One good way for low cost products (or for added protection on higher cost ones) is to add the little non-conductive ring to the end of the batter compartment so that only the "nub" on the positive end of the battery can touch the contact in the battery holder.
Wagner Lipnharski says:
Ok, use a reversed diode to the [circuit] with a low value resistor in series to the cells. It means, that if reversing cells polarity it will be drained by the resistor+diode. In real you would be short circuiting the cells, but not in a dangerous current (limited by the resistor). Calculate the resistor to drop only 50mV on normal operation, but when cells are inverted it will dissipate heat, leaving only the diode voltage drop to the [circuit].
For example, if the normal operation current is around 10mA, the resistor should be 50mV/10mA = 5 Ohms. If cells are assembled reversed, the diode will conduct directly over the resistor, so it will be 3V-0.6V(diode) / 5 Ohms = 480mA, enough to keep the reversed voltage over the [circuit] at 0.6V without exploding the cells.
For better protection, you could use a Schottky diode or even an "old germanium diode" that drops only 0.2Vdc. If your circuit drain pulses of high current, bigger than 10mA, you can add an electrolytic capacitor in parallel to the 5 Ohms diode, as a bypass to those high surge periods.
For sure, if you can afford to reduce 0.2Vdc from the [circuit], install only the germanium diode in series with the cells... :)
Steve Baldwin says
Zetex ZVP4424A low Vgs, P-channel, enhancement mode FET. Just tie the Gate to your negative terminal. When the circuit is connected backwards, the FET is biased off and the Drain/Source diode is reverse biased. When it is the right way around, the diode is forward biased and gets shunted by the FET turning on when the voltage starts to pass the Vgs threshold.
Harold Hallikainen says
[use] a series PTC then a shunt diode. The diode will conduct if the batteries are put in backwards, opening the PTC.
Nick Taylor says:[this] can be improved by using an appropriate valued zener for the diode ... also gives overvoltage protection.
Over Voltage protection@, Isolation@
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<A HREF="http://massmind.org/techref/power/polarity.htm"> Polarity Inversion protection</A>
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