piclist 2002\10\05\164859a >
Thread: Protecting Against Hooking Up a Battery in Reverse
www.piclist.com/techref/power/batterys.htm?key=battery
BY : Russell McMahon email (remove spam text)

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

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