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a busy cat http://www.learningaboutelectronics.com/images/Overvoltage-protection-circuit-schematic.png

Design Operating Characteristics

Current Draw: 2 amps

Operating Voltage: 13.5 V

This design will operate in an automotive environment and we would like to implement over voltage and over current protection utilizing a zener diode and fuse to accomplish this, respectively. Is this the proper way to go about doing this? And if so, what kind of voltage characteristics should the zener diode feature?

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  • \$\begingroup\$ Honestly, instead of putting a protection circuit with a fuse which may blow in front of a 2A load, I would just put a low-dropout linear regulator there. \$\endgroup\$ – Janka Oct 10 '16 at 18:11
  • \$\begingroup\$ The difference in construction of a TVS from a zener is low C (2pF) suitable for RF data rates and yet clamp high current spikes. Typically Polyfuse (tm) and thermally protected High side switches or regulators are used in automotive accessories to avoid maintenance of fuses with surge dump, 2x battery voltage and reverse battery voltage protection without blowing a fuse or at least causing damage. \$\endgroup\$ – Sunnyskyguy EE75 Oct 10 '16 at 19:08
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What you are suggesting is a bad idea. Normal automotive operating voltage with the battery charging is usually around 13.6 V. However, large spikes of 10s of V above that happen. Your fuse will blow regularly, which will be a hassle to the users.

A better strategy is to design the circuit to withstand occasional spikes instead of giving up and shutting itself off from the world when they happen.

There is no single answer for this that fits all needs. For low currents, you could use a series resistor followed by a clamp. For high currents, the resistor gets in the way or causes too much dissipation.

If your current is low and the voltage you want is somewhat less than 12 V, then you can use a automotive-rated linear regulator.

For higher power, use a buck regulator that can handle sufficiently high input voltage. There are also ways to use a P channel MOSFET as a series pass element that looks like a short except when a spike comes a long.

Again, there are lots of strategies, but without knowing the voltage and current your load needs, there is no way to decide which one makes the most sense.

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enter image description hereThe maximum voltage from a lead acid battery in this case would be 14.4 V. So such a crowbar protection at a voltage level of 15 V would be nice. As can be seen from the graph a zener does not have a complete sharp on/off caracteristic. This situation could lead to an unwanted situation before the fuse blows.

With a little more effort you could use an SCR crowbar to be triggered when the voltage level surpasses 15 V or whatever level the system is set. An even better system can be reached with a socalled thyrector. A protection diode with transient voltage suppression (TVS) properties.

Additional information about TVS diodes can be found on Wikipedia: https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode

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  • \$\begingroup\$ The proper phrase is "crowbar circuit". \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 10 '16 at 17:06
  • \$\begingroup\$ There is a whole specification for automotive, which includes "load dumps" that can momentarily provide over 100 V. Good protection requires more. In general, load dump protection will require high-energy TVS diodes. \$\endgroup\$ – jonk Oct 10 '16 at 17:15

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