Can the 3.3v output on atmega328p sink current? Is it safe to use the 3.3v and 5v pins to create a 1.7v source?
No.
What are you trying to supply 1.7V to? Do you realize that if you got this to work, the 3.3V line would be "ground" to the load?
It seems like a bad idea. And of course, your 1.7V isn't referenced to ground.
1.7V referenced to 3.3V would be an odd requirement. And if you accidentally ground it... poof!
I believe the current will "sink" through the existing 3.3V circuitry on the board. A few microamps through a multimeter should work fine and you should be able to measure ~1.7V between the two points. How far you can push that.... I don't know...
li20034:
Can the 3.3v output on atmega328p sink current? Is it safe to use the 3.3v and 5v pins to create a 1.7v source?
There are better ways to get a 1.7v source without risking the longevity of your Arduino.
Assuming you mean the pin marked 3.3v on most Arduino boards (which is not connected to the ATMega328p at all), no. It's just a bog standard LDO regulator, they can't sink current, and may be damaged if you try to do that.
If you mean an output pin on an 3.3v Atmega328p, you can let a very small amount of current flow through the protection diodes. You generally shouldn't, but you can if the current is low enough (IIRC, atmel suggests not more than 1mA - they certainly do that in some of their app notes) - but this wouldn't give you 1.7v, since there's voltage drop across the protection diode, and it would be referenced to the 3.3v+diode drop... (which varies with current)...
To get 1.7v, use an external 1.7v regulator, or a DC-DC stepdown converter depending on your needs.
OK thanks for helping me keep my Arduino safe. But why are linear voltage regulators unable to sink current? Also, can a switching voltage regulator sink current? I also want to use the 5v and 12v rail on computer psu to get 7v for powering a bunch of computer fans (12v too loud, 5v spins too slowly).
LDOs typically don't have a way to sink current because this is neither necessary nor desirable for typical use cases; specialty parts which can also sink current may exist, or you could use external components (I think that's what's typically done - a zener or something).
Switching power supplies may or may not, depending on their design.
You can definitely do that on a computer power supply. It's in fact very common to use the 5v and 12v to get 7v for fans, and commercial products (typically computer fans too cheap to support PWM speed control) which do this are readily available.
li20034:
I also want to use the 5v and 12v rail on computer psu to get 7v for powering a bunch of computer fans (12v too loud, 5v spins too slowly).
Better to power them from the 12volt rail with a cheap DC/DC buck converter from ebay (LM2596).
One of those boards can drive a bunch of fans.
And they have a trimpot to set the voltage (speed).
Leo..
DrAzzy:
You can definitely do that on a computer power supply. It's in fact very common to use the 5v and 12v to get 7v for fans, and commercial products (typically computer fans too cheap to support PWM speed control) which do this are readily available.
But only when the 5V supply is heavily loaded by the computer motherboard.
We still have not heard the excuse for wanting 1.7V.
The output of a linear regulator can never sink current in a safe manner (you will cause the voltage
to rise and risk anything else powered from that line). Certain switchmode topologies can run current
in either direction, but you need to check chip-by-chip, and you need the incoming supply to sink
current too as a switch-mode device is simply passing the buck (pun intended).
As Paul__B says, running a computer fan between the 12V and 5V line on a PC power supply only works because there are loads on the 5V line. The design of the power supply is such that it won't work if there is no load on 5V.
In fact, I was using this method to run a large fan slower, then I replaced the power supply and the new one would not even start up with the fan connected.
Look at the schematic of any linear or switch mode power supply and you can see that they are not designed to sink current and are not able.
I'm not counting simple zener or shunt regulators.