Interesting question, something that also puzzled me when I was starting out. Maybe you know what I saying here, but then at least it can help others who don’t know this.
Even though the microcontroller chip can source and sink current in the milliampere (mA) range you mustnt forget that milliamperes are still big compared to the small currents like mircoamperes (uA) and nanoamperes (nA) you can work with in small signal electronics.
According to the way a veasurement equipment must work:
An ideal voltmeter draws no current at all and therefor has an infinite resistance.
An ideal ammeter will have zero resistance.
We don’t live in an ideal mode, but when they design multimeters they aim is to get as close as possible to having the meter draw no current.
In the old days there were moving coil meters and you still get them, these have to draw small currents to actually move the needle.
Nowadays thanks to digital electronics we have digital multimeters (DMM) which draw almost no current and certainly that is drawn is very very low, I am not sure of the exact current but it will vary across brands. Regardless it will be low enough for almost any DMM/DVOM to be used for digital electronics.
The reason for this is that the DMM’s use FET transistor inputs and FET’s have very high input “resistances”.
You can be sure that the cheaper DVOM’s will probably draw a bit more current than the better quality ones. If you can afford it go for the higher priced ones.
I am relatively new to the Arduino and Atmel land, having come from PIC land. I have never had a problem with PICs even with some of the cheaper meters.
Like pluggy said make sure you don’t measure it on current setting, but thankfully most meters require you to change the socket for the lead when switching to current measuring mode.
I have destroyed a couple of PICs already but accidently shorting pins with the multimeter leads. Even though I am aware of it still happens ;D
Are you driving the triacs directly from the port pin or are you using a driver stage inbetween.