Lock Screen going black

Hi, sorry if this has been answered before, but I have just got a Mac Mini and using an HDMI monitor. The issue I am having is when I lock my screen to take time away from the computer. I use ctrl+CMD+Q to lock the screen and it goes to the log in screen awaiting my password, but within a few seconds the screen goes black and this puts my monitor to sleep. Is there anyway to prevent this? Say to keep the lock screenon lock screen for at least 30 minutes?

Thanks

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Also before anyone says monitor settings, I've also used the computer through HDMI with my TV and the same thing happened. So I'm assuming it's a setting within the Mac itself?

Yup, that's the way it works, with macOS High Sierra.

So, given that's how it works, you should look to see if there is a monitor setting that will wake up the monitor when a signal is present. That way, if you tap a key on the keyboard, the Mini wakes up and then the monitor sees a signal and wakes up. That's how my wife's mini is set up.