Remote Programming 127
PTC10 Programmable Temperature Controller
Drive a solid state relay with the digital IO lines
In some high-power applications, the current to a heating or cooling unit is provided by an
external power supply and modulated with an external solid state relay (SSR). To modulate the
heater or cooler power and obtain accurate temperature control, a variable duty cycle square
wave, similar to pulse width modulation but typically with a much longer cycle time, is required
from the PTC10. For example, to supply half of the maximum power to the heater, the PTC10
would need to turn the relay on for 5 seconds, off for 5 seconds, on for 5 seconds, etc.
The following procedure transforms the output of a PID feedback loop into a variable duty cycle
square wave that can be output on t tate
relay. The macro works well as long as a period of about 10 seconds or longer and a resolution of
0.1 seconds is acceptable. If a much shorter period or greater resolution is needed, it would be
better to fabricate an external analog-to-PWM circuit and drive it with an analog I/O channel.
First, make channel V1 the feedback output, and make it produce a value between 0 and 100.
To do this, select channel V1 and set the following parameters:
lmt: 0
Next, select channel DIO and set the following parameters:
ould be blank; or, the PID mode should be off.
Now run the following macro by sending it over a serial port (in which case it all has to be on
one line) or by copying it onto a USB stick (save it as a .txt file in
[
waitForSample
#d = 0
if (#V1>#t) { #d = 1 }
DIO = #d
#t += 1
if (#t>100) { #t = 0 }
]-1
To test the macro, set V1's value to 50 and plot channel DIO. You should see a square wave
with a duty cycle of 50% and a period of 10 seconds: high for 5 seconds, low for 5 seconds, high
for 5 seconds, etc. Reduce V1 to 25 and the duty cycle should go to 25%.
Before the feedback can be used, the PID gain factors will need to be set by using the automatic
tuning feature on channel V1. If tuning is successful, the feedback should now operate normally.
If more than one feedback loop is required, set up channels V2 and/or V3 as described for
channel V1, and add these lines after the { #d = 1 } statement:
if (#V2>#t) { #d+=2 }
if (#V3>#t) { #d+=4 }
To Next Page