Hot end heating up then dying

I've built my first mendel and...

The first issue I had was the hot end heating up. I set the temp in pronterface to 100 and immediately watched the temp climb up quickly to about 88 degrees then it stopped and slowly came back down. I turned it all off and went to bed and tested it again this morning. Dead - no heating up at all.

I decided that the only way to fix it was build another hot end which I did (I have two spare for the tricolour). I kept the hot end and bowden etc but changed the fan, the thermistor and most importantly, changed the heating element for a new one from another hot end kits supplied. I would have just changed the heating element but I couldn't remove it from the connectors!!

So I turned it all on again and did the same test. This time I set the temp to 50 degrees and immediately the temp rose to 55 and then slowly came back down - but kept going back down to room temperature again. I set it to 100 degrees. Nothing. Dead again.

Any ideas?

Your PSU might not be big enough or your electronics might not have a large enough MOSFET or the right MOSFET or a polyfuse issue. Heating up and dying is a symptom of those failures. Some ATX supplies want you to have a load on one of the 5 volt rails for reliable operation though I've never had one that needed that. If you got an inexpensive controller board the components might not be to spec. If the polyfuse is faulty once the current reaches a point the fuse opens and the heating stops. Difficult to say what it is without either seeing it or having more info.

To help others help you, post what kind of electronics and PSU you are using in addition to what firmware type you are running just in case.