When a circuit requires reverse-voltage protection on the power input, the usual approach is a series diode (see the figure, a). But if the input voltage input is low—say, two or three AA batteries—the 0.5-V loss of a Schottky diode may represent a significant loss in useful battery life. An FET can serve as a series diode with significantly less voltage drop (see the figure, b).
A dual IRF7342 FET was used (p-channel, with both sides wired in parallel) using a 3-V source, resulting in a voltage drop of only 100 mV into a 100-mA load. With a 4.5-V input, the loss is only 50 mV with a 100-mA load. If a low-side switch topology will work in the application, an n-channel FET can be employed (see the figure, c). A dual IRF7341 FET was used (n-channel, with both sides wired in parallel) with a 3-V source, resulting in a voltage drop of only 40 mV into a 100-mA load. A 4.5-V input results in a loss of only 25 mV with a 100-mA load.
If a "soft-start" feature is needed, it can be added with only one capacitor and one resistor (see the figure, d). The values indicated add about 100 ms of turn-on delay, as the FET goes from off through the linear mode to fully on.
I WANT TO PROTECT MY BATTERY OPERATED SYSTEM FROM 12V/7AH DC PLEASE PROVIDE THE SOLUTION FOR THE SAME
Anonymous -June 07, 2008
This article is useful but need to be expanded.
Ahmed Mohammed Nassar -February 11, 2007 (Article Rating: )
Is it posible to build a reverse voltage protection circuit with a bipolar transistor and two resistors. Thanks!
Anonymous -September 13, 2006
in the paperback 1993 reprint (ISBN 0-7506-9499-8) only the lowside version is given. The lowside version is not usuable in many situations; the highside pfet design here is a valuable trick.
Anonymous -July 20, 2005 (Article Rating: )
Analog guru Bob Pease put the same circuit in many places, like one of his best seller "Trouble Shooting Analog Circuit" (ISBN 0-7506-9184-0), Fig 13.5, P164. If Mr. Walker is a novice designer and did see this, someone ought to see it before it appears in the issue. You guys are ignoring the "authority"!
Anonymous -June 06, 2005
OK now - the images show up. Thanks!
Anonymous -April 08, 2005
This article would be a lot more useful if you could load the images. They are lost.
Anonymous -April 05, 2005 (Article Rating: )
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