Jim, You've brought up some very good points, I've seen Zeners used like you said, when there is a spike big enough to trigger them, they become a solid short to ground. SO they need to be replaced after they trigger, but they do save more expensive electronics from DEATH. And yes I've replaced more than a few Zeners that were used like this. It's cheap insurance for your electronics.
The SCR crowbar will trigger and stay triggered until you remove power, so the fuse type will not mater. I've had to test this circuit in some ery expensive Mil-Spec Avionics. Bill Higdon > Any alternator will deliver a very high voltage spike when it first comes on > line. I have measured these to be as high as 90 volts, using an > oscilloscope. > It only lasts a few milliseconds, but that is enough to fry electronics. > > The battery will absorb a lot of the spike, if the cables from the alternator > to > the battery are large diameter and short. > > Circuit breakers can take up to half a second to open when there is an > overload, > because they are a thermal device and have to heat up before they open. > > If you are using fuses to protect your electronics, make sure you are using > the > quick blow type. Some fuses are time delayed so they won't blow on a spike, > but > open when the average current goes high. > > The best approach to limit the spike is to put an 18 volt, 5 watt or larger > zener diode from the alternator output connection directly to ground. It > doesn't take a large one to swallow the spike. Any electronics supply store > should have them in stock. > > When the voltage regulator goes belly up, there's little that can be done, > since > most electronics will go before the fuse or circuit breaker blows. I'm > breadboarding up a low voltage indicator (with a flashing light to get my > attention), since I'm using the standard VW ignition system and want maximum > time to get on the ground before my battery quits. I will probably play with > a > high voltage warning circuit, too, but that is a good winter project. > > Jim Vance > va...@hbcomm.net > _______________________________________ > Search the KRnet Archives at http://www.maddyhome.com/krsrch/index.jsp > to UNsubscribe from KRnet, send a message to krnet-le...@mylist.net > please see other KRnet info at http://www.krnet.org/info.html
