# resistors

The book I read says to use 560 ohm resistors but they cannot be purchased at .25 watt only .5 watt
would this blow the 0.40 ma limit… 560 ohm .5 watt ???

8lenn:
The book I read says to use 560 ohm resistors but they cannot be purchased at .25 watt only .5 watt
would this blow the 0.40 ma limit… 560 ohm .5 watt

While I feel your pain, you evidently need a bit of education in basic electrical theory.

As we know not only half watt, but the more common quarter as well as one eighth Watt and SMD resistors are perfectly readily available, the question gets back to - from which supermarket or grocery store are you trying to purchase these?

OK, so eBay is the slow boat from China (2 to 3 weeks!), but what we use a lot.

You should have read this first:

How to use this forum

Get some 470 Ω

0.5W RATING is not necessarily what it takes.
Indeed you should always make sure you stay well below that, preferably have the resistor not dissipate more than half its rated power if used continuously.

And as long as you keep the voltage the pin outputs at less than 22.4V you’re not going to get more than 40 mA to flow through that resistor.

8lenn:
The book I read says to use 560 ohm resistors but they cannot be purchased at .25 watt only .5 watt
would this blow the 0.40 ma limit… 560 ohm .5 watt ???

0.5W is the maximum power the resistor can safely handle. Less than that is perfectly acceptable.

And it’s the resistance (Ohms) and voltage that decide the current in a circuit. A 560 Ohm resistor is still 560 Ohms whether it could handle 1/4W, 1/2W or 20W.

BTW if you’re talking about the maximum Arduino pin current it’s 40mA (or 0.04A) not 0.40mA.

Steve