5V Rail on Polaris GPUs

Here we'll talk about the 5V rail on Polaris cards, how it works, what it is used for, and the potential problems with this rail and how you could fix them.

The Controller Circuit
Polaris cards most often use a simple LDO (Low Drop Out) regulator to create 5V out of 12V_BUS.

Different manufacturers use different LDOs but the reference one is "MC78M05CDT" Which has 3 pins, Vin, Vout, and GND.

5V is the first rail to turn on. In some high end Polaris cards, 5V is created by 2 LDOs or in rare cases, a step down converter(s) similar to the one used in Pascal series cards.

Usage
5V is used to power other ICs on the card through their Vcc pins.

5V is also used for HDMI power through F400 fuse as shown in Figure 1.

No Voltage on the 5V rail
Quite possibly the most common issue, and most common culprit is the LDO itself if the 5V rail is not shorted to ground.

5V Short to ground
5V supplies the VCC voltage for almost all the ICs on the board, thus you should look there first. If not, a capacitor might be shorted.

To find the source of the short, Follow the steps in this guide: Base Voltage Rail Short on Pascal GPUs.