@jremington,
Bear with me. I am trying to get the OP to see that he needs a REGULATED 28V supply AND resistors to connect to it and
the current through each resistor to ground (switch by anything from a transistor to an analog switch) will be equal to the value
calculated by ohm's law. I was hoping the OP would EVENTUALLY see what you are pointing out , which is it.that he needs a REGULATED SUPPLY .