Electronic – Resistor placement for Zener diode for over voltage protection

You seem to be confusing the Zener action with a 'crowbar' circuit. A Zener will limit the voltage whereas a crowbar circuit will short out the supply in the event of an over-voltage. The crowbar short can usually only be removed by cycling the power.

The Zener diode will start to conduct in the reverse direction when the voltage across it reaches about 5 V. From then on very small increases in voltage will cause it to pass an exponentially greater current. At the same time the voltage will increase slightly above 5 V.

The problem with your arrangement is that if the regulator can pass, say 1 A, before going into limiting then your Zener has to handle all that power. \$P = V \cdot I = 5 \cdot 1 = 5~W\$. It will get hot.

Crowbar circuit

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Crowbar circuit.

This is a basic crowbar circuit. When the supply voltage rises above 4.7 V the Zener starts to conduct. When the voltage across R1 gets high enough SCR1 will be triggered and pretty-well short out the supply. The thyristor has to pass all the current (1 A in our example above) but this time the voltage across the device is only about 0.5 V so \$P = V \cdot I = 0.5 \cdot 1 = 0.5~W\$. Cool!

The thyristor will stay on until the current though it is reduced to zero by interrupting the supply.

History lesson: the term crowbar protection comes from the railway third-rail electrification. If the lineman saw a problem and needed an emergency isolation of the track power supply he would throw his crowbar to short the third rail to one of the running rails and trip out the breaker.

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.