Voltage spike protection for regulator

[Veketti]

Dear All,

I heard that in vehicle use it's good idea to protect regulator from voltage spikes. So my question is, does this circuit work?


Idea is that the Zener diode will ground through that 60ohm resistor voltage over 18V which is way before the maximum limit of 25V of LM7805 regulator voltage limit.

 

[barry]

1) Get rid of the resistor. Think about it. Say you've got 2000 volt spike; you'll have 18volts across your zener, 1982 volts across your resistor and 2000 volts into your regulator.

2) Get rid of the zener. Use a specially designed overvoltage device like a transorb (transient voltage suppressor). An ordinary zener may not be fast enough.

 

[Audioguru]

Why not use a voltage regulator that is designed to be used in a vehicle like most low dropout regulators?

 

[schmitt trigger]

An ordinary zener may not be fast enough.

100% agree with you.
Additionally zeners are not powerful enough. Transorbs are rated for very high (although very short) current clamping capabilities.

Also in a vehicle, is important to add reverse polarity protection.

 

[Audioguru]

Here is the description of only one of many low dropout regulators designed to be used in an automobile:

 

[Veketti]

Great, thanks for all of you for being so helpful again. I've learned bunch of new things again. I didn't knew about those low dropout regulators before and I have already bought bunch of LM7805's as I didn't know any more suitable products so I have to stick with what I have. I'll get those transorbs and if was it so that I'll put one of those eg. P6KE22A instead of that resistor and zener and I should be safe? I'm not sure which should I follow, Breakdown voltage or Stand-off voltage, but on that particular unit, both are under 25V.

 

[schmitt trigger]

Standoff voltage: voltage that can be applied indefinitely at the Tranzorb, without any degradation or stress. For a "12"v automotive system, choose 14v.

Breakdown voltage: voltage at which a small current (usually 1 mA) starts flowing thru the device. It is larger than the standoff.

Clamp voltage: maximum voltage level to which the device will clamp the surge, when it is subjected to an external spike whose waveshape and duration are defined by regulatory bodies. Usually the 10X1000 u waveshape. Many amps flow thru the device in that condition, and therefore if subjected to repetitive iterations in a short period of time, the device could fail.

 

[chuckey]

I read the IC as a 7805, 5V @1.5A, so under most operating conditions you could drop 5V across a series element. So depending on the current, this is what I would do. Use a resistor that is marginally rated, and a ceramic capacitor to earth. The reason that I say marginally rated is that an alternator with a fault will try and produce a constant current, so produce 50V or so until it burns out the ignition coil.
Frank