Trying to build a temperature controlled heater

Discussion in 'The Projects Forum' started by mikeyg123, Jun 13, 2018.

  1. mikeyg123

    Thread Starter New Member

    Jun 13, 2018

    I am new to building circuits, and could use some help getting a project off the ground. I work in a lab and am trying to heat a reaction vessel to a desired temperature using an aluminum block, equipped with 4 heating cartridges (see attached image).

    I already own a temperature controller that I can use with a commercial heater. The temperature controller receives input from the mains (label says AC input, 120V, 8A, 60Hz). It is hooked up to a thermocouple (Type J) which is inserted into my aluminum block heater, which provides temperature feedback to the temperature controller. The temperature controller, based on the temperature feedback, then provides power to four heating catrdiges (each are 600W, 5A). The heating cartridges are equally spaced around the aluminum block and are wired in parallel. The power cord I used to connect to my circuit is rated for 13A (which seems like it could be a little low).

    When I turn the temperature controller on, it immediately blows the fuse within the controller. The fuse is rated 8A, 250 V. The commercial heater I own that actually works with this temperature controller says 500W, 115V on it.

    I am wondering how I need to adjust my setup to have it work properly. It seems that perhaps my heating cartridges are trying to draw to much power from the controller? Unfortunately I do not know enough about this stuff to really work it out, but would be interested in learning.

    If I can provide more information, please let me know. I have attached some pictures of the setup if that helps.
  2. wayneh


    Sep 9, 2010
    Bingo. You're putting a 20A load on an output rated to less than 7A. The 800 VA rating equates to 800W in this case, or 800VA/120V = 6.7A. You can be glad the designer put in the fuses to protect you from yourself! :eek:

    There are a few ways to modify things and your choice may depend on what else you have on hand and the approach you want to take. Me? I'd stick the whole thing in a circulating water bath (or oil, if you need a higher temperature).
    -live wire- likes this.
  3. mikeyg123

    Thread Starter New Member

    Jun 13, 2018
    Hi, thanks for your response! I guess I should have read a little bit more about this stuff before I went ahead and built this...

    Unfortunately I am going up to 200-300 oC, which is above what most oil baths can sustain, especially for prolonged periods of time. The commercial heater I have is capable of heating reactions up to this temperature, so I guess it must be possible with some redesign.

    Ideally I would like to keep using the heating cartridges and temperature controller, since I already had or bought them. I'm happy to spend a little more on new components to get this setup working. From a quick google search, it seems I may need to incorporate some resistors into this circuit. Would you mind elaborating on some of the other approaches I could take, or point me to some nice resources where I can learn more?

    Thanks again.
  4. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
  5. wayneh


    Sep 9, 2010
    As noted above you can use everything you have (with appropriate wiring) by using the controller to control a relay beefy enough to switch 20A without concern. This will take the load off of the controller. The heavy wiring is needed for everything up to the relay and between the relay and the heaters. The controller wiring could be kept as it is. Make sure the circuit breaker and wiring in your lab can accept a 20A load.

    Resistors (or lightbulbs) in the circuit could reduce the load, but that would reduce the heating capacity of your heaters. And, the resistors or bulbs would make heat also. In short, you don't want to pursue that route. I though about recommending putting the heaters in series/parallel, meaning two parallel strings containing two heaters each in series. They would draw much less current (perhaps 5A total) and make much less heat, and would therefore have a much lower capacity. If you actually need the full wattage, that's not viable.
  6. Kjeldgaard


    Apr 7, 2016
    An intermediate step could be to see what 600W can provide in temperature.

    This can be done by switching the heaters in parallel two and two, and then in series as:
    The dotted line is not necessary, but may help to understand what happens.
  7. ebeowulf17

    AAC Fanatic!

    Aug 12, 2014
    Depending on how the temperature controller behaves, it may cycle heat on and off quite frequently, in which case solid state might be preferable to mechanical switching. There are countless options, but here's one SSR that I'm already familiar with that can do the job:

    At 20A load you might be able to get away with minimal heat sinking, just bolting it to a large metal surface that stays cool... or you might need real heat sinking. We've run 10A through them with negligible heat sinking with good luck, but 20A might require a real heat sink.

    As others have said, you can keep all of your existing parts, but you'll need to use the output of your controller as a signal for a switch/relay/contactor/SSR that can handle more power, along with upgrading power cable, plug, etc.