I am using this heater along with this thermocouple and this amplifier and am just trying to be able to set and hold a consistent temperature. I have the heaters connected to a transistor circuit and I'm running a simple PID loop using a Raspberry Pi so that when the temperature is lower than the setpoint, the pin connected to the transistor switches to HIGH and the heater heats up and vice versa.
I'm having a really weird issue however where when I first start running the program and set a temperature (that's higher than room temp), I see the Raspberry Pi pins pulled high as expected, but the heater's temperature doesn't change (i.e. they're not getting any/enough voltage). HOWEVER, when I go to measure the voltage difference across the two heater wires using my multimeter, it suddenly starts working (before you ask, the multimeter pins don't touch anything other than their specific wire lead, and those don't touch each other or anything else at any point). It doesn't matter how long I wait before I set the temperature or before I measure the voltage, it always just triggers everything to work perfectly.
I've tried using other multimeters and the same thing happens. I have zero clue how measuring a voltage with a multimeter does anything to impact the functionality of my circuit and don't really know where to go from here. Any ideas?