imac 24 powers off by itself

First time poster. My imac 24 powers off by itself. after long periods of being off (overnight) it will start-up fine and run for about 4 minutes - then shuts off after a very brief moment of the spinning colour wheel.
After re-starting it shuts off within 1 minute and after re-restart it shuts off within 20 seconds. Someone suggested a logic board problem ($1100+ to repair - yikes). Someone else suggests power supply. Anyone with this experience? Thanks to all!

Download a program that monitors the heat and fan speeds to see if perhaps your cooling fan is not working. A non functional cooling fan will cause the machine to overheat very quickly and shut itself off.

Download SMC Fan Control from here.

Hopefully, your machine will stay on long enough for you to install the program and get some readings.

Regards.