My father and I have done this in the past with good results. As the
filament heats, its resistance increases and the circuit is automatically
protected. Under normal operation, below the operational current of the
bulb, very little power is dissipated. It does not protect will against
spikes, however.

---
James Newton mailto:jamesnewton@geocities.com 1-619-652-0593
http://techref.massmind.org NEW! FINALLY A REAL NAME!
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-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Robert Rolf
Sent: Monday, March 06, 2000 23:13
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: virtual car


Tony Nixon wrote:
>
> Whew!!
>
> Finally got somewhere with this project.
>
> I've now got a PIC16F877 talking to another PIC16F877 over a radio link.
>
> The first PIC monitors steering and throttle positions from the virtual
> car, plus some other stuff and relays all the info to the other 877.
>
> This then tells two 12C672's to move the real thing via some rather
> large servos.
>
> I just blew a 10A fuse trying to do a quick steering direction change,
> but I noticed the motor manufacturer specified up to 20A when this
> happens, so maybe a 30A fuse is needed. The motor is braked when told to
> stop or change direction, so I may have to put in a bigger dead time as
> well. It's pretty hairy the first time you apply power to these things
> considering the power behind them and not exactly knowing what's going
> to happen. You can take all the precautions in the world, but there's
> always some little thing you forgot :-)

So use a high wattage light bulb as your self resetting fuse. It has a
low resistance as long as the current is low enough, and then develops a
very high resistance once that threshold is crossed (I.E. quite
non-linear just like a varistor). It also flashes to let you know
that you've gone overcurrent.