My project uses a Arduino controller. My project box has 5V, 12V , 24V and 120VC. All of which can fry the IO pins and the latter 3 the power input. I have never designed or learned about overvoltage protecting pins. A loose wire or bad connection would be disasters and being careful sometimes isnt safe enough. I would like to make sure that the power input (5V) will be protected by any of the higher voltages. Also protecting the input pins would be nice.

Can someone help me?

 

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Cover the pins with hot glue?

If you are talking about a loose wire, that is about it..

If you want a circuit to protect against over-voltage, just in case you tried to connect a 120v line into the 5v input, that is a different story.

Really, it would be cheaper to buy another $3 uC and be more careful next time.

Without ACTUALLY covering and sealing the pins and solder joints..even potting the board, you wont be able to protect it from stray "falling wires" and the such. You will just have to work that into you design. Keep the analog signals separate from the digital ones, and keep the high voltages away from the low.

 

I actually want to do both. I blew put a $150 board because a 120 line became loose and hit ground. I'm hoping with separating the connections better, hot glue, and tape will help. However a overvoltage protection circuit would be great as well.

 

A metal oxide varistor will help.

Such a resistor becomes conductive when the voltage across it exceeds its threshold.

If you were to use a 30V MOV, then 120V would be shorted to ground. Your MOV would absorb the surge.

Sounds great, but absorbing that energy is difficult. The MOV can only survive a few seconds at best with a direct short before it starts burning.

The best solution is to just separate the signals with a decent boundary. Keep high voltage isolated from low.

 

I dont think that will help in his case.

If his 120v line touched ground and blew the board, the MOV would just conduct the 120v over to the 5v line.

Hrm...

 

I should probably add this. While breadboarding my latest project, I have 5V and 3.3V rails. I run a dsPIC33F micro on 3.3V, and if the 5V were to short with the 3.3V - bang goes the PIC, which has an absolute maximum rating of 4V. The solution to this is to use a short circuit protected PSU (in my case a computer/ATX power supply worked very well) and have a 3.9V zener across the rails.

The 3.9V zener will short out if presented with more than 3.9V. In a few seconds it will be toast and useless, so you will have to buy a new one (about 2 pence.) But it will continue to short until it either breaks in half from the heat or the power supply trips.

Either way, it keeps the voltage at around 3V when in the shorted state. And the smoke and smell gives enough warning to grab the power lead. There is a brief 5V pulse <1μs (measured with a scope) but in my testing this didn't do any harm to the micro. Although the zener is toasted, and needs to be replaced, it saves the £4 microcontroller.

@retched: the MOV would be on the 5V line. Attaching 120V hot to the 5V line would short the MOV, protecting the device. However, damage would most likely still occur, as the MOV does not engage instantly.

There are other devices you can use. Spark gaps and TVS devices come to mind. These activate very quickly.

 

There is nothing (circuitwize) you can do to protect a chip from a falling 120v line.

A diode or tsv or MOV, whatever, will not do squat if the current doesn't follow the "protected" path.

If the 120v wire falls onto the pins of the uC, no protection circuitry will save the uC.

The little diodes will reverse WELL before 120v.

Did you destroy the board or just a component or two on the board?

If you fried the uC, $4 gets you another. Maybe a few caps... But you shouldn't have to replace the whole $150 board.

What board is it?

 

Its a GHI Cobra. I found the 2 onboard regulators were shot so I replaced them. After that the EMX uC would get immediattly hot. There isnt too many other things on this board. Potentially the regulators and EMX could be replaced but that is a $95 investment which "might" work. I deciced to get a new board. Im going to physcially seperate my lines better and make sure my PINs are well protected. Oh well. Was hoping I could do better then just not being a knucklehead!

-Bixter

 

Ahh... Well.. Thats one expensive uC.. I can see your urge to want to protect it.

GEEZ.

I hope you are using this thing for the power it has and not just to light leds

Now you've really got my interest. What are you implementing?

Is this what you are talking about? The "hyper arduino-like" rapid prototyping COBRA?

http://www.tinyclr.com/hardware/12/fez-cobra/

 

One solution would be to have two separate boards (or at least isolated sections), one carrying 120VAC and the other dealing with low voltage logic. The MCU pins could be protected by putting hot glue around them. But the solution is just to be careful in this instance.

 

Im using it to control a uBrewery I am building. It web enabled and all. It is a little over powered but , hey, this is for beer!

 

Well, for beer.... Its got to be done.

Why is their a 120v line in the area of this board?

 

Thats what I thought when I blew it up! Everything is physically well protected now so I shouldn't make another blunder. Its my first real physical build and a tough learning experience. Everything works like a charm though. Matter of fact I have another question Im going to post shortly.

Thanks for all the help!