From:   Kelly Miller[SMTP:fkmiller@COMCAST.NET]
Sent:   Monday, September 22, 2003 10:10 PM
To:     PICLIST@MITVMA.MIT.EDU
Subject:        [EE:] Newbie Question: Electrical protection for PICs

> 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?

> It doesn't sound to me like anyone does this, so I'm sure it is a flawed
> idea.  Why?  If it is a good idea, what is an example of some component
> to use.  I want to keep the magic blue smoke contained in the shiny
> little components.

I have seen 5.6V zener diodes between +5V and ground on some digital
projects to protect logic. The usual motivation is fear that the linear
regulator will short, putting the full unregulated DC on the load. This could
happen; the supply could also be connected backwards, and the zener
would also protect against this, limiting the reverse voltage to one diode
drop. Of  course, the zener must be able to handle enough current (power)
(in both directions) to protect the load until the fuse blows (if there is a fuse).

It seems that the real issue here concerns how much you should worry
about things like this. The issues are the cost of the parts being protected,
the need for reliability, the probability that the power supply will fail or be
connected backwards, and any other factor which would cause you to
expect something bad to happen. In the absence of any of these issues,
there would most likely not be a reason to worry about this problem.

Putting a resistor in the +5V line is risky. A 300 ohm resistor with 5mA
through it drops 1.5V, reducing your +5V to +3.5V which is not
acceptable. Even the zener diode trick requires a fuse if the unregulated
source has enough current to fry the zener.

Some commercial linear power supplies are constructed with this problem
in mind. They have a "crowbar" circuit across the output which is activated
if the output voltage exceeds a preset maximum. The crowbar is usually an
SCR rated for more current than the supply can put out. Once fired, it stays
on until the supply is turned off. The supply may then go into current
limiting until the circuit is reset. If you were making a large project which
had to be reliable and which could justify the cost, this would be the way
to go.

John Power

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