spike protection in automotive applications???

[findlay234]

Hi guys, just want some help in providing surge protection to a circuit im designing for use in a car. The ciruit uses a pic controller so runs on 5v through a voltage regulator(L78S05CV). What i need is a cost effective choke/filter or another way of doing the job. Also will there be any problems when starting the engine as there is usually a drop in the current to the rest of the car?

Cheers for any help.
Fin

 

[Skogsgurra]

Watch out for load dumps! They can reach well above 50 volts, some say up to 80 V, in a 12 V car system. A 78L05 will not survive and a filter that takes care of such a transient (it is hundreds of milliseconds long) will be a large one, indeed. I think that I would use a resistor and a zener if it is a PIC controller with low current consumption. The resistor does not get killed by the load dump, but you have to see to it that the zener isn't zapped. Look for short term overload and select accordingly. As for low voltage, you have to select a low enough resistor value to deliver the needed current even at lowest existing voltage and with enough power handling to survive the load dump. Load dumps do not happen very often, but when it happens, you better be ready for it.

 

[melone]

Look for a device called a transorb....... they are in a very large number of automotive applications.

 

[Skogsgurra]

Also try tranzorb.

 

[BrianR]

skogsgurra, You are absolutely right. I have connected a storage CRO to the terminals a car battery and recorded transients of several hundred volts. A good technique for filtering which also protects against voltage droop is a series diode in the incoming power line followed by a low ESR electro to ground, then add a tranzorb.

 

[Comcokid]

See the following other recent discussions related to transients and vehicle systems.
thread67-98147
thread67-94753
thread240-98170

 

[MacGyverS2000]

Brian,

When did you measure these spikes... during every start and/or stop, only during component failures like load dumps, etc.? Essentially, did you find these to be typical or once-in-a-blue-moon type of occurances?

 

[BrianR]

MacGyverS2000, this test was done some time ago when designing electronic ignition units and I recalled the magnitudes. From memory they were worst case measured within an hour or so of trying to create worst conditions: starting, switch off, high ignition voltage requirements and accessory switching. I no longer have the details of what and when but recall that alternator load dump, starter motor transients and high ignition voltage were major contributors.

 

[felixc]

A mix of the above suggestions will provide the needed protection:
- a forward series diode will eliminate the reverse spikes or connections.
- a current limiting device to control the overload inrush to the overvoltage protection device.
- an overvoltage protection device like a tranzorb. The wattage rating of the tranzorb is a function of the max current that the curent limiter will give during the energy spike. Set the voltage of the Vz high enough so that just the high spikes are absorbed, but within the regulator's range.
- LC filtering. Will also help keeping the juice (with the series diode) during crank-up. Watch if the capacitor has a max current surge rating. Many tantalums do.
- you may also want to fuse the line.

The protection circuits for an automotive application can jack up the component count and the product cost as well. It depends how serious your design is, meaning hobby or high volume. Balancing all these to have a good and cost competitive product is the engineer's job.

 

[melone]

If you want to see a nasty load dump, turn off your headlamps!

 

[2dye4]

If your circuit draws very little current the problem is
made easier. The regulator will work with 7V on its input.
Calculate a resistor that will give 5 volts of drop at the
maximum current your circuit will draw and place this in
series with the suppply.
Then a zener from regulator side of the resistor to
ground will limit the voltage.
This will limit the stress the zener endures and
maximize the transient voltage-duration product your
circuit will survive with.
Check your circuit for trouble with slowly rising power
supply though.
At one company I worked for they protected engine control
units with a 5W 27Volt zener across the power input.