Reverse polarity protection

[AllenBV]

I have a small battery operated device using 4.5 volts DC. I want to protect against the user from putting the batteries in backwards. Would putting a diode (1n4004) in series with the load on the + side of the battery solve this problem or do I have to do something a bit more complicated?<p>Thanks

[k7elp60]

A diode in the positive lead should do it. I would recommend using a 1N5817 thou, it is a schottky diode and has about half the voltage drop of the 1N4004.

[philba]

A Schottky would be your best bet for a diode but keep in mind that the '17 still has a drop of about .3V or higher - look at the data sheet graph of Vf vs If for the expected drop for your application.<p>If your circuit can't handle the lower voltage, use a p channel mosfet inline (for + polarity input). The gate is fed via 1 M resistor from ground. I believe (though never done it) that you can get almost no voltage drop.<p>here's a page that talks about this technique - http://www.geofex.com/Article_Folders/m ... switch.htm<p>Phil<p>[ March 17, 2005: Message edited by: philba ]</p>

[AllenBV]

My thanks to both posters for your help. When I asked the question I was concerned about the voltage drop. I really like the slick idea about using the FET for this issue. <p>Bradley

[k7elp60]

The voltage drop across the protection diode is certainaly an issue. Most battery operated devices quit working right, when the battery voltage reduces to a certain amount. I would try the schokky diode first. You may notice very little difference with the diode and without it.<p>The P channel power Mosfet is certainly another option, but it will no doubt be larger in size than the diode.

[AllenBV]

There is a brownout circuit on the micro that I plan to use but I would like the circuit to run as long as possible. I think that I will use the P FET example instead of using the Schottky diode for this reason.

[Dean Huster]

Don't get too agitaged about the the diode drop. Don't forget that the 1N4001 is specified for a forward drop of 0.9 volts, but that's at its full rated forward current of 1 amp. The drop for lower currents is less. It's the same for a Schottky. Keep the current well-below the rated current of the Schottky in use and you can find realistic forward drops as low as 0.1 volts.<p>If the drop is a severe issue, you can put the diode across the supply connections with a fuse in series with the supply. Backward battery results in a blown fuse and no forward drop at all.<p>Dean