What you have to do is:
1-put a power source first (torch, redstone block...)
2-put a comparator after the power source so that it powers that comparator
3-put a block in front of a comparator so that a comparator powers that block.
4-put a redstone dust infront of that block.
5-destroy the comparator

What should happen is that the redstone dust is supposed to turn off like if you would use repeater instead of comparator, but instead the redstone stays on until you 'update' it like putting a block next to it.