Hello!

I am trying to develop a small heating project where I will create 5 different temperature zones across an area that is about 5 inches long and 1 inch wide, with a temperature zone every inch (temperatures ranging from about 35C - 75C). Each zone will have a dedicated thermocouple and PID loop to control heat in that zone. The heating elements I'm using are custom made and as such have slightly different resistances. In the past I've used kapton thin-film heaters from Omega which had matched properties so wiring them in parallel was no problem. Here though Im finding that the differences in resistance in my custom elements are causing some elements to heat much more than others, which would, of course, lead to even worse variability as each zone switches on and off in an attempt to regulate to the setpoint. I'm wondering what approaches I might use to overcome this issue -- it seems like it should be an easy problem and I'm missing something obvious, but short of having a dedicated supply for each zone (which wouldn't be the end of the world given the scale and current requirements [ ~ 800 mA per element ]), I'm at a loss. I assume there's some kind of scheme for delivering constant current for each heater individually but I'm still pretty new to this stuff and most of my experience is with embedded circuits.

Thanks in advance for any input!

 

You need 5 DC to DC converters that will cost just a few dollars each and add your circuit to the output of each.

 

I knew I was missing something obvious. This should get the footprint down where I'd like it. Thanks!

 

I am trying to develop a small heating project where I will create 5 different temperature zones across an area that is about 5 inches long and 1 inch wide, with a temperature zone every inch (temperatures ranging from about 35C - 75C).

While I don't have an answer I do have a question. How are you going to insulate the different zones from heat migration? The transfer of heat to a lower temperature zone from adjacent higher temperature zones? That, to me . sounds like a big temperature difference in a small length of space.

 

Do you have thermal barriers between the zones? If not, the 5 controllers might fight each other.

 

There will definitely be some insulation between zones. To start probably just an air gap coupled with thermocouple placement nearer the center of each zone, but there may be some experimentation with that if air doesn't do the trick. We also may draw back a bit on that temperature range and do something more like 30 - 60 or 30 - 50C, it will depend on how the samples react and what gradient we find works.

 

Just to add on to that so it makes a bit more sense -- the heating elements double as the sample holders so there doesn't have to be any physical contact between them, they just need to be compact enough to fit within the working range of a moveable microscope stage. There will be some heat loss/transfer on the ends of each zone where they contact whatever mount I come up with, but I will likely machine this in something that will insulate the elements from one another and the metal stage, in addition to having the air gap between each element. Hopefully that makes sense, haha