> The idea was that if +V was greater than 30V (say 40V) than the excess voltage (10V) would be dropped over the internal resistance of the power supply.  If the power supply is a battery than this would result in a very large current (since the internal resistance is very small) which would blow the fuse.  Similarly, if the battery was connected in reverse, than the zener would act like a regular diode  and the excess voltage (~39V) would be dropped over the internal resistance leading to a large
>
> Although this will probably work I now realize it has several flaws.  First, if I am using a DC power supply I will probably blow the fuse in the supply.  Secondly, it requires a zener diode that can handle large currents (I've already melted one testing out the above circuit).  Third, there is almost certainly a better way to do this.
>
> How is this typically done?  How would you do it?


Hi Donovan, your email was probably missed by most
people on the list as you had no proper tag set.
I changed it to [EE]: for you.

Your fuse and zener is not the best way to do this.
You asked about a 40v input being "limited" to 30v,
which is the max input to your circuit.

This is called a "series voltage regulator" and
can be done with a transistor, resistor and zener
diode and will allow your circuit to keep running
safely even with the input over-voltage applied.

If you post more details on the range of voltages
you expect from your "battery" input, the current
requirements etc of your circuit i'm sure it will
be easy to find a good solution. :o)
-Roman

PS. Can you wordwrap your lines too? If people can
read your posts without having to scroll sideways
they are more likely to do so. :o)

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2002\10\06@174007 by Donovan Parks

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Hello,

Hopefully, this is now wordwrapped! :)

After thinking about my application some more I've decided it isn't worth
the time/effort/space to implement a series voltage regulator.  I'm simply
going to stat the maximum input voltage as 30V and assume people will not be
foolish enough to run it at higher voltages.  I would however still like to
protect from people inadverantly hooking up their power source in reverse.
I'm in the process of looking over the circuits people have sent me.
Thanks.

Donovan

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2002\10\06@182931 by Roman Black

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Donovan Parks wrote:
>
> Hopefully, this is now wordwrapped! :)

Yes, perfectly. :o)


> After thinking about my application some more I've decided it isn't worth
> the time/effort/space to implement a series voltage regulator.  I'm simply
> going to stat the maximum input voltage as 30V and assume people will not be
> foolish enough to run it at higher voltages.  I would however still like to
> protect from people inadverantly hooking up their power source in reverse.
> I'm in the process of looking over the circuits people have sent me.

If you no longer have need for over-voltage
protection you could simply use a series diode?
-Roman

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2002\10\06@193541 by Spehro Pefhany

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At 08:24 AM 10/7/02 +1000, you wrote:

>If you no longer have need for over-voltage
>protection you could simply use a series diode?

If the drop is acceptable, that's one of the easiest
choices. Or add a bridge rectifier, then it becomes polarity
insensitive. Add a capacitor too, and it is AC/DC polarity
insensitive.

Or drop in an LM2930 (about half a buck in quantity), which
is protected against -12V inputs or +40V inputs. but that might
not be enough reverse voltage if your maximum input is 30VDC.

Best regards,

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Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2002\10\06@204134 by Jinx

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> After thinking about my application some more I've decided it
> isn't worth the time/effort/space to implement a series voltage
> regulator.  I'm simply going to state the maximum input voltage
> as 30V and assume people will not be foolish enough to run it
> at higher voltages

I understand where you're coming from, but that's an assumption
that may come back to haunt you. Firstly, there are people who
will be negligent enough to try > 30V "just to see if it works" and
secondly, as has been discussed before, labels on power supplies,
especially wall-warts, are not wholly trustworthy. Neither of these
are your responsibility of course, but you may end up with repair
work. Better than stating that the max  is 30V, perhaps have a
safety margin and state it as 20V or 25V, if that's possible with
that circuit. If marked as 20V, someone would have to be truly mad
to exceed 30V and deserves a hefty lesson-teaching repair bill

It happens - I've just repaired a unit I made for someone who bypassed
the 5V regulator "to make it run better" !!!!!!!! and took out 3 PICs and
an LCD with 12V battery. I doubled the repair bill to teach them not
to f*** with my circuit again

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2002\10\06@213649 by Bill Westfield

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   Or drop in an LM2930 (about half a buck in quantity), which
   is protected against -12V inputs or +40V inputs.

Don't any of the semiconductor manufacturers MAKE a regulator that protects
against full reverse voltage and/or AC, at least to the point of blowing an
input fuse?  Seems like it would be a natural for consumer products, and
shouldn't be that hard to implement (I mean, an LDO with a power input diode
would still have lower dropout than a 7805-style regulator.)  Or are the
dies just too small for any additional power device?

BillW

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2002\10\07@125612 by Robert E. Griffith

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Hi Russell,

I like this because it seems to be a simple, inexpensive solution that will
work with an external regulated PS (since there is no diode V drop).  A
quick search at digikey came up with ZXMN2A01F ($0.30@100) Vgsmax = +-12v,
Rds = 0.12,   Vth = ~2.4v, Idmax=2A, Vgdmax 20V. All this in a little
SOT-23.

If I understand, right, this simple circuit protects against reverse
polarity PS up to -12V.  I did not understand where you put the zener and
resistor to protect the MOSFET against higher reverse voltages.

Vin (+5V regulated) --.------------ Vout
                     |
                     |   .- ZXMN2A01F (N-ch MOSFET)
                     |  /
                     G
Vin reference--------D S----------- Gnd

Now I am wondering if you could add something to turn the gate off if Vin is
over a specified value (say 5.5V).  That circuit would have to be tolerant
to reverse and overvoltage, though, independently.


--BobG

{Original Message removed}

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