reflow (toaster) oven now earning its keep

RE: profile.jpg

Are you using PID to control the temperature?

Have you considered adding a servo connected to the oven door to speed up the cool-down cycle? How about a fan?

Nice work mmcp42 - I'll get mine assembled one of these months :)

[quote author=Coding Badly link=topic=148358.msg1114872#msg1114872 date=1360626827]

RE: profile.jpg

Are you using PID to control the temperature?

Have you considered adding a servo connected to the oven door to speed up the cool-down cycle? How about a fan?

[/quote] not using a PID, yet need to get the temperature ramping fast enough at the moment there's nothing to control!

and yes, I have seriously considered a servo to open the door also a fan watch this space...

@CrossRoads cheers - and thanks for the initial inspiration :)

Thanks, but I am just following in the footsteps of many others here.

that's what I like about this forum so many generous people leaving footprints for lesser mortals to follow :)

shiny new temperature probe arrived today :slight_smile:
doesn’t work :frowning:

took it to pieces (what else! :slight_smile: )

found the problem

the two wires are insulated with what looks like glass insulation
then slipped inside a stainless steel “knitted” (thank you Mrs Noah) outer jacket

trouble is at the business end the trimming was awful
with strands of wire touching the probe
and shorting it out intermittently

so - trimmed it back - much easier said than done stainless steel is tough!
wrapped the end of the metal in heat shrink - just to hold it while I re-assembled

temperature readings now rock steady :slight_smile:

but :frowning:

as the probe is inside a metal sleeve, there does seem to be some inertia in the measurement now
the graph below shows the problem

  • (apparent) delay in getting started
  • overshoot at the intermediate temperature points

a naked thermocouple is much more responsive
(see the original curve posted earlier)

I may well add PID processing - which should stop the overshoot
next decision after that’s all working

oh and for amusement here’s a picture of the first probe
not completely heat proof methinks!

added link to probe

mmcp42: as the probe is inside a metal sleeve, there does seem to be some inertia in the measurement now

"Dead-time" in PID jargon.

I may well add PID processing - which should stop the overshoot

Not necessarily. There is a conflict between getting the response you want, the dead-time, and overshoot. If possible, you should first strive to reduce the dead-time as much as possible. That will make working with PID much much easier.

That stinks. Guess I'll stick with bare probe when I get around to assembling.

[quote author=Coding Badly link=topic=148358.msg1118019#msg1118019 date=1360790198]

mmcp42: as the probe is inside a metal sleeve, there does seem to be some inertia in the measurement now

"Dead-time" in PID jargon. [/quote] I knew that :coughs: !!

I may well add PID processing - which should stop the overshoot

Not necessarily. There is a conflict between getting the response you want, the dead-time, and overshoot. If possible, you should first strive to reduce the dead-time as much as possible. That will make working with PID much much easier.

indeed I'm looking at removing the end of the metal sleeve that way the thermocouple will be nicely inside the oven but still in free air (aka dangling!)

Probably breaks the budget but could you use a non contact sensor like this pointed at the PCB?

Riva: Probably breaks the budget but could you use a non contact sensor like this pointed at the PCB?

good thought a) cost exceeds £$€ spent so far! b) will it work through the glass door? c) actually measuring PCB temp would be really neat! still good thought though :)

Have been trawling eBay and found this that may hack to your needs.

I decided to “adjust” the temperature sensor…

stripped it down again
the plan was to “simply” saw 1/3" or so off the sealed end,
so the thermocouple would hang in the middle of the oven
took a hacksaw to it - no effect
looks like it’s stainless steel

tried a Dremel with one of the little “parting” discs
knife through butter!

cleaned the end with the same disc
so now have a slightly shorter tube
with thermostat out in the open

tried a profile run last night
MUCH closer to the first graph (several posts ago)
close enough that the dead time seems to have gone away

so next step is to look at stirring PID control into the equation(s)

modified probe and resultant profile shown below

IMG_0114b.jpg

ok PID now implemented much nicer looking profile you can see target and actual temp and also the PID control turning the heater on and off :)

I'm also wondering about using some loft insulation to pack the sides of the oven might make the up-ramps faster?

need to get cooldown quicker

time to get out the metal cutting tools and install a fan shaped hole :)

profile5.jpg|588x579

ok took an axe to the oven to day well almost

decided to move the bottom heaters up to the top of itself it didn't make much difference

big bonus is I can now put what they call the crumb tray on the top shelf effectively cutting the heated volume to around a third

by fiddling with the PID parameters I now get this curve (thanks to retrolefty for a steer there!)

so I'll probably stop playing now and get on with the day job (apart from adding a fan in the bottom, which now stays at room temperature :) )

profile8.jpg|595x581

I thought your other curve followed your desired profile better.
Am guessing its not all that critical if it can be controlled by hand just rotating a temperature control knob and a thermocouple stuck it the tines of a fork to hold in place near the card being reflowed …

further update on the probe that I had to fix
before leaving eBay feedback I emailed the vendor
very responsive - bottom line got £1.00 refund
I’m happy - the probe works and it (now) only cost me £1.52
the fix was dead easy
here’s a picture of the problem/fix

the red circle shows the stainless steel outer after I cut it back
the blue circle is where the outer stopped originally
with the frayed wires shorting out the probe

cheers
Mike

IMG_0115.JPG

been a while
tinkered a bit (as I tend to)

here’s the latest EAGLE files and sketch if anyone is considering going down the same path :slight_smile:
enjoy

if anyone does find it useful or makes any changes - do let me know :slight_smile:

cheers
Mike

ReflowOven.zip (58.9 KB)

ReflowOvenPID.zip (14.5 KB)

Awesome work~!

mmcp42: here's the final circuit (went through several iterations to get there - as you do!) and here's the triac heater control

In your circuit, what's the purpose of R5? Also, I see you have the mains hot connected to MT2 on the triac. Does it matter which way the triac is wired? Would it make a difference if MT1 went to mains hot and MT2 went to the oven?

heater.jpg|900x404