Kelly,

On Mon, 22 Sep 2003 21:25:18 -0400, Kelly Miller wrote:

>...<

> I understand that 5 volt PICs work with what seems low voltage and
> limited current.  Is it a good idea to put some sort of electrical
> "fusing" between a regulated 5 volt power supply and the PIC input
> pin(s)?  (There is a 3 Amp circuit breaker in the upstream 12-16 volt
> power supply that I regulated down to 5 volts.)  Would it serve to
> protect the PIC, associated components, and possibly safety?

The regulated 5V supply will usually include thermal/overcurrent protection - the common 3-terminal regulators
certainly do, and this protects themselves and to some extent the devices they are powering, so your project
is unlikely to catch fire unless you are *really* clumsy!  :-)  3 Amps at 12V would let quite a lot of smoke
out of anything drawing it, but only a failure of the regulator would let that happen.

> It doesn't sound to me like anyone does this, so I'm sure it is a flawed idea.  Why?

The currents on a typical PIC project are small, usually no more than a hundred mA or so, with most of that
being used by LEDs and such.  A device that would trip at that sort of level would be awkward to get right,
and wouldn't do much more than the in-built protection in the regulator.  There's very little physical danger
at that current level, and as for protecting the devices themselves it would be difficult to choose a current
trip-level that would stop damage to components in a fault condition but with enough headroom to stop nuisence
trips.

>If it is a good idea, what is an example of some component to use.

I don't know of anything that would do any good!  Overvoltage protection is easy (a single zener diode, at the
simplest), overcurrent at those levels isn't.

>I want to keep the magic blue smoke contained in the shiny little components.  :-o

An admirable aim, but one that's better achieved by being careful in design and construction, rather than
having safety devices everywhere.  The thing is, where would you stop?  A current-limiter on every chip's
power supply pin?  On every input pin on every chip?  On every resistor?  :-)  Best to do it at the board
level (as you have with a 78x05 for example) unless there is a particular need that warrants going further
than this.

I must go - it's stuffy in here so I shall have to prop open the fire-check door with a fire extinguisher...
:-D

Cheers,

Howard Winter
St.Albans, England

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