Help with transient protection

[john2k]

I need some help and advice on how I can add transient protection on the following setup i've drawn up. On the input of my arduino I have a L7809CV regulator board which can handle voltages from 11 to 25v but as I understand if there is faults with the rectifying diodes on alternator then voltages can spike really high which can fry the L7809CV. So firstly I want a way to protect against this. This power regulator module will be powering the arduino 24/7 so needs to be reliable. Then secondly, I want a way to be able to add transient protection on the CANH and CANL of both CAN transceiver modules. I've come across the PESD1CAN, which if i'm not mistaken is some sort of diode with 3 pins. Does anyone know how I can incorporate this into my design?

Any advice or help would be very much appreciated. Below is my diagram.

Thanks

 

[shrtrnd]

I've seen 3-Pin devices that are basically back-to-back diodes, but there are 2-Pin MOVs (Metal Oxide Varistors) available, you
might want to look into them. They're called 'varistors' for short. They're designed to absorb transient voltage spikes, and I use
them a lot of initial in-rush voltage spike situations (in my case, relay coils).
I don't remember trying them in DC circuits, and if they're not applicable in your application, someone here will probably comment on that. Determine the voltage range you need, varistors come in all voltages.
Just giving you an option to look into for your application.

 

[Harald Kapp]

For suppressing transient voltage spikes a bunch of different methods can be employed:

• MOVs (varistors)
• Suppressor diodes
• Zener diodes
• low pass filters (LC, RC)
• A combination thereof

One thing to note with voltage limiting elements like MOVs or suppressor diodes is that these elements require a series impedance to work properly. When such an element is activated by overvoltage, its impedance falls drastically. Without a series impedance this means a high current through the protection device without a considerable drop in voltage. Therefore a series resistor or inductor is required to drop the overvoltage. As a side effect this series impedance can reduce the stress on the 7809 as it reduces the input voltage to the 7809 due to the voltage drop across teh series impedance caused by the operating current. And using an inductor provides an additional level of EMC filtering.
See this simplified example:

The input voltage is at 12 V with a spike to 24 V at t = 10 ms.
The upper trace shows the voltage across D2 which spikes to 24 V.
The lower trace shows the voltage across D1 which is duly limited to ~15 V.

 

[john2k]

Sorry i'm a beginner. what would be the best and easiest solution? Could I not put a Zener diode on the input before the 7809 and maybe use a PESD1CAN on the CAN lines?

 

[Harald Kapp]

The L7809 is barely suited for your application as it works at the limits of its parameters when the input voltage from the battery is low (11 V or less). So adding a resistive serial element is not an option. Use a choke of 10 mH or more with very low DC resistance (<0.5 Ω) and a 15 V zener diode (choke in place of R1, zener in place of D1 in my schematic).

Or use an off the shelf line filter for automobile application (like this one, sorry the page turns up in German, but you can easily find equivalent modules online in your region). That should keep away any voltage spikes from the regulator.