Richard,


It is a good idea to always provide some sort of reversal polarity
protection to circuits. It is very common for equipment to be connected
with the wrong polarity or a wrong power adapter.

There are several approaches, suitable for various circuit requirements.

1) Just use a diode in series with the power input. Suitable when your
circuit consumes a moderate current and you have a voltage regulator in
the input or can tolerate a small voltage drop.

2) Use a fuse in series with the power input and a powerful diode
connected cathode after the fuse and anode to ground. Your diode must be
strong enough to ensure the fuse will blow before the diode is damaged.
The advantage is that you don't have the Vf loss from the diode when the
circuit is operating.

3) Use a power P-Channel MOSFET with its Drain pin connected to the
power supply, Gate to ground and Source as output to the circuit. This
circuit can be used for voltages up to approx. 20V, which usually is the
Gate-Source breakdown voltage. With the addition of a Zener diode and a
resistor this limit can be raised.
This circuit can be used for really high currents with very little
energy loss. If you add another MOSFET in series in the opposite
direction plus a TL431, another smaller MOSFET and some resistors, you
have overvoltage protection also.

4) Use a relay with its coil with a diode in series connected to the
power input. Use the normally-open contact to power your circuit. This
method can be used for any current and voltage ranges and the only loss
is the relay's coil current.

Cheers,

Isaac


Em 17/09/2015 03:43, Richard R. Pope escreveu:
> Bob and et al,
>      Well I just make a goof! I was very fortunate. I hooked the power=20
> to my control board up the wrong way. The PICs are tough little=20
> monsters. Even though the power was hooked up the wrong way for almost=20
> twenty minutes all of the PICs are fine. It looks like all that I=20
> damaged were the tantalum capacitors. Wow! Good news. I'll be more=20
> careful from now on. I appreciate all of the help that everyone has=20
> given me.
> Thanks,
>



--=20
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