Toaster Oven PWM Control - Page 1
Forum section: electronics / projects

author: StuUK
author_rank: Frequent Contributor
timestamp: January 31, 2016, 02:52:43 pm
content: HiI've been investigating various examples of converting toaster ovens for SMD reflow. Lots of examples out there utilising various techniques but one of the most common problems seems to be controlling the temperature profile accurately.It struck me that PWM control would be a better approach rather than full on/off of the heating elements but what I am unsure of is the best approach i.e. controlling Triacs or IGBT's for example... With PWM I could control the ramp of the temperature with more accuracy using appropriate feedback. In addition I'd rather just keep the whole controller separate, i.e. just plug the toaster in and put the thermocouple in the oven to avoid hacking the oven.Any thoughts?

author: dannyf
author_rank: Super Contributor
timestamp: January 31, 2016, 03:24:18 pm
content: That would be possible. Given the thermal mass, it is likely even a solid state relay would work, significantly reducing the design difficulties.

author: Paul Price
author_rank: Super Contributor
timestamp: January 31, 2016, 07:37:06 pm
content: Any attempt to measure oven temperature in a non-adiabatic apparatus, aka toaster oven, ends up being an historically semi-accurate spot measurement.Just use a simple TRIAC zero-crossing optocoupler combination to control power on a full cycle by cycle basis, the whole thing orchestrated by MCU timing and temp control. You don't need PWM.In every case to get something like this to work as your want, remember "Practice makes perfect."Every cook learns to carefully save their best recipes.

author: StuUK
author_rank: Frequent Contributor
timestamp: February 02, 2016, 09:19:54 am
content: Thanks for the reply guys, still mulling this over....

author: Codemonkey
author_rank: Regular Contributor
timestamp: February 02, 2016, 09:59:04 am
content: I built a toaster oven PID controller that does PWM control of a solid state relay with built in zero crossing switching. Its a while since I built it, but I think I used a pulse repetition rate of a few seconds and it seems to work well. Originally like you I fancied building something a bit more complex and was going to add on phase control using triacs but in reality, the thing reacts so slowly to the heaters being on it really doesn't need more fine grained control.The benefit of doing it this way meant I was able to build it completely separately in its own box with a 13A socket on the back to plug the completely unmodified oven into.I've done a few boards in it since and it seems to follow the profile I programmed into it (lead free) really well.

author: ve7xen
author_rank: Super Contributor
timestamp: February 02, 2016, 10:05:21 am
content: This may be obvious and not what you're after, but you can purchase a prebuilt PID controller, SSR and thermocouple from China for about $30. Put in a plastic box with mains cable and wall outlet, do a bit of tuning, and you're done.

author: mjkuwp
author_rank: Unknown
timestamp: February 02, 2016, 11:16:09 am
content: The primary difficulty is in getting the feedback. Here is one wayhttp://themzlab.tumblr.com/tarnishingThe oven responds so slowly that high-frequency PWM is not required. My controller works at 2Hz so I probably could have even used a mechanical relay.I do all surface mount in the oven and any misc. thru-hole by hand after I remove the board from the oven. This allows the use of this blackened copper sheet. I've done many, many boards now without any damaged ones and the only quality defects come me sometimes having too much paste on the pcb.

author: StuUK
author_rank: Frequent Contributor
timestamp: February 03, 2016, 12:34:14 pm
content: Quote from: ve7xen on February 02, 2016, 10:05:21 amYes, looked at PIDs but I want direct control over software I will write and want the challenge of building it as well...

author: StuUK
author_rank: Frequent Contributor
timestamp: February 03, 2016, 12:38:02 pm
content: Quote from: Codemonkey on February 02, 2016, 09:59:04 amMakes sense although where my head is currently at is writing software which learns the particular dynamics of the toaster oven being used and then using that to develop the necessary profile automatically and using feedback simply to determine if things are on track. This is why I'm thinking that PWM makes a bit more sense than simple on wait off....

author: mjkuwp
author_rank: Unknown
timestamp: February 03, 2016, 06:33:38 pm
content: PWM is on/off, it is just a matter of speed. The thermal time constant of the heating element and the material around it, as well as the oven itself is what leads me to believe the PWM frequency can be really low. It can also be related to the precision of your temperature measurement. The chip I used only had 0.25C resolution so I thought there was no point in trying to do adjustments in control really quickly.I documented the constants I arrived at here:http://mjkuwp94.tumblr.com/PIDTempandhttp://mjkuwp94.tumblr.com/PIDSlopeIt is hard to communicate this kind of thing and my method was to use real physical units in my documentation so that it could be transferred to any type of code.I have two modes (preheat) and (reflow profile) and that is why I developed two sets of constants. For the reflow soldering I do not control temperature with the PID loop, I control the slope of temperature as I found this much easier to do. A state machine determines when to switch between the different parts of the cycle.I fully expect almost every person to build their oven with their own unique ideas. I see this is as a 'thing' in the community. Tons of people make reflow ovens and every one is just a little bit different. I certainly don't expect anyone to duplicate mine but I wanted to try to document it completely as if that would happen. My point was to practice my documentation skills and try to contribute at least one new thing.I'm sure you'll have fun and learn a lot : )

author: mubes
author_rank: Regular Contributor
timestamp: February 04, 2016, 02:20:16 pm
content: Folks,If it's useful there's a complete oven implementation athttp://www.marples.net/Leater....it's open source and you're more than welcome to use it, or bits of it!DAVE

author: Kjelt
author_rank: Super Contributor
timestamp: February 04, 2016, 02:23:39 pm
content: PWM is not the big issue with a reflow oven. The heating elements should be quick enough to follow the profile esp. at the end where in a short amount of time the temperature has to be risen from 160oC to 230oC is a real challenge. I have a decent factory made oven with 6 IR 500W ceramic heating elements and that last part is the difficult one, it takes too long to reach the 230oC and time runs out to follow the curve. So you have to start the heating before to match the curve and put in a lot of targetpoints in the curve so it stays on heating but does not overshoot too much.

author: StuUK
author_rank: Frequent Contributor
timestamp: February 05, 2016, 10:23:02 am
content: Thanks for the input, seems I need to think about this more.. The point about the quick rise at the end is really interesting...