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
>
>
>
>