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West Sussex, UK
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Hi, a newb question...

I understand that there is a 40mA limit on the digital outputs, but if I have for experimental purposes a pot reading into an analog input connected to GND and 5V rails, what is a safe value for the pot? I have seen examples of 10k and 47k, but how do I know what is safe? I have just started playing with an UNO...

thanks, newbyte
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Hi, a newb question...

I understand that there is a 40mA limit on the digital outputs, but if I have for experimental purposes a pot reading into an analog input connected to GND and 5V rails, what is a safe value for the pot? I have seen examples of 10k and 47k, but how do I know what is safe? I have just started playing with an UNO...

thanks, newbyte

That is dependent on the physical size and rating of your specific pot. A datasheet would show maximum wattage dissipation rating. In the old days the rather large size volume control pots were rated at around 2 watts maximum. When wiring to a low voltage source like +5vdc and ground, I would think any pot of 500 ohms or higher would not have be a concern. On the high side of pot values, the Arduino Analog input pins were designed to be feed with an impedance of 10,000 ohms or lower, so 1K-10K is a good compromise between current consumption and impedance drive for an arduino analog input usage.

Lefty
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I understand that there is a 40mA limit on the digital outputs,

............
 but if I have for experimental purposes a pot reading into an analog input connected to GND and 5V rails, what is a safe value for the pot?

Those two things are not related in any way, they are two totally different things.
One is how much current an arduino OUTPUT can source and sink. The other I suspect is asking about INPUTS these are totally different conditions even though they use the same pins.

When a pin is an input (analogue or digital), it takes in very little current for 5V supplied to it so don't worry about putting too much current into an input, you can't unless you take it over 5V which will blow it anyway. For the analogue input testing a 10K pot (or less) is best because it gives the input capacitor enough time to charge when switching input channels.
As lefty says what it does to the pot is another matter.
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Thanks for the replies,

I should not have confused matters by mentioning digital outputs. I put that in merely to show that I had read some documentation at least...
I assumed that if I had a very low resistance pot, that I would draw a lot of current, either wasteful or maybe causing damage to my UNO. I am not worried about damaging my collection of large sized volume pots that you correctly imagined!

many thanks,

lowbyte (must get my handle right)
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I assumed that if I had a very low resistance pot, that I would draw a lot of current,

As I said you can't draw too much current into an input your arduino is safe.
If you think about it the ultimate is connecting the input to 5V and that happens no matter what value the pot is when it is at the hot end.
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The only sort of volume pots that you might have trouble with are the type sometimes used to control volume by direct connection to the speaker, rather than the amplifier input stages.  The direct speaker pots are very low value, typically around 10 ohms and invariably wire wound.
First thing all experimenters should do is buy themselves a digital test meter.  Provided you are not looking for a professional model you can buy these for around £10/$15.  A basic meter permits you to measure resistance values as well as voltages and currents.
jack
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Thanks guys, I've got it straight in my head now. I have a basic multimeter, so I can continue in the knowledge that I won't break anything. When they arrive, my VG400 (vegetronix) soil moisture sensors will replace my experimental potentiometer. For some reason they are being held up by UK customs.

The pot is to imitate the sensor so that I can get some of the coding sorted.

lowbyte
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These are good starter thinkings.

If you look it more clearly, you can see that analog inputs takes direct shorcuts, both ends, either to GND or +5V. Result is immediate, 0 or 255 (8-bit resolution systems), and 0 or 1023 (10-bit systems).

But if you have a load controlled by digital pin, you better learn ohms law, which is actually the simpliest thing in the world, ones you draw it on a paper and do few simple maths.

So, google for ohms law and other power calculations, you will sleep your nights better.

Cheers,
Kari
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The only law for me; Ohms Law: U=R*I       P=U*I
Note to self: "Damn! Why don't you just fix it!!!"

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