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 ----- From: Dale Botkin To: Sent: Sunday, October 06, 2002 9:20 AM Subject: Re: [EE]: Protecting Against Hooking Up a Battery in Reverse > 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 > --- > We are Dyslexia of Borg. > Fusistance is retile. > Your ass will be laminated. > > On Sat, 5 Oct 2002, Donovan Parks wrote: > > > How is this typically done? How would you do it? > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > > > > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics