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Protecting inputs

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Scarr

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Hi,

My project is for use in a vehicle 12-24v I want to have four to eight I/O for things like ignition etc. but I want to ensure these are protected, is there a IC ideal for this job? I would have thought with all the vehicle electronics an IC manufacturer would have made a SPI/I2C I/O type chip that will handle automotive voltages.

If there is not a chip what is best way to do this?

Thanks

Scarr
 

RadioRon

Well-Known Member
There are a few discrete protection devices out there. We often use transzorbs rated at about 30 volts. These protection devices act a bit like zener diodes except with a softer V-I characteristic and that these can absorb more power for a brief time. Usually you want to protect against over-voltage and reverse voltage transients that might last up to around 10mS. Google on "load dump" for a bit of background about the worst case transients. They can go pretty high in voltage. I also usually put some RF filtering to keep VHF radio interference in and out, using ferrite beads and possibly common mode EMI filters for DC power supply lines.
 
Hi

It's unlikely that you would get a 'chip' to do this as some supression activity may require the ability to dissipate a relatively large amount of heat and by the time you have 8 such inputs your 'chip' will be a considerable size.

Note that most relay contacts (a possible input to your system) are only reliable if you draw a reasonable current through them, the Rail Industry use 10mA. This is possibly a simple resistor load or that drawn by your protection circuitry. Using a relatively low input impedance like this also drains the power from many noise sources.

Your main problem is actually working out what sort of voltages and currents you will need to protect against, only then can you work out the right/optimum protection.

As you rightly say, vehicle manufacturers must have taken this into account in all the many systems that sit under the bonnets/hoods of our cars, so the best and most authorititive step would be to get some circuits diagrams from such systems. Hopefully there will be an automotive expert on this site.

I design equipment to go onto trains and we have the EN50155 standard that defines what you are likely to meet on Rail Vehicles ranging from fast burst transients to surges. You may find that the Automotive Industry has similar standards.

Failing that you can just expect the worst and go for a sledgehammer approach, some suggestions:

1. Put in a current limiting device, could be just the input voltage divider - transients/surges are easier to handle like this - after that.....
2. Protect against reverse polarity connection - normally a simple diode at this point - another diode could be used to 'clamp' the input to your circuits supply rail. (Both of these are often in chips being intrinsic to the design, however no point in protecting an input that will suffer at -0.4V with a forward voltage diode of 0.7V)
3. Protect against fast, high voltage, low power transients from ignition circuits - use a capacitor or a fast zener/transient supressor - don't forget that fast signals will get through your circuits using stray capacitance neatly sidestepping that big resistor you put in its way!

I'm sure that there will be other suggestions!


James
 
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