I'm trying to use an Arduino Pro Micro with a wifi module, which operates on 3.3V
I'm using an LM317 to regulate 5V to 3V, and it works fine on an Arduino Uno, but doesn't work at all on the Pro Micro.
What appears to be happening is that the Vcc available on the Pro Mico is less than 5V (around 4.7V) and is collapsing as soon as the wifi takes its start up power
Can anyone think why this is happening, because as far as I'm aware the only thing between the USB 5V and the RAW and Vcc pins on the pro micro is the resettable fuse
The most likely cause is that you have overlooked the very well known fact that linear voltage regulators
need headroom (additional input voltage above and beyond the output voltage to run the circuit ) so
5V-3.3V is what ? (ans: 1.7V) There is no way 1.7V is enough headroom for an LM317.
There are such things as LDO (Low Dropout regulators) but the LM317 is not one of them.
The datasheet term for headroom is "Input-Output Differential Voltage (minimum)"
If you knew what to look for you would have found it on the LM317 datasheet
on page 4 , Section 6.3 Recommended Operating Conditions:
Input-Output Differential Voltage (minimum)
the first number (3) is under the "Min" labell
VI – VO Input-to-output differential voltage 3 40 V
That makes you 3-1.7 = 1.3V SHORT for the LM317 input voltage.
In short, you need more input voltage for the LM317. I would think 6 to 9V.
It's just not going to work with only 5V input.
Are you trying to power it from "Vin" and using the on-board regulator? That is a very bad idea; you need a proper regulated 5 V supply for any serious project, likely from a switchmode regulator.
If you are feeding it by the USB connector, there is a diode in series with this, dropping a significant part of a Volt.
In any case, if you have a power supply with a higher voltage, that is what you should be using to feed the LM317.