I have run into a snag

. I get the panel powered up, got 35v output (phidgets handle a max output of 30v) on one light, ok, no problem just add a resistor and be done with it. Problem is, the more lights I turn on the more it loads and the lower my voltage goes. I need a variable type of resistor that adjusts itself to maintain 28-30v no matte what the load is.
I know that people will say "change the i/o card""buy a new power supply", etc. i dont have a lot of money to get stuff like that, I am trying to do this as cheaply as possible.
What I am asking, is if anyone has any ideas how I can control current cheaply?