Re: Student Question on using a PID as a Dimmer to Control a Light Bulb!!! This is a fun one!!!!!!

deriso@xxxxxxxxx wrote:
So, I would:

1) find a PID that has a continuous analog output (0-10v dc)
(can you explain what that is a little more :) ...

Yes, the controller has to have either a voltage output or a current output (that can be converted to a voltage with a load resistor) that is compatible with the phase power controller relay. It also has to accept the type of thermocouple you have or can get.

does it output a voltage from 0-10v dc according to what it thinks
my lamp needs to
heat to?

Yes. 0% output is zero volts, and 100% output is 10 volts.

ex. set temp is 200deg C
current object temp is 150deg C

the PID will give a voltage output of lets say 8.789 volts
(my guess) and decrease
the voltage to say 5.565 as the temp nears the setpoint (or
however the graph
goes...). The idea is that it gives its approximation of how
far my temperature is
from my set temp in terms of an analog (dc?) voltage. i might
be very lost here...

The PID algorithm compares the present value of the input (temperature) with the stored setpoint value and computes the three terms (proportional, integral and derivative) and combines the three factors to produce the output. You give the controller a model of the response of the system, so it uses the correct amount of each factor to push the measured value toward setpoint without overshooting or oscillating. If you want to read a nontechnical tutorial about how to tune a PID controller and how the three factors act, one I wrote is available at:
http://www.tcnj.edu/~rgraham/PID/popelish.html

2) use an SSR that converts the analog voltage output to a phase
control (dont really
know what that is either... but it looks expensive) What is a phase
control??

It is a solid state switch that holds back the line voltage for some part of each half line cycle and then turns on and conducts for the rest of the half cycle. The fraction of each half cycle that line voltage is applied to the load is controlled by the DC input voltage. So, if the power comes from a 60 Hz line frequency, you get 120 power pulses per second, which is fast enough for the lamp to have very little flicker.

3) take that phase output and put it into some kind of dimmer circuit
like this one
http://www.epanorama.net/documents/lights/lightdimmer.html#advanced

What goes to the lamp? the phase output? is that a voltage potential?
please explain...

If you get one of the phase controlled relays, it controls the power to the lamp, directly. you just wire its two output terminals in a break in the lamp circuit.

That link shows you approximately what is inside the phase control relay, if you would rather buy than build.
.

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