Maximum Current/Voltage into an analog pin on an Arduino Uno

Grumpy_Mike: If you are only using the analogue inputs as analogue inputs then simple don't do anything it gets set automatically. If you are using them as a digital input or output then use pin number 14 for A0, 15 for A1 and so on.

The 5V input limit only applies to an arduino that is powered from 5V, lower voltage arduinos like those that run from 3.3v then that is the limit. This applies to not only analogue pins but also to digital pins.

Hi, I have a question here. Do I understand you correctly that when you power your arduino with a 12V voltage source, then it is allowed to feed up to 12V to your analog in?

Regards

No. Because the chip is only running from 5V, the 12V from the power jack gets regulated to 5V before it powers the chip.

I'm just now getting my MEGA hooked up, and looking at what kind of input protection to put on it. Has anyone had problems with their 'duino getting fried (like from parasitic powering) when they already had 47K ohm resistors and zeners protecting the inputs? (if not, then that's what I'll be using)

What voltage zener do you plan to use?

What fault current and duration do you expect the zener to protect against?

Have you heard about TVS diodes?

MorganS:
What voltage zener do you plan to use?

What fault current and duration do you expect the zener to protect against?

Have you heard about TVS diodes?

I figured on using a 4.7v zener since I’ve read 5v is all it can take, and the next zener value up is 5.1v. The plan is to use it to test TTL and CMOS circuits that are in unknown (and possibly malfunctioning) conditions. But I don’t know how much fault current is safe for the duino, so I’m trying to have it as low as I can. I attached the planned zener circuit.

but if mere microamps can cause latch-up where the duino isn’t on when input is applied, I’ll have to go with transistors to protect the input, which will be more circuitry. Because those will reduce the unpowered fault current to zero.

I have considered TVS, but their clamp voltages just aren’t consistent enough. Besides, a half-watt zener is more than enough to clamp 1mA to 5v.

duinoin.jpg

That is fine for digital inputs.
It caused a problem on analogue inputs though because a Zener starts to limit before the quoted knee value.

If your fault current is 1mA then the internal protection diodes in the Arduino are sufficient. You don't need either of those diodes you drew. That will keep your pin voltage below 5.3 and above -0.3. The datasheet isn't clear on what current you can put through those diodes but the consensus on the forum here a year or two ago was that 2mA was OK.

I would be worried that the circuit as drawn will lose too much current to the zener (operating "below the knee") and valid HIGH inputs won't be able to draw the Arduino pin to HIGH. Work out what voltage you need on the left-hand end of the 47K to achieve a valid HIGH on the Arduino pin. I predict it is over 5V.

TVS diodes are intended to clamp transients. Spikes and whatnot. They have a limited amount of energy they can absorb before they explode. So they can't take continuous faults and you don't design them "close" to the normal operating voltage. A 7V TVS may be used on a 5V system. Using the circuit you showed, a 7V TVS plus the 4.7K resistor will limit the fault current into the Arduino pin to under half a milliamp and it can handle short term spikes of extraordinarily high voltages and currents.

Another useful reference is the "Ruggeduino". They used polyfuses on all the inputs. A polyfuse can withstand a continuous fault because it doesn't try to absorb the energy. It disconnects (goes to high resistance) with high current.

"The datasheet isn't clear on what current you can put through those diodes but the consensus on the forum here a year or two ago was that 2mA was OK."

The only place a number has been seen is in the Zero Cross Detection application note where it says not to exceed 1mA. Has anyone done a long duration test to see how much they can really withstand? I don't know.

MorganS: (snip) I would be worried that the circuit as drawn will lose too much current to the zener (operating "below the knee") and valid HIGH inputs won't be able to draw the Arduino pin to HIGH. Work out what voltage you need on the left-hand end of the 47K to achieve a valid HIGH on the Arduino pin. I predict it is over 5V. urrent.

I tried the circuit I drew. the input went from 0 to 1 at about 2.5v on the left end of the 47k. Basically, at anything less than 4.7v, the zener draws no current at all. So, if 1mA of unpowered-parasite current per input is fine, then I'll go ahead and build 16 input buffers and see how it goes.....

vivekkameshwaran: I am using Arduino UNO , by mistake A0 analog pin is got connected with 15V AC for few seconds , then my laptop got off , I disconnected everything. But now my Arduino Uno is not shown in the Port , but it is connected with my laptop and the power Led in Arduino is ON , how to solve this problem?

15V is too much for ATMEGA 328P microcontroller. Additionally, it was AC voltage, which probably destroyed the internal circuitry as well. Actually its surprising that the power LED was OK.

Just out of curiosity, did you fix your Arduino Uno?