making a bench top psu

Yes, I think it's a faulty PSU, you can do one thing to verify that.

You can buy three 12V halogen bulbs, those are 60W each, and you can put three in parallel to load test the PSU. The point is to load test the thing so you can use any methods you want.

if you find that your PSU is unable to drive the load, its probably caused by the driver circuit inside the PSU

Attach a photo and I can verify that.

The problem is not with the PSU but with the load line in 5V.

ATX based Power supply units will trip in 2 seconds or less if you turn it on by shorting the Greena and Black wire and getting output from other than 5V supply line.

Because, the computer motherboard uses the 5V line and the PSU first check the 5V load line and start to regulate the 12V line afterwards. I dont know why it is done but I can assume that the first priority is given to the Processor and motherboard peripherals to fulfill the inrush huge currrent.

However, this problem can be solved by connecting a load in the 5V line and accesing the 12V output. It will work as expected.

Make sure to connect a 5W resistor on the 5V line and connect the load in the 12V line.

After that, if you want to test the 12V output you could follow the way as described by Debasish Das.

A bench power supply is an extremely handy bit of kit to have around for electronics hobbyists, but they can be expensive when purchased from the market. In this Instructable, I will show you, how to make a variable lab bench power supply with a limited budget.It is a great DIY project for beginners as well as any one interested in Electronics.
The main objective of the project is to learn how a linear power supply unit works.In the beginning,to explain the working principle of a Linear Power Supply, I have taken an example of LM 317 based power Supply.For making the final Power Supply, I purchased a Power Supply kit from Banggood and assembled it.This is a high quality stabilized voltage supply with which the voltage can be regulated continuously, and the range in which to regulate the voltage is 0-30V. It even contains a current limit circuit which can effectively control the output current from 2mA to 3A with the ability to regulate the current continuously, and this unique feature makes this device an indispensably powerful tool in the circuit lab.