Over-voltage protection uses the zener and fuse.Question is: how to have the over voltage clamping protecting with the simple scheme of relying on the voltage drop across a resistor to switch on Q1?
I thought it used the combination of the zener plus the resistor? Technically termed 'crowbar' protection in textbooks.Over-voltage protection uses the zener and fuse.
In reality the zener will get very hot and fail and the fuse will not blow if an over voltage transient occurs. You need a TVS device that is designed for high surge currents.
That is crowbar protection. The problem is, a zener won't conduct enough current to blow the fuse unless it's designed to do so. Most are designed to conduct a few mA. You could use a 16-20V TVS device. These are designed like zener diodes but can absorb huge surges without failure.I thought it used the combination of the zener plus the resistor? Technically termed 'crowbar' protection in textbooks.
For Q1, the theory is that current of 600mA passing through it will cause a voltage drop of 0.6 across it, therefore forward biasing Q1 into conduction to series pass current into the load.
So a 1Ω resistor?
TVS device: http://parts.digikey.com/1/parts/719517-diode-tvs-16v-600-watt-bi-dir-p6ke16ca.htmlAhh I see. I forget about the fuse for a moment.
I studied this like 20-years ago @ trade level. Have never practically applied it though.TVS device: http://parts.digikey.com/1/parts/719517-diode-tvs-16v-600-watt-bi-dir-p6ke16ca.html
Capable of dissipating a 600W surge, that's 37.5 amps peak. If the fuse blows in 1 second at 4A, it will blow in about 11.3 ms at 37.5A.
I^2 * t is a good approximation. If 4A blows it in 1 second, it uses 16 "fuse units" of life. (I forget the units!) 37.5A^2 * 0.0113 also uses 16 lives, so the fuse blows as well.I studied this like 20-years ago @ trade level. Have never practically applied it though.
During a failure, yes. But if it's allowed to cool down a fuse will often survive a high current event. Eventually metal fatigue from thermal expansion may get to it.So it has a working life is this what you mean?
What have you got a science degree or something? I mean I did 2 to 3 learning all this stuff and we never went this far into it.During a failure, yes. But if it's allowed to cool down a fuse will often survive a high current event. Eventually metal fatigue from thermal expansion may get to it.
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by Lianne Frith