Read page 18 and 19 of this post. Danf has explained the problem clearly.
Meanwhile I am using a 24V PSU only for the heatbed.
Thanks :-) after posting my pictures i went back a couple of pages and read the whole discussion.
I Already looked up some Solid State Relays and i think i will go with a SSR-40DD Relay like @tommeysen is trying to implement. http://www.ebay.de/itm/141023030534?_trksid=p2060353.m2763.l2649&ssPageName=STRK%3AMEBIDX%3AIT
That should be good up to 40A and thats well over the 23A the heatbed is taking at 15v and see how this will work out, because at the moment buying another psu and new cables and all is out of the
question at this point from the budget side of things ;-) and i dont really want 2 powersupplys hanging under my printer.
Still a SSR-40DD Relay should be a good replacement for the powerexpander, we will see how it goes.
Meanwhile i think i will order a new power expander as a backup plan if i consider switching to a 24v powersupply.