Homebrew PCBs

Tony,
You wrote, "I figure I can do it by monitoring the heater, and when it 
switches off for the first time use that to start the timer."

No, that won't work.

  Any decent PID controller properly set up will start to cycle as it 
approaches the setpoint temperature. That cycling point is controlled  
by the need to decrease the input energy to the heater in order to not 
overshoot the temperature significantly.  Optimum would be a 
proportional, phase controlled SSR, but these generate more RFI, require 
an analog input that brings in the need for scaling in the interface and 
cost more per amp controlled.  In practice the time constant of the 
average application is so long, simply switching the SSR on and off for 
tenths and seconds at a time is sufficient.  But it's important to keep 
in mind that the cycling start point varies according to all the 
variables that include among others, input line voltage, heater 
apparatus time constant, PID damping setting, and ambient temperature 
(heat loss) among others. This cycling starts minutes in advance on just 
a simple hot plate that  I built a controller for using a controller I 
got on Ebay a few years ago.  I just searched Ebay using "digital 
temperature controller" and the first  item for $39 looks to be the same 
one I used.  I had an SSR I used, but multi-amp SSR's from MPJA 
Electronics are available for $5 to $8.  I built the whole thing into a 
small  plastic electrical box from Home Depot along with a duplex 
receptacle.  That way I can plug in a hot plate, electric skillet or 
some other heater with ease. A great feature is that this controller  
can self-tune.  That saves a lot of time.  Tuning a PID can be a long, 
tedious process if tried by hit or miss.  The controller is started, 
allowed to heat up.  It internally measures how long it took, and the 
amount of overshoot and time to return back to the setpoint.  After that 
it internally sets the constants needed, loads them to flash memory so 
powering the unit down doesn't lose the information,  and does a great 
job.    It would be nice if there were a way to externally control the 
setpoint then the idea of thermal profile reflow would be possible, but 
that controller would be quite a bit more money.

I've seen hacker solutions where a uP is used to decode the display 
signals to the device to be controlled (such as this temperature 
controller)   Then input to the control switches to change its 
settings.  However the level of that task is almost the same as just 
starting from scratch and making PID controller with the chips you 
mentioned.

Regards,
Charles R. Patton

On 3/13/2011 6:32 AM, Tony Smith wrote:
>
> > David, You could try making one. The REPRAP group makes pid temperature
> > controls for the heated bed and extruder.
>
> Ok, I looked up the RepRap PID, and they use the 'usual chip' for the
> thermocouple, the AD595. Sparkfun - http://www.sparkfun.com/products/306 -
> have them for $18. The other 'usual chip' is the MAX6675, and it's $12.
> Ouch. You still need a display, controls, microcontroller, PID coding,
> power supply and a few other bits & pieces.
>
> I think I'll take my chances with the $25 eBay unit.
>
> My only problem with that unit is it doesn't (I think) send a signal 
> when it
> reaches temperature, you could use that to start a timer. It has an alarm,
> but that's only for under/over-temperature. I figure I can do it by
> monitoring the heater, and when it switches off for the first time use 
> that
> to start the timer.
>
> Oh, and I should mention it doesn't have a SSD built-in, but it can 
> drive an
> SSD. It also has a relay that can handle 240v @ 3 amps.
>
> Tony
>
>
>
>