12vdc polarity protection

[notlikelytoknow]

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?

 

[Ron Russell]

You can use a rectifier diode in series on the positive side of the supply,
but you will lose about .7 vdc.
Also a small electrolitic cap, say 10uF on the cathode side, should be used
to de-couple any resulting AC.

 

[Roger Hamlett]

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 two different 'approaches'.
The first is just to put a diode in series with the incoming line. Problem
here is the voltage drop induced, and the ESR of the diode, can increase
the need for smoothing on the unit itself.
The second, is to have an incoming fuse, or thermal trip (which _should_
be present anyway, for safety), and put a large diode after this, reverse
biased across the supply. Then if the supply is reversed the diode
conducts, and blows the fuse/trip. The advantage of this is that you only
have the single voltage drop associated with the fuse, and gain overload
protection, as well as reverse supply connection. Depending on the
currents involved, a device like a 'Polyswitch' self resetting fuse, is
ideal for this configuration.

Best Wishes

 

[Dave Platt]

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.

 

[James T. White]

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?

Here is a good article that covers several options:

http://www.edn.com/archives/1996/030196/05df3.htm

 

[Jamie]

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?

you can use a diode to protect you from that problem how ever,
the diode will cause a voltage loss of about .7 volts.
you can how ever, use that same method to drive a relay coil
which will turn on a 12 volt relay which then will connect the
+ source to your output via a set of relay contactors.
with a diode driving the relay coil, if the polarity is
wrong, the current will not flow to activate the relay coil and
thus the contacts won't close.
you would want a double pole single throw type. pass both the
- and + source through the relay. you can use a double throw
relay if you can't find a single throw , it will just cost a bit
more.

 

[Vanheesbeke Stefaan]

The second, is to have an incoming fuse, or thermal trip (which _should_
be present anyway, for safety), and put a large diode after this, reverse
biased across the supply. Then if the supply is reversed the diode
conducts, and blows the fuse/trip. The advantage of this is that you only
have the single voltage drop associated with the fuse, and gain overload
protection, as well as reverse supply connection. Depending on the
currents involved, a device like a 'Polyswitch' self resetting fuse, is
ideal for this configuration.

Best Wishes

Just an idea

If you use the polyswitch, and replace the diode suggested with a zener
diode (or better a transorb diode), you have also an overvoltage protection.

 

[Roger Hamlett]

Just an idea

If you use the polyswitch, and replace the diode suggested with a zener
diode (or better a transorb diode), you have also an overvoltage
protection.

The only 'downside', is that for reverse protection, the voltage drop of a
zener may be somewhat larger than you want, and it's current handling
ability less than mind-blowing. It does all come down to caeful selection
of the parts.

Best Wishes