Hi, I've been doing some measurements and serial observing.
First of all; when connected through USB, I can turn it up as high as I want and turn the nichrome wire into a candle (so the PSU isn't the problem). This makes me think that there might be a voltage drop causing the Arduino to malfunction. My assumption is that the buck converter doesn't compensate for a voltage drop on the PSU.
So I tested this assumption, and it turns out that the PWM signal cuts out when the buck converter's output drops below 4.5V
Some stats on the PSU:
12V primary (drops to 9.7V under load) with 14.2A max current
5V secondary with 1A max current (shorting this rail turns the PSU on)
Stats on the nichrome wire:
3 Ohms of resistance
1.1A~1.2A of current across the wire when the PWM signal drops
I noticed the PSU has a secondary rail with 5V, so I'm going to attempt to use that rail for 5V as I assume this rail doesn't have a voltage drop across it. I'm not sure if the Arduino will turn on the PSU, but I guess we'll see. If the Arduino isn't enough I might try to add a resistor across the 5V rail to see if that helps turning the PSU on (is this a bad idea?).
This is what I've found out so far. The PSU can easily push that wire to the point of melting without breaking a sweat so this also means I can probably use a slightly thicker nichrome wire if the time ever comes where I want to cut thicker foams, but by that time I'll also need a sturdier frame 