I put together a solar thermal differential controller that uses a Pro Mini to read two pt1000 sensors and control a relay (more details following). I first powered it with an old linear "7.5v" power supply (actually measures close to 12v), and everything worked perfectly. I then bought a new 9v switching power supply, figuring it would last longer. When I power it with the new supply, my pt1000 readings fluctuate wildly, +/- at least 10 degrees F. My software uses the median of 10 readings to smooth out noise. I've since tried two other switching power supplies, with the same results: big fluctuations that make my controller useless. With the two linear 9-12 v supplies I've tried, there's no problem.
Other details The whole system consists of: An inside unit consisting of two Pro Minis (from SparkFun): one 5v master that reads two pt1000s and two ds18b20s (located in the outside unit) and writes the data to an attached SD card; and a 3.3v slave (communicating via i2c through a logic level converter) that displays readings on a Nokia display. The inside unit is connected via a 20 foot long cat5 cable to an outside box that houses the relay (powered externally) and sensor connections. The switching supplies I've tried are a 9v 600mA one and a 9v 1A one. All power is connected to the raw inputs of both Pro Minis.
Anyone know why the switching supplies would be giving me trouble?