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  #1  
Old 11-26-2011, 04:14 PM
jblackston78 jblackston78 is offline
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Default MCU 5v Input Protection

I have a single wire based communication bus which handle both rx/tx communication at 5v. I've tied this directly to the MCU input and placed TVS diodes to help under and over voltage protection. This is a bus that gets exposed to the outside world and while being used in the field it has come into contact to 12v on the bus and a high current run, which intern blows my TVS diodes and then pops my MCU.

Does anybody know a circuit to handle over voltage protection on this type of circuit which can with stand 12v. I've tried to find some kind of transceivers which mimic my bus network, but have not found any.

I imagine this is probably a common circuit and hope someone can point me in the right direction.

Thanks in advance,
Jason
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Old 11-26-2011, 04:20 PM
Yako Yako is offline
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Most MCUs already have protection built in.
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Old 11-27-2011, 05:18 AM
jblackston78 jblackston78 is offline
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I'm using a MicroChip part and the input on that is not rated to go too high over 5v. The high voltage placed on the bus has the ability for a large current drain so when the TVS heats up and pops the entire 12v is pushed to the MCU which only can protect to 5v. To protect it self the MCU just blows that pin. Depending upon what happens the rest of the chip is fine except for that pin. So how do I protect that pin from over voltage? The TVS diode protection isn't enough.
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Old 11-27-2011, 06:50 AM
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No MCU pin should be used directly on a communication bus. Use an RS-232, RS-422 or RS-485 device instead.
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Old 11-27-2011, 08:18 AM
Yako Yako is offline
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Quote:
Originally Posted by jblackston78 View Post
The TVS diode protection isn't enough.
Isn't it?

Can you show us what you've done?
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Old 11-27-2011, 08:33 AM
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Yako, he has already told us that the TVS diodes and MCU blow. He must use proper high voltage bus transceivers. He can install isolated bus transceivers that give 5KV protection.

Here are some links:

http://www.bb-elec.com/tech_articles...protection.asp

http://www.bb-elec.com/tech_articles...ion_theory.asp

http://www.bb-elec.com/tech_articles...ection.asp#top
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Last edited by MrChips; 11-27-2011 at 08:55 AM.
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Old 11-27-2011, 12:40 PM
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THE_RB THE_RB is offline
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Quote:
Originally Posted by jblackston78 View Post
I have a single wire based communication bus which handle both rx/tx communication at 5v. I've tied this directly to the MCU input and placed TVS diodes to help under and over voltage protection. This is a bus that gets exposed to the outside world and while being used in the field it has come into contact to 12v on the bus and a high current run, which intern blows my TVS diodes and then pops my MCU.

Does anybody know a circuit to handle over voltage protection on this type of circuit which can with stand 12v. I've tried to find some kind of transceivers which mimic my bus network, but have not found any.
...
Why not just use a resistor? A resistor on the data line will limit the max current according to the fault voltage, so a 1k resistor and 12v fault voltage (7v over 5v) will limit fault current to about 7v / 1000 ohms = 7mA.
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Old 11-28-2011, 01:30 AM
jblackston78 jblackston78 is offline
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I had thought about placing a resistor on the data line, but wasn't sure if that could completely protect it. My real goal if possible is to not only prevent over voltage, but also include other protection like short-to-ground, ect...

If you had to connect to a 5v bus bidirectional line what would you use to protect this data line? Is there a transceiver available to handle this? The RS-422 is a differential signal line which doesn't work. Is it possible to modify an existing transceiver to get the bus connection I need?
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Old 11-28-2011, 02:21 AM
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Putting a 33 to 49-ohm resistor in series with the bus line may save burning your diodes and MCU.

Is the MCU part something of your design? The I/O pin is being switched from input to output if it is a bidirectional data bus. Do you have a spare output pin that you can use as a transmit/receive control signal. If that is the case you may be able to use a RS-485 transceiver such as 75LBC179.
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Old 11-28-2011, 02:43 AM
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I'm assuming the PIC and bus wire are sharing the ground with the 12V system?

You could be out of luck unless you switch to an HV PIC and simply run it at 12V with good power conditioning for both PIC and signals.
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