Re: 12vdc polarity protection

In article <1131593763.682033.93030@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
notlikelytoknow <mobilsurg@xxxxxxx> wrote:

>I am looking for a component or construct of components that will trip
>and could be reset if polarity is reversed in the positive and negative
>source wires in a 12vdc circuit (automotive) Obviously, I am not an
>engineer. Any help?

There are a number of ways to provide this sort of protection.

One is to put a forward-biased diode in series with the 12-volt
supply. It won't trip upon a polarity reversal - it'll just block any
significant amount of current from flowing in the reverse direction.
This is the easiest approach if your circuit can tolerate the amount
of voltage lost in the diode's forward-biased junction - this is
usually on the rough order of 1 volt for a typical 3-to-5-amp
rectifier diode. You'll want a diode rated at 50 volts or more (100
volts is better) in order to ensure that it'll survive voltage spikes
caused by a phenomenon called "load dumping".

A slight modification to this approach, devised by Bob Pease of
National Semiconductor, uses a power MOSFET located in one of the
power supply lines. The MOSFET is hooked up "backwards" from its
traditional arrangement (the source and drain are exchanged) so that
both the MOSFET itself and its intrinsic "body" diode are turned
"on" normal operation and switched off during a reverse hookup.
Details can be found in his excellent text "Troubleshooting Analog
Electronics."

Another approach is to place a fuse (or fast-acting circuit breaker)
in the 12-volt supply line, and then place a _reverse_ biased diode
(as above) between the circuit breaker's output and ground. If the
circuit is hooked up correctly, the diode doesn't conduct, and nothing
happens. If the circuit is hooked up backwards, the diode goes into
hard conduction, draws lots of current, limits the reverse voltage
seen by the load to a volt or so, and the fuse blows almost instantly
or the breaker trips. This approach doesn't cause a 1-volt decrease
in supply voltage (as the diode approach does), but may require you to
replace a fuse after goofing.

A slight modification to the second approach uses a self-resetting
thermal fuse of some sort... the Raychem "Polyswitch" is the
best-known example.

--
Dave Platt <dplatt@xxxxxxxxxxxx> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
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.

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